yaml-cpp

cook_book.md (146528B)

---

 # gMock Cookbook
 
 <!-- GOOGLETEST_CM0012 DO NOT DELETE -->
 
 You can find recipes for using gMock here. If you haven't yet, please read
 [this](for_dummies.md) first to make sure you understand the basics.
 
 **Note:** gMock lives in the `testing` name space. For readability, it is
 recommended to write `using ::testing::Foo;` once in your file before using the
 name `Foo` defined by gMock. We omit such `using` statements in this section for
 brevity, but you should do it in your own code.
 
 ## Creating Mock Classes
 
 Mock classes are defined as normal classes, using the `MOCK_METHOD` macro to
 generate mocked methods. The macro gets 3 or 4 parameters:
 
 ```cpp
 class MyMock {
  public:
   MOCK_METHOD(ReturnType, MethodName, (Args...));
   MOCK_METHOD(ReturnType, MethodName, (Args...), (Specs...));
 };
 ```
 
 The first 3 parameters are simply the method declaration, split into 3 parts.
 The 4th parameter accepts a closed list of qualifiers, which affect the
 generated method:
 
 *   **`const`** - Makes the mocked method a `const` method. Required if
     overriding a `const` method.
 *   **`override`** - Marks the method with `override`. Recommended if overriding
     a `virtual` method.
 *   **`noexcept`** - Marks the method with `noexcept`. Required if overriding a
     `noexcept` method.
 *   **`Calltype(...)`** - Sets the call type for the method (e.g. to
     `STDMETHODCALLTYPE`), useful in Windows.
 
 ### Dealing with unprotected commas
 
 Unprotected commas, i.e. commas which are not surrounded by parentheses, prevent
 `MOCK_METHOD` from parsing its arguments correctly:
 
 ```cpp {.bad}
 class MockFoo {
  public:
   MOCK_METHOD(std::pair<bool, int>, GetPair, ());  // Won't compile!
   MOCK_METHOD(bool, CheckMap, (std::map<int, double>, bool));  // Won't compile!
 };
 ```
 
 Solution 1 - wrap with parentheses:
 
 ```cpp {.good}
 class MockFoo {
  public:
   MOCK_METHOD((std::pair<bool, int>), GetPair, ());
   MOCK_METHOD(bool, CheckMap, ((std::map<int, double>), bool));
 };
 ```
 
 Note that wrapping a return or argument type with parentheses is, in general,
 invalid C++. `MOCK_METHOD` removes the parentheses.
 
 Solution 2 - define an alias:
 
 ```cpp {.good}
 class MockFoo {
  public:
   using BoolAndInt = std::pair<bool, int>;
   MOCK_METHOD(BoolAndInt, GetPair, ());
   using MapIntDouble = std::map<int, double>;
   MOCK_METHOD(bool, CheckMap, (MapIntDouble, bool));
 };
 ```
 
 ### Mocking Private or Protected Methods
 
 You must always put a mock method definition (`MOCK_METHOD`) in a `public:`
 section of the mock class, regardless of the method being mocked being `public`,
 `protected`, or `private` in the base class. This allows `ON_CALL` and
 `EXPECT_CALL` to reference the mock function from outside of the mock class.
 (Yes, C++ allows a subclass to change the access level of a virtual function in
 the base class.) Example:
 
 ```cpp
 class Foo {
  public:
   ...
   virtual bool Transform(Gadget* g) = 0;
 
  protected:
   virtual void Resume();
 
  private:
   virtual int GetTimeOut();
 };
 
 class MockFoo : public Foo {
  public:
   ...
   MOCK_METHOD(bool, Transform, (Gadget* g), (override));
 
   // The following must be in the public section, even though the
   // methods are protected or private in the base class.
   MOCK_METHOD(void, Resume, (), (override));
   MOCK_METHOD(int, GetTimeOut, (), (override));
 };
 ```
 
 ### Mocking Overloaded Methods
 
 You can mock overloaded functions as usual. No special attention is required:
 
 ```cpp
 class Foo {
   ...
 
   // Must be virtual as we'll inherit from Foo.
   virtual ~Foo();
 
   // Overloaded on the types and/or numbers of arguments.
   virtual int Add(Element x);
   virtual int Add(int times, Element x);
 
   // Overloaded on the const-ness of this object.
   virtual Bar& GetBar();
   virtual const Bar& GetBar() const;
 };
 
 class MockFoo : public Foo {
   ...
   MOCK_METHOD(int, Add, (Element x), (override));
   MOCK_METHOD(int, Add, (int times, Element x), (override));
 
   MOCK_METHOD(Bar&, GetBar, (), (override));
   MOCK_METHOD(const Bar&, GetBar, (), (const, override));
 };
 ```
 
 **Note:** if you don't mock all versions of the overloaded method, the compiler
 will give you a warning about some methods in the base class being hidden. To
 fix that, use `using` to bring them in scope:
 
 ```cpp
 class MockFoo : public Foo {
   ...
   using Foo::Add;
   MOCK_METHOD(int, Add, (Element x), (override));
   // We don't want to mock int Add(int times, Element x);
   ...
 };
 ```
 
 ### Mocking Class Templates
 
 You can mock class templates just like any class.
 
 ```cpp
 template <typename Elem>
 class StackInterface {
   ...
   // Must be virtual as we'll inherit from StackInterface.
   virtual ~StackInterface();
 
   virtual int GetSize() const = 0;
   virtual void Push(const Elem& x) = 0;
 };
 
 template <typename Elem>
 class MockStack : public StackInterface<Elem> {
   ...
   MOCK_METHOD(int, GetSize, (), (override));
   MOCK_METHOD(void, Push, (const Elem& x), (override));
 };
 ```
 
 ### Mocking Non-virtual Methods {#MockingNonVirtualMethods}
 
 gMock can mock non-virtual functions to be used in Hi-perf dependency
 injection.<!-- GOOGLETEST_CM0017 DO NOT DELETE -->
 
 In this case, instead of sharing a common base class with the real class, your
 mock class will be *unrelated* to the real class, but contain methods with the
 same signatures. The syntax for mocking non-virtual methods is the *same* as
 mocking virtual methods (just don't add `override`):
 
 ```cpp
 // A simple packet stream class.  None of its members is virtual.
 class ConcretePacketStream {
  public:
   void AppendPacket(Packet* new_packet);
   const Packet* GetPacket(size_t packet_number) const;
   size_t NumberOfPackets() const;
   ...
 };
 
 // A mock packet stream class.  It inherits from no other, but defines
 // GetPacket() and NumberOfPackets().
 class MockPacketStream {
  public:
   MOCK_METHOD(const Packet*, GetPacket, (size_t packet_number), (const));
   MOCK_METHOD(size_t, NumberOfPackets, (), (const));
   ...
 };
 ```
 
 Note that the mock class doesn't define `AppendPacket()`, unlike the real class.
 That's fine as long as the test doesn't need to call it.
 
 Next, you need a way to say that you want to use `ConcretePacketStream` in
 production code, and use `MockPacketStream` in tests. Since the functions are
 not virtual and the two classes are unrelated, you must specify your choice at
 *compile time* (as opposed to run time).
 
 One way to do it is to templatize your code that needs to use a packet stream.
 More specifically, you will give your code a template type argument for the type
 of the packet stream. In production, you will instantiate your template with
 `ConcretePacketStream` as the type argument. In tests, you will instantiate the
 same template with `MockPacketStream`. For example, you may write:
 
 ```cpp
 template <class PacketStream>
 void CreateConnection(PacketStream* stream) { ... }
 
 template <class PacketStream>
 class PacketReader {
  public:
   void ReadPackets(PacketStream* stream, size_t packet_num);
 };
 ```
 
 Then you can use `CreateConnection<ConcretePacketStream>()` and
 `PacketReader<ConcretePacketStream>` in production code, and use
 `CreateConnection<MockPacketStream>()` and `PacketReader<MockPacketStream>` in
 tests.
 
 ```cpp
   MockPacketStream mock_stream;
   EXPECT_CALL(mock_stream, ...)...;
   .. set more expectations on mock_stream ...
   PacketReader<MockPacketStream> reader(&mock_stream);
   ... exercise reader ...
 ```
 
 ### Mocking Free Functions
 
 It's possible to use gMock to mock a free function (i.e. a C-style function or a
 static method). You just need to rewrite your code to use an interface (abstract
 class).
 
 Instead of calling a free function (say, `OpenFile`) directly, introduce an
 interface for it and have a concrete subclass that calls the free function:
 
 ```cpp
 class FileInterface {
  public:
   ...
   virtual bool Open(const char* path, const char* mode) = 0;
 };
 
 class File : public FileInterface {
  public:
   ...
   virtual bool Open(const char* path, const char* mode) {
      return OpenFile(path, mode);
   }
 };
 ```
 
 Your code should talk to `FileInterface` to open a file. Now it's easy to mock
 out the function.
 
 This may seem like a lot of hassle, but in practice you often have multiple
 related functions that you can put in the same interface, so the per-function
 syntactic overhead will be much lower.
 
 If you are concerned about the performance overhead incurred by virtual
 functions, and profiling confirms your concern, you can combine this with the
 recipe for [mocking non-virtual methods](#MockingNonVirtualMethods).
 
 ### Old-Style `MOCK_METHODn` Macros
 
 Before the generic `MOCK_METHOD` macro was introduced, mocks where created using
 a family of macros collectively called `MOCK_METHODn`. These macros are still
 supported, though migration to the new `MOCK_METHOD` is recommended.
 
 The macros in the `MOCK_METHODn` family differ from `MOCK_METHOD`:
 
 *   The general structure is `MOCK_METHODn(MethodName, ReturnType(Args))`,
     instead of `MOCK_METHOD(ReturnType, MethodName, (Args))`.
 *   The number `n` must equal the number of arguments.
 *   When mocking a const method, one must use `MOCK_CONST_METHODn`.
 *   When mocking a class template, the macro name must be suffixed with `_T`.
 *   In order to specify the call type, the macro name must be suffixed with
     `_WITH_CALLTYPE`, and the call type is the first macro argument.
 
 Old macros and their new equivalents:
 
 <a name="table99"></a>
 <table border="1" cellspacing="0" cellpadding="1">
 <tr> <th colspan=2> Simple </th></tr>
 <tr> <td> Old </td> <td> `MOCK_METHOD1(Foo, bool(int))` </td> </tr>
 <tr> <td> New </td> <td> `MOCK_METHOD(bool, Foo, (int))` </td> </tr>
 
 <tr> <th colspan=2> Const Method </th></tr> <tr> <td> Old </td> <td>
 `MOCK_CONST_METHOD1(Foo, bool(int))` </td> </tr> <tr> <td> New </td> <td>
 `MOCK_METHOD(bool, Foo, (int), (const))` </td> </tr>
 
 <tr> <th colspan=2> Method in a Class Template </th></tr> <tr> <td> Old </td>
 <td> `MOCK_METHOD1_T(Foo, bool(int))` </td> </tr> <tr> <td> New </td> <td>
 `MOCK_METHOD(bool, Foo, (int))` </td> </tr>
 
 <tr> <th colspan=2> Const Method in a Class Template </th></tr> <tr> <td> Old
 </td> <td> `MOCK_CONST_METHOD1_T(Foo, bool(int))` </td> </tr> <tr> <td> New
 </td> <td> `MOCK_METHOD(bool, Foo, (int), (const))` </td> </tr>
 
 <tr> <th colspan=2> Method with Call Type </th></tr> <tr> <td> Old </td> <td>
 `MOCK_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))` </td> </tr> <tr>
 <td> New </td> <td> `MOCK_METHOD(bool, Foo, (int),
 (Calltype(STDMETHODCALLTYPE)))` </td> </tr>
 
 <tr> <th colspan=2> Const Method with Call Type </th></tr> <tr> <td> Old</td>
 <td> `MOCK_CONST_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))` </td>
 </tr> <tr> <td> New </td> <td> `MOCK_METHOD(bool, Foo, (int), (const,
 Calltype(STDMETHODCALLTYPE)))` </td> </tr>
 
 <tr> <th colspan=2> Method with Call Type in a Class Template </th></tr> <tr>
 <td> Old </td> <td> `MOCK_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo,
 bool(int))` </td> </tr> <tr> <td> New </td> <td> `MOCK_METHOD(bool, Foo, (int),
 (Calltype(STDMETHODCALLTYPE)))` </td> </tr>
 
 <tr> <th colspan=2> Const Method with Call Type in a Class Template </th></tr>
 <tr> <td> Old </td> <td> `MOCK_CONST_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE,
 Foo, bool(int))` </td> </tr> <tr> <td> New </td> <td> `MOCK_METHOD(bool, Foo,
 (int), (const, Calltype(STDMETHODCALLTYPE)))` </td> </tr>
 
 </table>
 
 ### The Nice, the Strict, and the Naggy {#NiceStrictNaggy}
 
 If a mock method has no `EXPECT_CALL` spec but is called, we say that it's an
 "uninteresting call", and the default action (which can be specified using
 `ON_CALL()`) of the method will be taken. Currently, an uninteresting call will
 also by default cause gMock to print a warning. (In the future, we might remove
 this warning by default.)
 
 However, sometimes you may want to ignore these uninteresting calls, and
 sometimes you may want to treat them as errors. gMock lets you make the decision
 on a per-mock-object basis.
 
 Suppose your test uses a mock class `MockFoo`:
 
 ```cpp
 TEST(...) {
   MockFoo mock_foo;
   EXPECT_CALL(mock_foo, DoThis());
   ... code that uses mock_foo ...
 }
 ```
 
 If a method of `mock_foo` other than `DoThis()` is called, you will get a
 warning. However, if you rewrite your test to use `NiceMock<MockFoo>` instead,
 you can suppress the warning:
 
 ```cpp
 using ::testing::NiceMock;
 
 TEST(...) {
   NiceMock<MockFoo> mock_foo;
   EXPECT_CALL(mock_foo, DoThis());
   ... code that uses mock_foo ...
 }
 ```
 
 `NiceMock<MockFoo>` is a subclass of `MockFoo`, so it can be used wherever
 `MockFoo` is accepted.
 
 It also works if `MockFoo`'s constructor takes some arguments, as
 `NiceMock<MockFoo>` "inherits" `MockFoo`'s constructors:
 
 ```cpp
 using ::testing::NiceMock;
 
 TEST(...) {
   NiceMock<MockFoo> mock_foo(5, "hi");  // Calls MockFoo(5, "hi").
   EXPECT_CALL(mock_foo, DoThis());
   ... code that uses mock_foo ...
 }
 ```
 
 The usage of `StrictMock` is similar, except that it makes all uninteresting
 calls failures:
 
 ```cpp
 using ::testing::StrictMock;
 
 TEST(...) {
   StrictMock<MockFoo> mock_foo;
   EXPECT_CALL(mock_foo, DoThis());
   ... code that uses mock_foo ...
 
   // The test will fail if a method of mock_foo other than DoThis()
   // is called.
 }
 ```
 
 NOTE: `NiceMock` and `StrictMock` only affects *uninteresting* calls (calls of
 *methods* with no expectations); they do not affect *unexpected* calls (calls of
 methods with expectations, but they don't match). See
 [Understanding Uninteresting vs Unexpected Calls](#uninteresting-vs-unexpected).
 
 There are some caveats though (I dislike them just as much as the next guy, but
 sadly they are side effects of C++'s limitations):
 
 1.  `NiceMock<MockFoo>` and `StrictMock<MockFoo>` only work for mock methods
     defined using the `MOCK_METHOD` macro **directly** in the `MockFoo` class.
     If a mock method is defined in a **base class** of `MockFoo`, the "nice" or
     "strict" modifier may not affect it, depending on the compiler. In
     particular, nesting `NiceMock` and `StrictMock` (e.g.
     `NiceMock<StrictMock<MockFoo> >`) is **not** supported.
 2.  `NiceMock<MockFoo>` and `StrictMock<MockFoo>` may not work correctly if the
     destructor of `MockFoo` is not virtual. We would like to fix this, but it
     requires cleaning up existing tests. http://b/28934720 tracks the issue.
 3.  During the constructor or destructor of `MockFoo`, the mock object is *not*
     nice or strict. This may cause surprises if the constructor or destructor
     calls a mock method on `this` object. (This behavior, however, is consistent
     with C++'s general rule: if a constructor or destructor calls a virtual
     method of `this` object, that method is treated as non-virtual. In other
     words, to the base class's constructor or destructor, `this` object behaves
     like an instance of the base class, not the derived class. This rule is
     required for safety. Otherwise a base constructor may use members of a
     derived class before they are initialized, or a base destructor may use
     members of a derived class after they have been destroyed.)
 
 Finally, you should be **very cautious** about when to use naggy or strict
 mocks, as they tend to make tests more brittle and harder to maintain. When you
 refactor your code without changing its externally visible behavior, ideally you
 shouldn't need to update any tests. If your code interacts with a naggy mock,
 however, you may start to get spammed with warnings as the result of your
 change. Worse, if your code interacts with a strict mock, your tests may start
 to fail and you'll be forced to fix them. Our general recommendation is to use
 nice mocks (not yet the default) most of the time, use naggy mocks (the current
 default) when developing or debugging tests, and use strict mocks only as the
 last resort.
 
 ### Simplifying the Interface without Breaking Existing Code {#SimplerInterfaces}
 
 Sometimes a method has a long list of arguments that is mostly uninteresting.
 For example:
 
 ```cpp
 class LogSink {
  public:
   ...
   virtual void send(LogSeverity severity, const char* full_filename,
                     const char* base_filename, int line,
                     const struct tm* tm_time,
                     const char* message, size_t message_len) = 0;
 };
 ```
 
 This method's argument list is lengthy and hard to work with (the `message`
 argument is not even 0-terminated). If we mock it as is, using the mock will be
 awkward. If, however, we try to simplify this interface, we'll need to fix all
 clients depending on it, which is often infeasible.
 
 The trick is to redispatch the method in the mock class:
 
 ```cpp
 class ScopedMockLog : public LogSink {
  public:
   ...
   virtual void send(LogSeverity severity, const char* full_filename,
                     const char* base_filename, int line, const tm* tm_time,
                     const char* message, size_t message_len) {
     // We are only interested in the log severity, full file name, and
     // log message.
     Log(severity, full_filename, std::string(message, message_len));
   }
 
   // Implements the mock method:
   //
   //   void Log(LogSeverity severity,
   //            const string& file_path,
   //            const string& message);
   MOCK_METHOD(void, Log,
               (LogSeverity severity, const string& file_path,
                const string& message));
 };
 ```
 
 By defining a new mock method with a trimmed argument list, we make the mock
 class more user-friendly.
 
 This technique may also be applied to make overloaded methods more amenable to
 mocking. For example, when overloads have been used to implement default
 arguments:
 
 ```cpp
 class MockTurtleFactory : public TurtleFactory {
  public:
   Turtle* MakeTurtle(int length, int weight) override { ... }
   Turtle* MakeTurtle(int length, int weight, int speed) override { ... }
 
   // the above methods delegate to this one:
   MOCK_METHOD(Turtle*, DoMakeTurtle, ());
 };
 ```
 
 This allows tests that don't care which overload was invoked to avoid specifying
 argument matchers:
 
 ```cpp
 ON_CALL(factory, DoMakeTurtle)
     .WillByDefault(MakeMockTurtle());
 ```
 
 ### Alternative to Mocking Concrete Classes
 
 Often you may find yourself using classes that don't implement interfaces. In
 order to test your code that uses such a class (let's call it `Concrete`), you
 may be tempted to make the methods of `Concrete` virtual and then mock it.
 
 Try not to do that.
 
 Making a non-virtual function virtual is a big decision. It creates an extension
 point where subclasses can tweak your class' behavior. This weakens your control
 on the class because now it's harder to maintain the class invariants. You
 should make a function virtual only when there is a valid reason for a subclass
 to override it.
 
 Mocking concrete classes directly is problematic as it creates a tight coupling
 between the class and the tests - any small change in the class may invalidate
 your tests and make test maintenance a pain.
 
 To avoid such problems, many programmers have been practicing "coding to
 interfaces": instead of talking to the `Concrete` class, your code would define
 an interface and talk to it. Then you implement that interface as an adaptor on
 top of `Concrete`. In tests, you can easily mock that interface to observe how
 your code is doing.
 
 This technique incurs some overhead:
 
 *   You pay the cost of virtual function calls (usually not a problem).
 *   There is more abstraction for the programmers to learn.
 
 However, it can also bring significant benefits in addition to better
 testability:
 
 *   `Concrete`'s API may not fit your problem domain very well, as you may not
     be the only client it tries to serve. By designing your own interface, you
     have a chance to tailor it to your need - you may add higher-level
     functionalities, rename stuff, etc instead of just trimming the class. This
     allows you to write your code (user of the interface) in a more natural way,
     which means it will be more readable, more maintainable, and you'll be more
     productive.
 *   If `Concrete`'s implementation ever has to change, you don't have to rewrite
     everywhere it is used. Instead, you can absorb the change in your
     implementation of the interface, and your other code and tests will be
     insulated from this change.
 
 Some people worry that if everyone is practicing this technique, they will end
 up writing lots of redundant code. This concern is totally understandable.
 However, there are two reasons why it may not be the case:
 
 *   Different projects may need to use `Concrete` in different ways, so the best
     interfaces for them will be different. Therefore, each of them will have its
     own domain-specific interface on top of `Concrete`, and they will not be the
     same code.
 *   If enough projects want to use the same interface, they can always share it,
     just like they have been sharing `Concrete`. You can check in the interface
     and the adaptor somewhere near `Concrete` (perhaps in a `contrib`
     sub-directory) and let many projects use it.
 
 You need to weigh the pros and cons carefully for your particular problem, but
 I'd like to assure you that the Java community has been practicing this for a
 long time and it's a proven effective technique applicable in a wide variety of
 situations. :-)
 
 ### Delegating Calls to a Fake {#DelegatingToFake}
 
 Some times you have a non-trivial fake implementation of an interface. For
 example:
 
 ```cpp
 class Foo {
  public:
   virtual ~Foo() {}
   virtual char DoThis(int n) = 0;
   virtual void DoThat(const char* s, int* p) = 0;
 };
 
 class FakeFoo : public Foo {
  public:
   char DoThis(int n) override {
     return (n > 0) ? '+' :
            (n < 0) ? '-' : '0';
   }
 
   void DoThat(const char* s, int* p) override {
     *p = strlen(s);
   }
 };
 ```
 
 Now you want to mock this interface such that you can set expectations on it.
 However, you also want to use `FakeFoo` for the default behavior, as duplicating
 it in the mock object is, well, a lot of work.
 
 When you define the mock class using gMock, you can have it delegate its default
 action to a fake class you already have, using this pattern:
 
 ```cpp
 class MockFoo : public Foo {
  public:
   // Normal mock method definitions using gMock.
   MOCK_METHOD(char, DoThis, (int n), (override));
   MOCK_METHOD(void, DoThat, (const char* s, int* p), (override));
 
   // Delegates the default actions of the methods to a FakeFoo object.
   // This must be called *before* the custom ON_CALL() statements.
   void DelegateToFake() {
     ON_CALL(*this, DoThis).WillByDefault([this](int n) {
       return fake_.DoThis(n);
     });
     ON_CALL(*this, DoThat).WillByDefault([this](const char* s, int* p) {
       fake_.DoThat(s, p);
     });
   }
 
  private:
   FakeFoo fake_;  // Keeps an instance of the fake in the mock.
 };
 ```
 
 With that, you can use `MockFoo` in your tests as usual. Just remember that if
 you don't explicitly set an action in an `ON_CALL()` or `EXPECT_CALL()`, the
 fake will be called upon to do it.:
 
 ```cpp
 using ::testing::_;
 
 TEST(AbcTest, Xyz) {
   MockFoo foo;
 
   foo.DelegateToFake();  // Enables the fake for delegation.
 
   // Put your ON_CALL(foo, ...)s here, if any.
 
   // No action specified, meaning to use the default action.
   EXPECT_CALL(foo, DoThis(5));
   EXPECT_CALL(foo, DoThat(_, _));
 
   int n = 0;
   EXPECT_EQ('+', foo.DoThis(5));  // FakeFoo::DoThis() is invoked.
   foo.DoThat("Hi", &n);  // FakeFoo::DoThat() is invoked.
   EXPECT_EQ(2, n);
 }
 ```
 
 **Some tips:**
 
 *   If you want, you can still override the default action by providing your own
     `ON_CALL()` or using `.WillOnce()` / `.WillRepeatedly()` in `EXPECT_CALL()`.
 *   In `DelegateToFake()`, you only need to delegate the methods whose fake
     implementation you intend to use.
 
 *   The general technique discussed here works for overloaded methods, but
     you'll need to tell the compiler which version you mean. To disambiguate a
     mock function (the one you specify inside the parentheses of `ON_CALL()`),
     use [this technique](#SelectOverload); to disambiguate a fake function (the
     one you place inside `Invoke()`), use a `static_cast` to specify the
     function's type. For instance, if class `Foo` has methods `char DoThis(int
     n)` and `bool DoThis(double x) const`, and you want to invoke the latter,
     you need to write `Invoke(&fake_, static_cast<bool (FakeFoo::*)(double)
     const>(&FakeFoo::DoThis))` instead of `Invoke(&fake_, &FakeFoo::DoThis)`
     (The strange-looking thing inside the angled brackets of `static_cast` is
     the type of a function pointer to the second `DoThis()` method.).
 
 *   Having to mix a mock and a fake is often a sign of something gone wrong.
     Perhaps you haven't got used to the interaction-based way of testing yet. Or
     perhaps your interface is taking on too many roles and should be split up.
     Therefore, **don't abuse this**. We would only recommend to do it as an
     intermediate step when you are refactoring your code.
 
 Regarding the tip on mixing a mock and a fake, here's an example on why it may
 be a bad sign: Suppose you have a class `System` for low-level system
 operations. In particular, it does file and I/O operations. And suppose you want
 to test how your code uses `System` to do I/O, and you just want the file
 operations to work normally. If you mock out the entire `System` class, you'll
 have to provide a fake implementation for the file operation part, which
 suggests that `System` is taking on too many roles.
 
 Instead, you can define a `FileOps` interface and an `IOOps` interface and split
 `System`'s functionalities into the two. Then you can mock `IOOps` without
 mocking `FileOps`.
 
 ### Delegating Calls to a Real Object
 
 When using testing doubles (mocks, fakes, stubs, and etc), sometimes their
 behaviors will differ from those of the real objects. This difference could be
 either intentional (as in simulating an error such that you can test the error
 handling code) or unintentional. If your mocks have different behaviors than the
 real objects by mistake, you could end up with code that passes the tests but
 fails in production.
 
 You can use the *delegating-to-real* technique to ensure that your mock has the
 same behavior as the real object while retaining the ability to validate calls.
 This technique is very similar to the [delegating-to-fake](#DelegatingToFake)
 technique, the difference being that we use a real object instead of a fake.
 Here's an example:
 
 ```cpp
 using ::testing::AtLeast;
 
 class MockFoo : public Foo {
  public:
   MockFoo() {
     // By default, all calls are delegated to the real object.
     ON_CALL(*this, DoThis).WillByDefault([this](int n) {
       return real_.DoThis(n);
     });
     ON_CALL(*this, DoThat).WillByDefault([this](const char* s, int* p) {
       real_.DoThat(s, p);
     });
     ...
   }
   MOCK_METHOD(char, DoThis, ...);
   MOCK_METHOD(void, DoThat, ...);
   ...
  private:
   Foo real_;
 };
 
 ...
   MockFoo mock;
   EXPECT_CALL(mock, DoThis())
       .Times(3);
   EXPECT_CALL(mock, DoThat("Hi"))
       .Times(AtLeast(1));
   ... use mock in test ...
 ```
 
 With this, gMock will verify that your code made the right calls (with the right
 arguments, in the right order, called the right number of times, etc), and a
 real object will answer the calls (so the behavior will be the same as in
 production). This gives you the best of both worlds.
 
 ### Delegating Calls to a Parent Class
 
 Ideally, you should code to interfaces, whose methods are all pure virtual. In
 reality, sometimes you do need to mock a virtual method that is not pure (i.e,
 it already has an implementation). For example:
 
 ```cpp
 class Foo {
  public:
   virtual ~Foo();
 
   virtual void Pure(int n) = 0;
   virtual int Concrete(const char* str) { ... }
 };
 
 class MockFoo : public Foo {
  public:
   // Mocking a pure method.
   MOCK_METHOD(void, Pure, (int n), (override));
   // Mocking a concrete method.  Foo::Concrete() is shadowed.
   MOCK_METHOD(int, Concrete, (const char* str), (override));
 };
 ```
 
 Sometimes you may want to call `Foo::Concrete()` instead of
 `MockFoo::Concrete()`. Perhaps you want to do it as part of a stub action, or
 perhaps your test doesn't need to mock `Concrete()` at all (but it would be
 oh-so painful to have to define a new mock class whenever you don't need to mock
 one of its methods).
 
 The trick is to leave a back door in your mock class for accessing the real
 methods in the base class:
 
 ```cpp
 class MockFoo : public Foo {
  public:
   // Mocking a pure method.
   MOCK_METHOD(void, Pure, (int n), (override));
   // Mocking a concrete method.  Foo::Concrete() is shadowed.
   MOCK_METHOD(int, Concrete, (const char* str), (override));
 
   // Use this to call Concrete() defined in Foo.
   int FooConcrete(const char* str) { return Foo::Concrete(str); }
 };
 ```
 
 Now, you can call `Foo::Concrete()` inside an action by:
 
 ```cpp
 ...
   EXPECT_CALL(foo, Concrete).WillOnce([&foo](const char* str) {
     return foo.FooConcrete(str);
   });
 ```
 
 or tell the mock object that you don't want to mock `Concrete()`:
 
 ```cpp
 ...
   ON_CALL(foo, Concrete).WillByDefault([&foo](const char* str) {
     return foo.FooConcrete(str);
   });
 ```
 
 (Why don't we just write `{ return foo.Concrete(str); }`? If you do that,
 `MockFoo::Concrete()` will be called (and cause an infinite recursion) since
 `Foo::Concrete()` is virtual. That's just how C++ works.)
 
 ## Using Matchers
 
 ### Matching Argument Values Exactly
 
 You can specify exactly which arguments a mock method is expecting:
 
 ```cpp
 using ::testing::Return;
 ...
   EXPECT_CALL(foo, DoThis(5))
       .WillOnce(Return('a'));
   EXPECT_CALL(foo, DoThat("Hello", bar));
 ```
 
 ### Using Simple Matchers
 
 You can use matchers to match arguments that have a certain property:
 
 ```cpp
 using ::testing::NotNull;
 using ::testing::Return;
 ...
   EXPECT_CALL(foo, DoThis(Ge(5)))  // The argument must be >= 5.
       .WillOnce(Return('a'));
   EXPECT_CALL(foo, DoThat("Hello", NotNull()));
       // The second argument must not be NULL.
 ```
 
 A frequently used matcher is `_`, which matches anything:
 
 ```cpp
   EXPECT_CALL(foo, DoThat(_, NotNull()));
 ```
 <!-- GOOGLETEST_CM0022 DO NOT DELETE -->
 
 ### Combining Matchers {#CombiningMatchers}
 
 You can build complex matchers from existing ones using `AllOf()`,
 `AllOfArray()`, `AnyOf()`, `AnyOfArray()` and `Not()`:
 
 ```cpp
 using ::testing::AllOf;
 using ::testing::Gt;
 using ::testing::HasSubstr;
 using ::testing::Ne;
 using ::testing::Not;
 ...
   // The argument must be > 5 and != 10.
   EXPECT_CALL(foo, DoThis(AllOf(Gt(5),
                                 Ne(10))));
 
   // The first argument must not contain sub-string "blah".
   EXPECT_CALL(foo, DoThat(Not(HasSubstr("blah")),
                           NULL));
 ```
 
 ### Casting Matchers {#SafeMatcherCast}
 
 gMock matchers are statically typed, meaning that the compiler can catch your
 mistake if you use a matcher of the wrong type (for example, if you use `Eq(5)`
 to match a `string` argument). Good for you!
 
 Sometimes, however, you know what you're doing and want the compiler to give you
 some slack. One example is that you have a matcher for `long` and the argument
 you want to match is `int`. While the two types aren't exactly the same, there
 is nothing really wrong with using a `Matcher<long>` to match an `int` - after
 all, we can first convert the `int` argument to a `long` losslessly before
 giving it to the matcher.
 
 To support this need, gMock gives you the `SafeMatcherCast<T>(m)` function. It
 casts a matcher `m` to type `Matcher<T>`. To ensure safety, gMock checks that
 (let `U` be the type `m` accepts :
 
 1.  Type `T` can be *implicitly* cast to type `U`;
 2.  When both `T` and `U` are built-in arithmetic types (`bool`, integers, and
     floating-point numbers), the conversion from `T` to `U` is not lossy (in
     other words, any value representable by `T` can also be represented by `U`);
     and
 3.  When `U` is a reference, `T` must also be a reference (as the underlying
     matcher may be interested in the address of the `U` value).
 
 The code won't compile if any of these conditions isn't met.
 
 Here's one example:
 
 ```cpp
 using ::testing::SafeMatcherCast;
 
 // A base class and a child class.
 class Base { ... };
 class Derived : public Base { ... };
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(void, DoThis, (Derived* derived), (override));
 };
 
 ...
   MockFoo foo;
   // m is a Matcher<Base*> we got from somewhere.
   EXPECT_CALL(foo, DoThis(SafeMatcherCast<Derived*>(m)));
 ```
 
 If you find `SafeMatcherCast<T>(m)` too limiting, you can use a similar function
 `MatcherCast<T>(m)`. The difference is that `MatcherCast` works as long as you
 can `static_cast` type `T` to type `U`.
 
 `MatcherCast` essentially lets you bypass C++'s type system (`static_cast` isn't
 always safe as it could throw away information, for example), so be careful not
 to misuse/abuse it.
 
 ### Selecting Between Overloaded Functions {#SelectOverload}
 
 If you expect an overloaded function to be called, the compiler may need some
 help on which overloaded version it is.
 
 To disambiguate functions overloaded on the const-ness of this object, use the
 `Const()` argument wrapper.
 
 ```cpp
 using ::testing::ReturnRef;
 
 class MockFoo : public Foo {
   ...
   MOCK_METHOD(Bar&, GetBar, (), (override));
   MOCK_METHOD(const Bar&, GetBar, (), (const, override));
 };
 
 ...
   MockFoo foo;
   Bar bar1, bar2;
   EXPECT_CALL(foo, GetBar())         // The non-const GetBar().
       .WillOnce(ReturnRef(bar1));
   EXPECT_CALL(Const(foo), GetBar())  // The const GetBar().
       .WillOnce(ReturnRef(bar2));
 ```
 
 (`Const()` is defined by gMock and returns a `const` reference to its argument.)
 
 To disambiguate overloaded functions with the same number of arguments but
 different argument types, you may need to specify the exact type of a matcher,
 either by wrapping your matcher in `Matcher<type>()`, or using a matcher whose
 type is fixed (`TypedEq<type>`, `An<type>()`, etc):
 
 ```cpp
 using ::testing::An;
 using ::testing::Matcher;
 using ::testing::TypedEq;
 
 class MockPrinter : public Printer {
  public:
   MOCK_METHOD(void, Print, (int n), (override));
   MOCK_METHOD(void, Print, (char c), (override));
 };
 
 TEST(PrinterTest, Print) {
   MockPrinter printer;
 
   EXPECT_CALL(printer, Print(An<int>()));            // void Print(int);
   EXPECT_CALL(printer, Print(Matcher<int>(Lt(5))));  // void Print(int);
   EXPECT_CALL(printer, Print(TypedEq<char>('a')));   // void Print(char);
 
   printer.Print(3);
   printer.Print(6);
   printer.Print('a');
 }
 ```
 
 ### Performing Different Actions Based on the Arguments
 
 When a mock method is called, the *last* matching expectation that's still
 active will be selected (think "newer overrides older"). So, you can make a
 method do different things depending on its argument values like this:
 
 ```cpp
 using ::testing::_;
 using ::testing::Lt;
 using ::testing::Return;
 ...
   // The default case.
   EXPECT_CALL(foo, DoThis(_))
       .WillRepeatedly(Return('b'));
   // The more specific case.
   EXPECT_CALL(foo, DoThis(Lt(5)))
       .WillRepeatedly(Return('a'));
 ```
 
 Now, if `foo.DoThis()` is called with a value less than 5, `'a'` will be
 returned; otherwise `'b'` will be returned.
 
 ### Matching Multiple Arguments as a Whole
 
 Sometimes it's not enough to match the arguments individually. For example, we
 may want to say that the first argument must be less than the second argument.
 The `With()` clause allows us to match all arguments of a mock function as a
 whole. For example,
 
 ```cpp
 using ::testing::_;
 using ::testing::Ne;
 using ::testing::Lt;
 ...
   EXPECT_CALL(foo, InRange(Ne(0), _))
       .With(Lt());
 ```
 
 says that the first argument of `InRange()` must not be 0, and must be less than
 the second argument.
 
 The expression inside `With()` must be a matcher of type
 `Matcher< ::std::tuple<A1, ..., An> >`, where `A1`, ..., `An` are the types of
 the function arguments.
 
 You can also write `AllArgs(m)` instead of `m` inside `.With()`. The two forms
 are equivalent, but `.With(AllArgs(Lt()))` is more readable than `.With(Lt())`.
 
 You can use `Args<k1, ..., kn>(m)` to match the `n` selected arguments (as a
 tuple) against `m`. For example,
 
 ```cpp
 using ::testing::_;
 using ::testing::AllOf;
 using ::testing::Args;
 using ::testing::Lt;
 ...
   EXPECT_CALL(foo, Blah)
       .With(AllOf(Args<0, 1>(Lt()), Args<1, 2>(Lt())));
 ```
 
 says that `Blah` will be called with arguments `x`, `y`, and `z` where `x < y <
 z`. Note that in this example, it wasn't necessary specify the positional
 matchers.
 
 As a convenience and example, gMock provides some matchers for 2-tuples,
 including the `Lt()` matcher above. See [here](#MultiArgMatchers) for the
 complete list.
 
 Note that if you want to pass the arguments to a predicate of your own (e.g.
 `.With(Args<0, 1>(Truly(&MyPredicate)))`), that predicate MUST be written to
 take a `::std::tuple` as its argument; gMock will pass the `n` selected
 arguments as *one* single tuple to the predicate.
 
 ### Using Matchers as Predicates
 
 Have you noticed that a matcher is just a fancy predicate that also knows how to
 describe itself? Many existing algorithms take predicates as arguments (e.g.
 those defined in STL's `<algorithm>` header), and it would be a shame if gMock
 matchers were not allowed to participate.
 
 Luckily, you can use a matcher where a unary predicate functor is expected by
 wrapping it inside the `Matches()` function. For example,
 
 ```cpp
 #include <algorithm>
 #include <vector>
 
 using ::testing::Matches;
 using ::testing::Ge;
 
 vector<int> v;
 ...
 // How many elements in v are >= 10?
 const int count = count_if(v.begin(), v.end(), Matches(Ge(10)));
 ```
 
 Since you can build complex matchers from simpler ones easily using gMock, this
 gives you a way to conveniently construct composite predicates (doing the same
 using STL's `<functional>` header is just painful). For example, here's a
 predicate that's satisfied by any number that is >= 0, <= 100, and != 50:
 
 ```cpp
 using testing::AllOf;
 using testing::Ge;
 using testing::Le;
 using testing::Matches;
 using testing::Ne;
 ...
 Matches(AllOf(Ge(0), Le(100), Ne(50)))
 ```
 
 ### Using Matchers in googletest Assertions
 
 Since matchers are basically predicates that also know how to describe
 themselves, there is a way to take advantage of them in googletest assertions.
 It's called `ASSERT_THAT` and `EXPECT_THAT`:
 
 ```cpp
   ASSERT_THAT(value, matcher);  // Asserts that value matches matcher.
   EXPECT_THAT(value, matcher);  // The non-fatal version.
 ```
 
 For example, in a googletest test you can write:
 
 ```cpp
 #include "gmock/gmock.h"
 
 using ::testing::AllOf;
 using ::testing::Ge;
 using ::testing::Le;
 using ::testing::MatchesRegex;
 using ::testing::StartsWith;
 
 ...
   EXPECT_THAT(Foo(), StartsWith("Hello"));
   EXPECT_THAT(Bar(), MatchesRegex("Line \\d+"));
   ASSERT_THAT(Baz(), AllOf(Ge(5), Le(10)));
 ```
 
 which (as you can probably guess) executes `Foo()`, `Bar()`, and `Baz()`, and
 verifies that:
 
 *   `Foo()` returns a string that starts with `"Hello"`.
 *   `Bar()` returns a string that matches regular expression `"Line \\d+"`.
 *   `Baz()` returns a number in the range [5, 10].
 
 The nice thing about these macros is that *they read like English*. They
 generate informative messages too. For example, if the first `EXPECT_THAT()`
 above fails, the message will be something like:
 
 ```cpp
 Value of: Foo()
   Actual: "Hi, world!"
 Expected: starts with "Hello"
 ```
 
 **Credit:** The idea of `(ASSERT|EXPECT)_THAT` was borrowed from Joe Walnes'
 Hamcrest project, which adds `assertThat()` to JUnit.
 
 ### Using Predicates as Matchers
 
 gMock provides a [built-in set](#MatcherList) of matchers. In case you find them
 lacking, you can use an arbitrary unary predicate function or functor as a
 matcher - as long as the predicate accepts a value of the type you want. You do
 this by wrapping the predicate inside the `Truly()` function, for example:
 
 ```cpp
 using ::testing::Truly;
 
 int IsEven(int n) { return (n % 2) == 0 ? 1 : 0; }
 ...
   // Bar() must be called with an even number.
   EXPECT_CALL(foo, Bar(Truly(IsEven)));
 ```
 
 Note that the predicate function / functor doesn't have to return `bool`. It
 works as long as the return value can be used as the condition in in statement
 `if (condition) ...`.
 
 <!-- GOOGLETEST_CM0023 DO NOT DELETE -->
 
 ### Matching Arguments that Are Not Copyable
 
 When you do an `EXPECT_CALL(mock_obj, Foo(bar))`, gMock saves away a copy of
 `bar`. When `Foo()` is called later, gMock compares the argument to `Foo()` with
 the saved copy of `bar`. This way, you don't need to worry about `bar` being
 modified or destroyed after the `EXPECT_CALL()` is executed. The same is true
 when you use matchers like `Eq(bar)`, `Le(bar)`, and so on.
 
 But what if `bar` cannot be copied (i.e. has no copy constructor)? You could
 define your own matcher function or callback and use it with `Truly()`, as the
 previous couple of recipes have shown. Or, you may be able to get away from it
 if you can guarantee that `bar` won't be changed after the `EXPECT_CALL()` is
 executed. Just tell gMock that it should save a reference to `bar`, instead of a
 copy of it. Here's how:
 
 ```cpp
 using ::testing::ByRef;
 using ::testing::Eq;
 using ::testing::Lt;
 ...
   // Expects that Foo()'s argument == bar.
   EXPECT_CALL(mock_obj, Foo(Eq(ByRef(bar))));
 
   // Expects that Foo()'s argument < bar.
   EXPECT_CALL(mock_obj, Foo(Lt(ByRef(bar))));
 ```
 
 Remember: if you do this, don't change `bar` after the `EXPECT_CALL()`, or the
 result is undefined.
 
 ### Validating a Member of an Object
 
 Often a mock function takes a reference to object as an argument. When matching
 the argument, you may not want to compare the entire object against a fixed
 object, as that may be over-specification. Instead, you may need to validate a
 certain member variable or the result of a certain getter method of the object.
 You can do this with `Field()` and `Property()`. More specifically,
 
 ```cpp
 Field(&Foo::bar, m)
 ```
 
 is a matcher that matches a `Foo` object whose `bar` member variable satisfies
 matcher `m`.
 
 ```cpp
 Property(&Foo::baz, m)
 ```
 
 is a matcher that matches a `Foo` object whose `baz()` method returns a value
 that satisfies matcher `m`.
 
 For example:
 
 <!-- mdformat off(github rendering does not support multiline tables) -->
 | Expression                   | Description                              |
 | :--------------------------- | :--------------------------------------- |
 | `Field(&Foo::number, Ge(3))` | Matches `x` where `x.number >= 3`.       |
 | `Property(&Foo::name,  StartsWith("John "))` | Matches `x` where `x.name()` starts with  `"John "`. |
 <!-- mdformat on -->
 
 Note that in `Property(&Foo::baz, ...)`, method `baz()` must take no argument
 and be declared as `const`.
 
 BTW, `Field()` and `Property()` can also match plain pointers to objects. For
 instance,
 
 ```cpp
 using ::testing::Field;
 using ::testing::Ge;
 ...
 Field(&Foo::number, Ge(3))
 ```
 
 matches a plain pointer `p` where `p->number >= 3`. If `p` is `NULL`, the match
 will always fail regardless of the inner matcher.
 
 What if you want to validate more than one members at the same time? Remember
 that there are [`AllOf()` and `AllOfArray()`](#CombiningMatchers).
 
 Finally `Field()` and `Property()` provide overloads that take the field or
 property names as the first argument to include it in the error message. This
 can be useful when creating combined matchers.
 
 ```cpp
 using ::testing::AllOf;
 using ::testing::Field;
 using ::testing::Matcher;
 using ::testing::SafeMatcherCast;
 
 Matcher<Foo> IsFoo(const Foo& foo) {
   return AllOf(Field("some_field", &Foo::some_field, foo.some_field),
                Field("other_field", &Foo::other_field, foo.other_field),
                Field("last_field", &Foo::last_field, foo.last_field));
 }
 ```
 
 ### Validating the Value Pointed to by a Pointer Argument
 
 C++ functions often take pointers as arguments. You can use matchers like
 `IsNull()`, `NotNull()`, and other comparison matchers to match a pointer, but
 what if you want to make sure the value *pointed to* by the pointer, instead of
 the pointer itself, has a certain property? Well, you can use the `Pointee(m)`
 matcher.
 
 `Pointee(m)` matches a pointer if and only if `m` matches the value the pointer
 points to. For example:
 
 ```cpp
 using ::testing::Ge;
 using ::testing::Pointee;
 ...
   EXPECT_CALL(foo, Bar(Pointee(Ge(3))));
 ```
 
 expects `foo.Bar()` to be called with a pointer that points to a value greater
 than or equal to 3.
 
 One nice thing about `Pointee()` is that it treats a `NULL` pointer as a match
 failure, so you can write `Pointee(m)` instead of
 
 ```cpp
 using ::testing::AllOf;
 using ::testing::NotNull;
 using ::testing::Pointee;
 ...
   AllOf(NotNull(), Pointee(m))
 ```
 
 without worrying that a `NULL` pointer will crash your test.
 
 Also, did we tell you that `Pointee()` works with both raw pointers **and**
 smart pointers (`std::unique_ptr`, `std::shared_ptr`, etc)?
 
 What if you have a pointer to pointer? You guessed it - you can use nested
 `Pointee()` to probe deeper inside the value. For example,
 `Pointee(Pointee(Lt(3)))` matches a pointer that points to a pointer that points
 to a number less than 3 (what a mouthful...).
 
 ### Testing a Certain Property of an Object
 
 Sometimes you want to specify that an object argument has a certain property,
 but there is no existing matcher that does this. If you want good error
 messages, you should [define a matcher](#NewMatchers). If you want to do it
 quick and dirty, you could get away with writing an ordinary function.
 
 Let's say you have a mock function that takes an object of type `Foo`, which has
 an `int bar()` method and an `int baz()` method, and you want to constrain that
 the argument's `bar()` value plus its `baz()` value is a given number. Here's
 how you can define a matcher to do it:
 
 ```cpp
 using ::testing::Matcher;
 using ::testing::MatcherInterface;
 using ::testing::MatchResultListener;
 
 class BarPlusBazEqMatcher : public MatcherInterface<const Foo&> {
  public:
   explicit BarPlusBazEqMatcher(int expected_sum)
       : expected_sum_(expected_sum) {}
 
   bool MatchAndExplain(const Foo& foo,
                        MatchResultListener* /* listener */) const override {
     return (foo.bar() + foo.baz()) == expected_sum_;
   }
 
   void DescribeTo(::std::ostream* os) const override {
     *os << "bar() + baz() equals " << expected_sum_;
   }
 
   void DescribeNegationTo(::std::ostream* os) const override {
     *os << "bar() + baz() does not equal " << expected_sum_;
   }
  private:
   const int expected_sum_;
 };
 
 Matcher<const Foo&> BarPlusBazEq(int expected_sum) {
   return MakeMatcher(new BarPlusBazEqMatcher(expected_sum));
 }
 
 ...
   EXPECT_CALL(..., DoThis(BarPlusBazEq(5)))...;
 ```
 
 ### Matching Containers
 
 Sometimes an STL container (e.g. list, vector, map, ...) is passed to a mock
 function and you may want to validate it. Since most STL containers support the
 `==` operator, you can write `Eq(expected_container)` or simply
 `expected_container` to match a container exactly.
 
 Sometimes, though, you may want to be more flexible (for example, the first
 element must be an exact match, but the second element can be any positive
 number, and so on). Also, containers used in tests often have a small number of
 elements, and having to define the expected container out-of-line is a bit of a
 hassle.
 
 You can use the `ElementsAre()` or `UnorderedElementsAre()` matcher in such
 cases:
 
 ```cpp
 using ::testing::_;
 using ::testing::ElementsAre;
 using ::testing::Gt;
 ...
   MOCK_METHOD(void, Foo, (const vector<int>& numbers), (override));
 ...
   EXPECT_CALL(mock, Foo(ElementsAre(1, Gt(0), _, 5)));
 ```
 
 The above matcher says that the container must have 4 elements, which must be 1,
 greater than 0, anything, and 5 respectively.
 
 If you instead write:
 
 ```cpp
 using ::testing::_;
 using ::testing::Gt;
 using ::testing::UnorderedElementsAre;
 ...
   MOCK_METHOD(void, Foo, (const vector<int>& numbers), (override));
 ...
   EXPECT_CALL(mock, Foo(UnorderedElementsAre(1, Gt(0), _, 5)));
 ```
 
 It means that the container must have 4 elements, which (under some permutation)
 must be 1, greater than 0, anything, and 5 respectively.
 
 As an alternative you can place the arguments in a C-style array and use
 `ElementsAreArray()` or `UnorderedElementsAreArray()` instead:
 
 ```cpp
 using ::testing::ElementsAreArray;
 ...
   // ElementsAreArray accepts an array of element values.
   const int expected_vector1[] = {1, 5, 2, 4, ...};
   EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector1)));
 
   // Or, an array of element matchers.
   Matcher<int> expected_vector2[] = {1, Gt(2), _, 3, ...};
   EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector2)));
 ```
 
 In case the array needs to be dynamically created (and therefore the array size
 cannot be inferred by the compiler), you can give `ElementsAreArray()` an
 additional argument to specify the array size:
 
 ```cpp
 using ::testing::ElementsAreArray;
 ...
   int* const expected_vector3 = new int[count];
   ... fill expected_vector3 with values ...
   EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector3, count)));
 ```
 
 Use `Pair` when comparing maps or other associative containers.
 
 ```cpp
 using testing::ElementsAre;
 using testing::Pair;
 ...
   std::map<string, int> m = {{"a", 1}, {"b", 2}, {"c", 3}};
   EXPECT_THAT(m, ElementsAre(Pair("a", 1), Pair("b", 2), Pair("c", 3)));
 ```
 
 **Tips:**
 
 *   `ElementsAre*()` can be used to match *any* container that implements the
     STL iterator pattern (i.e. it has a `const_iterator` type and supports
     `begin()/end()`), not just the ones defined in STL. It will even work with
     container types yet to be written - as long as they follows the above
     pattern.
 *   You can use nested `ElementsAre*()` to match nested (multi-dimensional)
     containers.
 *   If the container is passed by pointer instead of by reference, just write
     `Pointee(ElementsAre*(...))`.
 *   The order of elements *matters* for `ElementsAre*()`. If you are using it
     with containers whose element order are undefined (e.g. `hash_map`) you
     should use `WhenSorted` around `ElementsAre`.
 
 ### Sharing Matchers
 
 Under the hood, a gMock matcher object consists of a pointer to a ref-counted
 implementation object. Copying matchers is allowed and very efficient, as only
 the pointer is copied. When the last matcher that references the implementation
 object dies, the implementation object will be deleted.
 
 Therefore, if you have some complex matcher that you want to use again and
 again, there is no need to build it everytime. Just assign it to a matcher
 variable and use that variable repeatedly! For example,
 
 ```cpp
 using ::testing::AllOf;
 using ::testing::Gt;
 using ::testing::Le;
 using ::testing::Matcher;
 ...
   Matcher<int> in_range = AllOf(Gt(5), Le(10));
   ... use in_range as a matcher in multiple EXPECT_CALLs ...
 ```
 
 ### Matchers must have no side-effects {#PureMatchers}
 
 WARNING: gMock does not guarantee when or how many times a matcher will be
 invoked. Therefore, all matchers must be *purely functional*: they cannot have
 any side effects, and the match result must not depend on anything other than
 the matcher's parameters and the value being matched.
 
 This requirement must be satisfied no matter how a matcher is defined (e.g., if
 it is one of the standard matchers, or a custom matcher). In particular, a
 matcher can never call a mock function, as that will affect the state of the
 mock object and gMock.
 
 ## Setting Expectations
 
 ### Knowing When to Expect {#UseOnCall}
 
 <!-- GOOGLETEST_CM0018 DO NOT DELETE -->
 
 **`ON_CALL`** is likely the *single most under-utilized construct* in gMock.
 
 There are basically two constructs for defining the behavior of a mock object:
 `ON_CALL` and `EXPECT_CALL`. The difference? `ON_CALL` defines what happens when
 a mock method is called, but <em>doesn't imply any expectation on the method
 being called</em>. `EXPECT_CALL` not only defines the behavior, but also sets an
 expectation that <em>the method will be called with the given arguments, for the
 given number of times</em> (and *in the given order* when you specify the order
 too).
 
 Since `EXPECT_CALL` does more, isn't it better than `ON_CALL`? Not really. Every
 `EXPECT_CALL` adds a constraint on the behavior of the code under test. Having
 more constraints than necessary is *baaad* - even worse than not having enough
 constraints.
 
 This may be counter-intuitive. How could tests that verify more be worse than
 tests that verify less? Isn't verification the whole point of tests?
 
 The answer lies in *what* a test should verify. **A good test verifies the
 contract of the code.** If a test over-specifies, it doesn't leave enough
 freedom to the implementation. As a result, changing the implementation without
 breaking the contract (e.g. refactoring and optimization), which should be
 perfectly fine to do, can break such tests. Then you have to spend time fixing
 them, only to see them broken again the next time the implementation is changed.
 
 Keep in mind that one doesn't have to verify more than one property in one test.
 In fact, **it's a good style to verify only one thing in one test.** If you do
 that, a bug will likely break only one or two tests instead of dozens (which
 case would you rather debug?). If you are also in the habit of giving tests
 descriptive names that tell what they verify, you can often easily guess what's
 wrong just from the test log itself.
 
 So use `ON_CALL` by default, and only use `EXPECT_CALL` when you actually intend
 to verify that the call is made. For example, you may have a bunch of `ON_CALL`s
 in your test fixture to set the common mock behavior shared by all tests in the
 same group, and write (scarcely) different `EXPECT_CALL`s in different `TEST_F`s
 to verify different aspects of the code's behavior. Compared with the style
 where each `TEST` has many `EXPECT_CALL`s, this leads to tests that are more
 resilient to implementational changes (and thus less likely to require
 maintenance) and makes the intent of the tests more obvious (so they are easier
 to maintain when you do need to maintain them).
 
 If you are bothered by the "Uninteresting mock function call" message printed
 when a mock method without an `EXPECT_CALL` is called, you may use a `NiceMock`
 instead to suppress all such messages for the mock object, or suppress the
 message for specific methods by adding `EXPECT_CALL(...).Times(AnyNumber())`. DO
 NOT suppress it by blindly adding an `EXPECT_CALL(...)`, or you'll have a test
 that's a pain to maintain.
 
 ### Ignoring Uninteresting Calls
 
 If you are not interested in how a mock method is called, just don't say
 anything about it. In this case, if the method is ever called, gMock will
 perform its default action to allow the test program to continue. If you are not
 happy with the default action taken by gMock, you can override it using
 `DefaultValue<T>::Set()` (described [here](#DefaultValue)) or `ON_CALL()`.
 
 Please note that once you expressed interest in a particular mock method (via
 `EXPECT_CALL()`), all invocations to it must match some expectation. If this
 function is called but the arguments don't match any `EXPECT_CALL()` statement,
 it will be an error.
 
 ### Disallowing Unexpected Calls
 
 If a mock method shouldn't be called at all, explicitly say so:
 
 ```cpp
 using ::testing::_;
 ...
   EXPECT_CALL(foo, Bar(_))
       .Times(0);
 ```
 
 If some calls to the method are allowed, but the rest are not, just list all the
 expected calls:
 
 ```cpp
 using ::testing::AnyNumber;
 using ::testing::Gt;
 ...
   EXPECT_CALL(foo, Bar(5));
   EXPECT_CALL(foo, Bar(Gt(10)))
       .Times(AnyNumber());
 ```
 
 A call to `foo.Bar()` that doesn't match any of the `EXPECT_CALL()` statements
 will be an error.
 
 ### Understanding Uninteresting vs Unexpected Calls {#uninteresting-vs-unexpected}
 
 *Uninteresting* calls and *unexpected* calls are different concepts in gMock.
 *Very* different.
 
 A call `x.Y(...)` is **uninteresting** if there's *not even a single*
 `EXPECT_CALL(x, Y(...))` set. In other words, the test isn't interested in the
 `x.Y()` method at all, as evident in that the test doesn't care to say anything
 about it.
 
 A call `x.Y(...)` is **unexpected** if there are *some* `EXPECT_CALL(x,
 Y(...))`s set, but none of them matches the call. Put another way, the test is
 interested in the `x.Y()` method (therefore it explicitly sets some
 `EXPECT_CALL` to verify how it's called); however, the verification fails as the
 test doesn't expect this particular call to happen.
 
 **An unexpected call is always an error,** as the code under test doesn't behave
 the way the test expects it to behave.
 
 **By default, an uninteresting call is not an error,** as it violates no
 constraint specified by the test. (gMock's philosophy is that saying nothing
 means there is no constraint.) However, it leads to a warning, as it *might*
 indicate a problem (e.g. the test author might have forgotten to specify a
 constraint).
 
 In gMock, `NiceMock` and `StrictMock` can be used to make a mock class "nice" or
 "strict". How does this affect uninteresting calls and unexpected calls?
 
 A **nice mock** suppresses uninteresting call *warnings*. It is less chatty than
 the default mock, but otherwise is the same. If a test fails with a default
 mock, it will also fail using a nice mock instead. And vice versa. Don't expect
 making a mock nice to change the test's result.
 
 A **strict mock** turns uninteresting call warnings into errors. So making a
 mock strict may change the test's result.
 
 Let's look at an example:
 
 ```cpp
 TEST(...) {
   NiceMock<MockDomainRegistry> mock_registry;
   EXPECT_CALL(mock_registry, GetDomainOwner("google.com"))
           .WillRepeatedly(Return("Larry Page"));
 
   // Use mock_registry in code under test.
   ... &mock_registry ...
 }
 ```
 
 The sole `EXPECT_CALL` here says that all calls to `GetDomainOwner()` must have
 `"google.com"` as the argument. If `GetDomainOwner("yahoo.com")` is called, it
 will be an unexpected call, and thus an error. *Having a nice mock doesn't
 change the severity of an unexpected call.*
 
 So how do we tell gMock that `GetDomainOwner()` can be called with some other
 arguments as well? The standard technique is to add a "catch all" `EXPECT_CALL`:
 
 ```cpp
   EXPECT_CALL(mock_registry, GetDomainOwner(_))
         .Times(AnyNumber());  // catches all other calls to this method.
   EXPECT_CALL(mock_registry, GetDomainOwner("google.com"))
         .WillRepeatedly(Return("Larry Page"));
 ```
 
 Remember that `_` is the wildcard matcher that matches anything. With this, if
 `GetDomainOwner("google.com")` is called, it will do what the second
 `EXPECT_CALL` says; if it is called with a different argument, it will do what
 the first `EXPECT_CALL` says.
 
 Note that the order of the two `EXPECT_CALL`s is important, as a newer
 `EXPECT_CALL` takes precedence over an older one.
 
 For more on uninteresting calls, nice mocks, and strict mocks, read
 ["The Nice, the Strict, and the Naggy"](#NiceStrictNaggy).
 
 ### Ignoring Uninteresting Arguments {#ParameterlessExpectations}
 
 If your test doesn't care about the parameters (it only cares about the number
 or order of calls), you can often simply omit the parameter list:
 
 ```cpp
   // Expect foo.Bar( ... ) twice with any arguments.
   EXPECT_CALL(foo, Bar).Times(2);
 
   // Delegate to the given method whenever the factory is invoked.
   ON_CALL(foo_factory, MakeFoo)
       .WillByDefault(&BuildFooForTest);
 ```
 
 This functionality is only available when a method is not overloaded; to prevent
 unexpected behavior it is a compilation error to try to set an expectation on a
 method where the specific overload is ambiguous. You can work around this by
 supplying a [simpler mock interface](#SimplerInterfaces) than the mocked class
 provides.
 
 This pattern is also useful when the arguments are interesting, but match logic
 is substantially complex. You can leave the argument list unspecified and use
 SaveArg actions to [save the values for later verification](#SaveArgVerify). If
 you do that, you can easily differentiate calling the method the wrong number of
 times from calling it with the wrong arguments.
 
 ### Expecting Ordered Calls {#OrderedCalls}
 
 Although an `EXPECT_CALL()` statement defined earlier takes precedence when
 gMock tries to match a function call with an expectation, by default calls don't
 have to happen in the order `EXPECT_CALL()` statements are written. For example,
 if the arguments match the matchers in the third `EXPECT_CALL()`, but not those
 in the first two, then the third expectation will be used.
 
 If you would rather have all calls occur in the order of the expectations, put
 the `EXPECT_CALL()` statements in a block where you define a variable of type
 `InSequence`:
 
 ```cpp
 using ::testing::_;
 using ::testing::InSequence;
 
   {
     InSequence s;
 
     EXPECT_CALL(foo, DoThis(5));
     EXPECT_CALL(bar, DoThat(_))
         .Times(2);
     EXPECT_CALL(foo, DoThis(6));
   }
 ```
 
 In this example, we expect a call to `foo.DoThis(5)`, followed by two calls to
 `bar.DoThat()` where the argument can be anything, which are in turn followed by
 a call to `foo.DoThis(6)`. If a call occurred out-of-order, gMock will report an
 error.
 
 ### Expecting Partially Ordered Calls {#PartialOrder}
 
 Sometimes requiring everything to occur in a predetermined order can lead to
 brittle tests. For example, we may care about `A` occurring before both `B` and
 `C`, but aren't interested in the relative order of `B` and `C`. In this case,
 the test should reflect our real intent, instead of being overly constraining.
 
 gMock allows you to impose an arbitrary DAG (directed acyclic graph) on the
 calls. One way to express the DAG is to use the [After](#AfterClause) clause of
 `EXPECT_CALL`.
 
 Another way is via the `InSequence()` clause (not the same as the `InSequence`
 class), which we borrowed from jMock 2. It's less flexible than `After()`, but
 more convenient when you have long chains of sequential calls, as it doesn't
 require you to come up with different names for the expectations in the chains.
 Here's how it works:
 
 If we view `EXPECT_CALL()` statements as nodes in a graph, and add an edge from
 node A to node B wherever A must occur before B, we can get a DAG. We use the
 term "sequence" to mean a directed path in this DAG. Now, if we decompose the
 DAG into sequences, we just need to know which sequences each `EXPECT_CALL()`
 belongs to in order to be able to reconstruct the original DAG.
 
 So, to specify the partial order on the expectations we need to do two things:
 first to define some `Sequence` objects, and then for each `EXPECT_CALL()` say
 which `Sequence` objects it is part of.
 
 Expectations in the same sequence must occur in the order they are written. For
 example,
 
 ```cpp
 using ::testing::Sequence;
 ...
   Sequence s1, s2;
 
   EXPECT_CALL(foo, A())
       .InSequence(s1, s2);
   EXPECT_CALL(bar, B())
       .InSequence(s1);
   EXPECT_CALL(bar, C())
       .InSequence(s2);
   EXPECT_CALL(foo, D())
       .InSequence(s2);
 ```
 
 specifies the following DAG (where `s1` is `A -> B`, and `s2` is `A -> C -> D`):
 
 ```text
        +---> B
        |
   A ---|
        |
         +---> C ---> D
 ```
 
 This means that A must occur before B and C, and C must occur before D. There's
 no restriction about the order other than these.
 
 ### Controlling When an Expectation Retires
 
 When a mock method is called, gMock only considers expectations that are still
 active. An expectation is active when created, and becomes inactive (aka
 *retires*) when a call that has to occur later has occurred. For example, in
 
 ```cpp
 using ::testing::_;
 using ::testing::Sequence;
 ...
   Sequence s1, s2;
 
   EXPECT_CALL(log, Log(WARNING, _, "File too large."))      // #1
       .Times(AnyNumber())
       .InSequence(s1, s2);
   EXPECT_CALL(log, Log(WARNING, _, "Data set is empty."))   // #2
       .InSequence(s1);
   EXPECT_CALL(log, Log(WARNING, _, "User not found."))      // #3
       .InSequence(s2);
 ```
 
 as soon as either #2 or #3 is matched, #1 will retire. If a warning `"File too
 large."` is logged after this, it will be an error.
 
 Note that an expectation doesn't retire automatically when it's saturated. For
 example,
 
 ```cpp
 using ::testing::_;
 ...
   EXPECT_CALL(log, Log(WARNING, _, _));                     // #1
   EXPECT_CALL(log, Log(WARNING, _, "File too large."));     // #2
 ```
 
 says that there will be exactly one warning with the message `"File too
 large."`. If the second warning contains this message too, #2 will match again
 and result in an upper-bound-violated error.
 
 If this is not what you want, you can ask an expectation to retire as soon as it
 becomes saturated:
 
 ```cpp
 using ::testing::_;
 ...
   EXPECT_CALL(log, Log(WARNING, _, _));                     // #1
   EXPECT_CALL(log, Log(WARNING, _, "File too large."))      // #2
       .RetiresOnSaturation();
 ```
 
 Here #2 can be used only once, so if you have two warnings with the message
 `"File too large."`, the first will match #2 and the second will match #1 -
 there will be no error.
 
 ## Using Actions
 
 ### Returning References from Mock Methods
 
 If a mock function's return type is a reference, you need to use `ReturnRef()`
 instead of `Return()` to return a result:
 
 ```cpp
 using ::testing::ReturnRef;
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(Bar&, GetBar, (), (override));
 };
 ...
   MockFoo foo;
   Bar bar;
   EXPECT_CALL(foo, GetBar())
       .WillOnce(ReturnRef(bar));
 ...
 ```
 
 ### Returning Live Values from Mock Methods
 
 The `Return(x)` action saves a copy of `x` when the action is created, and
 always returns the same value whenever it's executed. Sometimes you may want to
 instead return the *live* value of `x` (i.e. its value at the time when the
 action is *executed*.). Use either `ReturnRef()` or `ReturnPointee()` for this
 purpose.
 
 If the mock function's return type is a reference, you can do it using
 `ReturnRef(x)`, as shown in the previous recipe ("Returning References from Mock
 Methods"). However, gMock doesn't let you use `ReturnRef()` in a mock function
 whose return type is not a reference, as doing that usually indicates a user
 error. So, what shall you do?
 
 Though you may be tempted, DO NOT use `ByRef()`:
 
 ```cpp
 using testing::ByRef;
 using testing::Return;
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(int, GetValue, (), (override));
 };
 ...
   int x = 0;
   MockFoo foo;
   EXPECT_CALL(foo, GetValue())
       .WillRepeatedly(Return(ByRef(x)));  // Wrong!
   x = 42;
   EXPECT_EQ(42, foo.GetValue());
 ```
 
 Unfortunately, it doesn't work here. The above code will fail with error:
 
 ```text
 Value of: foo.GetValue()
   Actual: 0
 Expected: 42
 ```
 
 The reason is that `Return(*value*)` converts `value` to the actual return type
 of the mock function at the time when the action is *created*, not when it is
 *executed*. (This behavior was chosen for the action to be safe when `value` is
 a proxy object that references some temporary objects.) As a result, `ByRef(x)`
 is converted to an `int` value (instead of a `const int&`) when the expectation
 is set, and `Return(ByRef(x))` will always return 0.
 
 `ReturnPointee(pointer)` was provided to solve this problem specifically. It
 returns the value pointed to by `pointer` at the time the action is *executed*:
 
 ```cpp
 using testing::ReturnPointee;
 ...
   int x = 0;
   MockFoo foo;
   EXPECT_CALL(foo, GetValue())
       .WillRepeatedly(ReturnPointee(&x));  // Note the & here.
   x = 42;
   EXPECT_EQ(42, foo.GetValue());  // This will succeed now.
 ```
 
 ### Combining Actions
 
 Want to do more than one thing when a function is called? That's fine. `DoAll()`
 allow you to do sequence of actions every time. Only the return value of the
 last action in the sequence will be used.
 
 ```cpp
 using ::testing::_;
 using ::testing::DoAll;
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(bool, Bar, (int n), (override));
 };
 ...
   EXPECT_CALL(foo, Bar(_))
       .WillOnce(DoAll(action_1,
                       action_2,
                       ...
                       action_n));
 ```
 
 ### Verifying Complex Arguments {#SaveArgVerify}
 
 If you want to verify that a method is called with a particular argument but the
 match criteria is complex, it can be difficult to distinguish between
 cardinality failures (calling the method the wrong number of times) and argument
 match failures. Similarly, if you are matching multiple parameters, it may not
 be easy to distinguishing which argument failed to match. For example:
 
 ```cpp
   // Not ideal: this could fail because of a problem with arg1 or arg2, or maybe
   // just the method wasn't called.
   EXPECT_CALL(foo, SendValues(_, ElementsAre(1, 4, 4, 7), EqualsProto( ... )));
 ```
 
 You can instead save the arguments and test them individually:
 
 ```cpp
   EXPECT_CALL(foo, SendValues)
       .WillOnce(DoAll(SaveArg<1>(&actual_array), SaveArg<2>(&actual_proto)));
   ... run the test
   EXPECT_THAT(actual_array, ElementsAre(1, 4, 4, 7));
   EXPECT_THAT(actual_proto, EqualsProto( ... ));
 ```
 
 ### Mocking Side Effects {#MockingSideEffects}
 
 Sometimes a method exhibits its effect not via returning a value but via side
 effects. For example, it may change some global state or modify an output
 argument. To mock side effects, in general you can define your own action by
 implementing `::testing::ActionInterface`.
 
 If all you need to do is to change an output argument, the built-in
 `SetArgPointee()` action is convenient:
 
 ```cpp
 using ::testing::_;
 using ::testing::SetArgPointee;
 
 class MockMutator : public Mutator {
  public:
   MOCK_METHOD(void, Mutate, (bool mutate, int* value), (override));
   ...
 }
 ...
   MockMutator mutator;
   EXPECT_CALL(mutator, Mutate(true, _))
       .WillOnce(SetArgPointee<1>(5));
 ```
 
 In this example, when `mutator.Mutate()` is called, we will assign 5 to the
 `int` variable pointed to by argument #1 (0-based).
 
 `SetArgPointee()` conveniently makes an internal copy of the value you pass to
 it, removing the need to keep the value in scope and alive. The implication
 however is that the value must have a copy constructor and assignment operator.
 
 If the mock method also needs to return a value as well, you can chain
 `SetArgPointee()` with `Return()` using `DoAll()`, remembering to put the
 `Return()` statement last:
 
 ```cpp
 using ::testing::_;
 using ::testing::Return;
 using ::testing::SetArgPointee;
 
 class MockMutator : public Mutator {
  public:
   ...
   MOCK_METHOD(bool, MutateInt, (int* value), (override));
 }
 ...
   MockMutator mutator;
   EXPECT_CALL(mutator, MutateInt(_))
       .WillOnce(DoAll(SetArgPointee<0>(5),
                       Return(true)));
 ```
 
 Note, however, that if you use the `ReturnOKWith()` method, it will override the
 values provided by `SetArgPointee()` in the response parameters of your function
 call.
 
 If the output argument is an array, use the `SetArrayArgument<N>(first, last)`
 action instead. It copies the elements in source range `[first, last)` to the
 array pointed to by the `N`-th (0-based) argument:
 
 ```cpp
 using ::testing::NotNull;
 using ::testing::SetArrayArgument;
 
 class MockArrayMutator : public ArrayMutator {
  public:
   MOCK_METHOD(void, Mutate, (int* values, int num_values), (override));
   ...
 }
 ...
   MockArrayMutator mutator;
   int values[5] = {1, 2, 3, 4, 5};
   EXPECT_CALL(mutator, Mutate(NotNull(), 5))
       .WillOnce(SetArrayArgument<0>(values, values + 5));
 ```
 
 This also works when the argument is an output iterator:
 
 ```cpp
 using ::testing::_;
 using ::testing::SetArrayArgument;
 
 class MockRolodex : public Rolodex {
  public:
   MOCK_METHOD(void, GetNames, (std::back_insert_iterator<vector<string>>),
               (override));
   ...
 }
 ...
   MockRolodex rolodex;
   vector<string> names;
   names.push_back("George");
   names.push_back("John");
   names.push_back("Thomas");
   EXPECT_CALL(rolodex, GetNames(_))
       .WillOnce(SetArrayArgument<0>(names.begin(), names.end()));
 ```
 
 ### Changing a Mock Object's Behavior Based on the State
 
 If you expect a call to change the behavior of a mock object, you can use
 `::testing::InSequence` to specify different behaviors before and after the
 call:
 
 ```cpp
 using ::testing::InSequence;
 using ::testing::Return;
 
 ...
   {
      InSequence seq;
      EXPECT_CALL(my_mock, IsDirty())
          .WillRepeatedly(Return(true));
      EXPECT_CALL(my_mock, Flush());
      EXPECT_CALL(my_mock, IsDirty())
          .WillRepeatedly(Return(false));
   }
   my_mock.FlushIfDirty();
 ```
 
 This makes `my_mock.IsDirty()` return `true` before `my_mock.Flush()` is called
 and return `false` afterwards.
 
 If the behavior change is more complex, you can store the effects in a variable
 and make a mock method get its return value from that variable:
 
 ```cpp
 using ::testing::_;
 using ::testing::SaveArg;
 using ::testing::Return;
 
 ACTION_P(ReturnPointee, p) { return *p; }
 ...
   int previous_value = 0;
   EXPECT_CALL(my_mock, GetPrevValue)
       .WillRepeatedly(ReturnPointee(&previous_value));
   EXPECT_CALL(my_mock, UpdateValue)
       .WillRepeatedly(SaveArg<0>(&previous_value));
   my_mock.DoSomethingToUpdateValue();
 ```
 
 Here `my_mock.GetPrevValue()` will always return the argument of the last
 `UpdateValue()` call.
 
 ### Setting the Default Value for a Return Type {#DefaultValue}
 
 If a mock method's return type is a built-in C++ type or pointer, by default it
 will return 0 when invoked. Also, in C++ 11 and above, a mock method whose
 return type has a default constructor will return a default-constructed value by
 default. You only need to specify an action if this default value doesn't work
 for you.
 
 Sometimes, you may want to change this default value, or you may want to specify
 a default value for types gMock doesn't know about. You can do this using the
 `::testing::DefaultValue` class template:
 
 ```cpp
 using ::testing::DefaultValue;
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(Bar, CalculateBar, (), (override));
 };
 
 
 ...
   Bar default_bar;
   // Sets the default return value for type Bar.
   DefaultValue<Bar>::Set(default_bar);
 
   MockFoo foo;
 
   // We don't need to specify an action here, as the default
   // return value works for us.
   EXPECT_CALL(foo, CalculateBar());
 
   foo.CalculateBar();  // This should return default_bar.
 
   // Unsets the default return value.
   DefaultValue<Bar>::Clear();
 ```
 
 Please note that changing the default value for a type can make you tests hard
 to understand. We recommend you to use this feature judiciously. For example,
 you may want to make sure the `Set()` and `Clear()` calls are right next to the
 code that uses your mock.
 
 ### Setting the Default Actions for a Mock Method
 
 You've learned how to change the default value of a given type. However, this
 may be too coarse for your purpose: perhaps you have two mock methods with the
 same return type and you want them to have different behaviors. The `ON_CALL()`
 macro allows you to customize your mock's behavior at the method level:
 
 ```cpp
 using ::testing::_;
 using ::testing::AnyNumber;
 using ::testing::Gt;
 using ::testing::Return;
 ...
   ON_CALL(foo, Sign(_))
       .WillByDefault(Return(-1));
   ON_CALL(foo, Sign(0))
       .WillByDefault(Return(0));
   ON_CALL(foo, Sign(Gt(0)))
       .WillByDefault(Return(1));
 
   EXPECT_CALL(foo, Sign(_))
       .Times(AnyNumber());
 
   foo.Sign(5);   // This should return 1.
   foo.Sign(-9);  // This should return -1.
   foo.Sign(0);   // This should return 0.
 ```
 
 As you may have guessed, when there are more than one `ON_CALL()` statements,
 the newer ones in the order take precedence over the older ones. In other words,
 the **last** one that matches the function arguments will be used. This matching
 order allows you to set up the common behavior in a mock object's constructor or
 the test fixture's set-up phase and specialize the mock's behavior later.
 
 Note that both `ON_CALL` and `EXPECT_CALL` have the same "later statements take
 precedence" rule, but they don't interact. That is, `EXPECT_CALL`s have their
 own precedence order distinct from the `ON_CALL` precedence order.
 
 ### Using Functions/Methods/Functors/Lambdas as Actions {#FunctionsAsActions}
 
 If the built-in actions don't suit you, you can use an existing callable
 (function, `std::function`, method, functor, lambda as an action.
 
 <!-- GOOGLETEST_CM0024 DO NOT DELETE -->
 
 ```cpp
 using ::testing::_; using ::testing::Invoke;
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(int, Sum, (int x, int y), (override));
   MOCK_METHOD(bool, ComplexJob, (int x), (override));
 };
 
 int CalculateSum(int x, int y) { return x + y; }
 int Sum3(int x, int y, int z) { return x + y + z; }
 
 class Helper {
  public:
   bool ComplexJob(int x);
 };
 
 ...
   MockFoo foo;
   Helper helper;
   EXPECT_CALL(foo, Sum(_, _))
       .WillOnce(&CalculateSum)
       .WillRepeatedly(Invoke(NewPermanentCallback(Sum3, 1)));
   EXPECT_CALL(foo, ComplexJob(_))
       .WillOnce(Invoke(&helper, &Helper::ComplexJob));
       .WillRepeatedly([](int x) { return x > 0; });
 
   foo.Sum(5, 6);         // Invokes CalculateSum(5, 6).
   foo.Sum(2, 3);         // Invokes Sum3(1, 2, 3).
   foo.ComplexJob(10);    // Invokes helper.ComplexJob(10).
   foo.ComplexJob(-1);    // Invokes the inline lambda.
 ```
 
 The only requirement is that the type of the function, etc must be *compatible*
 with the signature of the mock function, meaning that the latter's arguments can
 be implicitly converted to the corresponding arguments of the former, and the
 former's return type can be implicitly converted to that of the latter. So, you
 can invoke something whose type is *not* exactly the same as the mock function,
 as long as it's safe to do so - nice, huh?
 
 **`Note:`{.escaped}**
 
 *   The action takes ownership of the callback and will delete it when the
     action itself is destructed.
 *   If the type of a callback is derived from a base callback type `C`, you need
     to implicitly cast it to `C` to resolve the overloading, e.g.
 
     ```cpp
     using ::testing::Invoke;
     ...
       ResultCallback<bool>* is_ok = ...;
       ... Invoke(is_ok) ...;  // This works.
 
       BlockingClosure* done = new BlockingClosure;
       ... Invoke(implicit_cast<Closure*>(done)) ...;  // The cast is necessary.
     ```
 
 ### Using Functions with Extra Info as Actions
 
 The function or functor you call using `Invoke()` must have the same number of
 arguments as the mock function you use it for. Sometimes you may have a function
 that takes more arguments, and you are willing to pass in the extra arguments
 yourself to fill the gap. You can do this in gMock using callbacks with
 pre-bound arguments. Here's an example:
 
 ```cpp
 using ::testing::Invoke;
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(char, DoThis, (int n), (override));
 };
 
 char SignOfSum(int x, int y) {
   const int sum = x + y;
   return (sum > 0) ? '+' : (sum < 0) ? '-' : '0';
 }
 
 TEST_F(FooTest, Test) {
   MockFoo foo;
 
   EXPECT_CALL(foo, DoThis(2))
       .WillOnce(Invoke(NewPermanentCallback(SignOfSum, 5)));
   EXPECT_EQ('+', foo.DoThis(2));  // Invokes SignOfSum(5, 2).
 }
 ```
 
 ### Invoking a Function/Method/Functor/Lambda/Callback Without Arguments
 
 `Invoke()` is very useful for doing actions that are more complex. It passes the
 mock function's arguments to the function, etc being invoked such that the
 callee has the full context of the call to work with. If the invoked function is
 not interested in some or all of the arguments, it can simply ignore them.
 
 Yet, a common pattern is that a test author wants to invoke a function without
 the arguments of the mock function. `Invoke()` allows her to do that using a
 wrapper function that throws away the arguments before invoking an underlining
 nullary function. Needless to say, this can be tedious and obscures the intent
 of the test.
 
 `InvokeWithoutArgs()` solves this problem. It's like `Invoke()` except that it
 doesn't pass the mock function's arguments to the callee. Here's an example:
 
 ```cpp
 using ::testing::_;
 using ::testing::InvokeWithoutArgs;
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(bool, ComplexJob, (int n), (override));
 };
 
 bool Job1() { ... }
 bool Job2(int n, char c) { ... }
 
 ...
   MockFoo foo;
   EXPECT_CALL(foo, ComplexJob(_))
       .WillOnce(InvokeWithoutArgs(Job1))
       .WillOnce(InvokeWithoutArgs(NewPermanentCallback(Job2, 5, 'a')));
 
   foo.ComplexJob(10);  // Invokes Job1().
   foo.ComplexJob(20);  // Invokes Job2(5, 'a').
 ```
 
 **`Note:`{.escaped}**
 
 *   The action takes ownership of the callback and will delete it when the
     action itself is destructed.
 *   If the type of a callback is derived from a base callback type `C`, you need
     to implicitly cast it to `C` to resolve the overloading, e.g.
 
     ```cpp
     using ::testing::InvokeWithoutArgs;
     ...
       ResultCallback<bool>* is_ok = ...;
       ... InvokeWithoutArgs(is_ok) ...;  // This works.
 
       BlockingClosure* done = ...;
       ... InvokeWithoutArgs(implicit_cast<Closure*>(done)) ...;
       // The cast is necessary.
     ```
 
 ### Invoking an Argument of the Mock Function
 
 Sometimes a mock function will receive a function pointer, a functor (in other
 words, a "callable") as an argument, e.g.
 
 ```cpp
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(bool, DoThis, (int n, (ResultCallback1<bool, int>* callback)),
               (override));
 };
 ```
 
 and you may want to invoke this callable argument:
 
 ```cpp
 using ::testing::_;
 ...
   MockFoo foo;
   EXPECT_CALL(foo, DoThis(_, _))
       .WillOnce(...);
       // Will execute callback->Run(5), where callback is the
       // second argument DoThis() receives.
 ```
 
 NOTE: The section below is legacy documentation from before C++ had lambdas:
 
 Arghh, you need to refer to a mock function argument but C++ has no lambda
 (yet), so you have to define your own action. :-( Or do you really?
 
 Well, gMock has an action to solve *exactly* this problem:
 
 ```cpp
 InvokeArgument<N>(arg_1, arg_2, ..., arg_m)
 ```
 
 will invoke the `N`-th (0-based) argument the mock function receives, with
 `arg_1`, `arg_2`, ..., and `arg_m`. No matter if the argument is a function
 pointer, a functor, or a callback. gMock handles them all.
 
 With that, you could write:
 
 ```cpp
 using ::testing::_;
 using ::testing::InvokeArgument;
 ...
   EXPECT_CALL(foo, DoThis(_, _))
       .WillOnce(InvokeArgument<1>(5));
       // Will execute callback->Run(5), where callback is the
       // second argument DoThis() receives.
 ```
 
 What if the callable takes an argument by reference? No problem - just wrap it
 inside `ByRef()`:
 
 ```cpp
   ...
   MOCK_METHOD(bool, Bar,
               ((ResultCallback2<bool, int, const Helper&>* callback)),
               (override));
   ...
   using ::testing::_;
   using ::testing::ByRef;
   using ::testing::InvokeArgument;
   ...
   MockFoo foo;
   Helper helper;
   ...
   EXPECT_CALL(foo, Bar(_))
       .WillOnce(InvokeArgument<0>(5, ByRef(helper)));
       // ByRef(helper) guarantees that a reference to helper, not a copy of it,
       // will be passed to the callback.
 ```
 
 What if the callable takes an argument by reference and we do **not** wrap the
 argument in `ByRef()`? Then `InvokeArgument()` will *make a copy* of the
 argument, and pass a *reference to the copy*, instead of a reference to the
 original value, to the callable. This is especially handy when the argument is a
 temporary value:
 
 ```cpp
   ...
   MOCK_METHOD(bool, DoThat, (bool (*f)(const double& x, const string& s)),
               (override));
   ...
   using ::testing::_;
   using ::testing::InvokeArgument;
   ...
   MockFoo foo;
   ...
   EXPECT_CALL(foo, DoThat(_))
       .WillOnce(InvokeArgument<0>(5.0, string("Hi")));
       // Will execute (*f)(5.0, string("Hi")), where f is the function pointer
       // DoThat() receives.  Note that the values 5.0 and string("Hi") are
       // temporary and dead once the EXPECT_CALL() statement finishes.  Yet
       // it's fine to perform this action later, since a copy of the values
       // are kept inside the InvokeArgument action.
 ```
 
 ### Ignoring an Action's Result
 
 Sometimes you have an action that returns *something*, but you need an action
 that returns `void` (perhaps you want to use it in a mock function that returns
 `void`, or perhaps it needs to be used in `DoAll()` and it's not the last in the
 list). `IgnoreResult()` lets you do that. For example:
 
 ```cpp
 using ::testing::_;
 using ::testing::DoAll;
 using ::testing::IgnoreResult;
 using ::testing::Return;
 
 int Process(const MyData& data);
 string DoSomething();
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(void, Abc, (const MyData& data), (override));
   MOCK_METHOD(bool, Xyz, (), (override));
 };
 
   ...
   MockFoo foo;
   EXPECT_CALL(foo, Abc(_))
       // .WillOnce(Invoke(Process));
       // The above line won't compile as Process() returns int but Abc() needs
       // to return void.
       .WillOnce(IgnoreResult(Process));
   EXPECT_CALL(foo, Xyz())
       .WillOnce(DoAll(IgnoreResult(DoSomething),
                       // Ignores the string DoSomething() returns.
                       Return(true)));
 ```
 
 Note that you **cannot** use `IgnoreResult()` on an action that already returns
 `void`. Doing so will lead to ugly compiler errors.
 
 ### Selecting an Action's Arguments {#SelectingArgs}
 
 Say you have a mock function `Foo()` that takes seven arguments, and you have a
 custom action that you want to invoke when `Foo()` is called. Trouble is, the
 custom action only wants three arguments:
 
 ```cpp
 using ::testing::_;
 using ::testing::Invoke;
 ...
   MOCK_METHOD(bool, Foo,
               (bool visible, const string& name, int x, int y,
                (const map<pair<int, int>>), double& weight, double min_weight,
                double max_wight));
 ...
 bool IsVisibleInQuadrant1(bool visible, int x, int y) {
   return visible && x >= 0 && y >= 0;
 }
 ...
   EXPECT_CALL(mock, Foo)
       .WillOnce(Invoke(IsVisibleInQuadrant1));  // Uh, won't compile. :-(
 ```
 
 To please the compiler God, you need to define an "adaptor" that has the same
 signature as `Foo()` and calls the custom action with the right arguments:
 
 ```cpp
 using ::testing::_;
 using ::testing::Invoke;
 ...
 bool MyIsVisibleInQuadrant1(bool visible, const string& name, int x, int y,
                             const map<pair<int, int>, double>& weight,
                             double min_weight, double max_wight) {
   return IsVisibleInQuadrant1(visible, x, y);
 }
 ...
   EXPECT_CALL(mock, Foo)
       .WillOnce(Invoke(MyIsVisibleInQuadrant1));  // Now it works.
 ```
 
 But isn't this awkward?
 
 gMock provides a generic *action adaptor*, so you can spend your time minding
 more important business than writing your own adaptors. Here's the syntax:
 
 ```cpp
 WithArgs<N1, N2, ..., Nk>(action)
 ```
 
 creates an action that passes the arguments of the mock function at the given
 indices (0-based) to the inner `action` and performs it. Using `WithArgs`, our
 original example can be written as:
 
 ```cpp
 using ::testing::_;
 using ::testing::Invoke;
 using ::testing::WithArgs;
 ...
   EXPECT_CALL(mock, Foo)
       .WillOnce(WithArgs<0, 2, 3>(Invoke(IsVisibleInQuadrant1)));  // No need to define your own adaptor.
 ```
 
 For better readability, gMock also gives you:
 
 *   `WithoutArgs(action)` when the inner `action` takes *no* argument, and
 *   `WithArg<N>(action)` (no `s` after `Arg`) when the inner `action` takes
     *one* argument.
 
 As you may have realized, `InvokeWithoutArgs(...)` is just syntactic sugar for
 `WithoutArgs(Invoke(...))`.
 
 Here are more tips:
 
 *   The inner action used in `WithArgs` and friends does not have to be
     `Invoke()` -- it can be anything.
 *   You can repeat an argument in the argument list if necessary, e.g.
     `WithArgs<2, 3, 3, 5>(...)`.
 *   You can change the order of the arguments, e.g. `WithArgs<3, 2, 1>(...)`.
 *   The types of the selected arguments do *not* have to match the signature of
     the inner action exactly. It works as long as they can be implicitly
     converted to the corresponding arguments of the inner action. For example,
     if the 4-th argument of the mock function is an `int` and `my_action` takes
     a `double`, `WithArg<4>(my_action)` will work.
 
 ### Ignoring Arguments in Action Functions
 
 The [selecting-an-action's-arguments](#SelectingArgs) recipe showed us one way
 to make a mock function and an action with incompatible argument lists fit
 together. The downside is that wrapping the action in `WithArgs<...>()` can get
 tedious for people writing the tests.
 
 If you are defining a function (or method, functor, lambda, callback) to be used
 with `Invoke*()`, and you are not interested in some of its arguments, an
 alternative to `WithArgs` is to declare the uninteresting arguments as `Unused`.
 This makes the definition less cluttered and less fragile in case the types of
 the uninteresting arguments change. It could also increase the chance the action
 function can be reused. For example, given
 
 ```cpp
  public:
   MOCK_METHOD(double, Foo, double(const string& label, double x, double y),
               (override));
   MOCK_METHOD(double, Bar, (int index, double x, double y), (override));
 ```
 
 instead of
 
 ```cpp
 using ::testing::_;
 using ::testing::Invoke;
 
 double DistanceToOriginWithLabel(const string& label, double x, double y) {
   return sqrt(x*x + y*y);
 }
 double DistanceToOriginWithIndex(int index, double x, double y) {
   return sqrt(x*x + y*y);
 }
 ...
   EXPECT_CALL(mock, Foo("abc", _, _))
       .WillOnce(Invoke(DistanceToOriginWithLabel));
   EXPECT_CALL(mock, Bar(5, _, _))
       .WillOnce(Invoke(DistanceToOriginWithIndex));
 ```
 
 you could write
 
 ```cpp
 using ::testing::_;
 using ::testing::Invoke;
 using ::testing::Unused;
 
 double DistanceToOrigin(Unused, double x, double y) {
   return sqrt(x*x + y*y);
 }
 ...
   EXPECT_CALL(mock, Foo("abc", _, _))
       .WillOnce(Invoke(DistanceToOrigin));
   EXPECT_CALL(mock, Bar(5, _, _))
       .WillOnce(Invoke(DistanceToOrigin));
 ```
 
 ### Sharing Actions
 
 Just like matchers, a gMock action object consists of a pointer to a ref-counted
 implementation object. Therefore copying actions is also allowed and very
 efficient. When the last action that references the implementation object dies,
 the implementation object will be deleted.
 
 If you have some complex action that you want to use again and again, you may
 not have to build it from scratch everytime. If the action doesn't have an
 internal state (i.e. if it always does the same thing no matter how many times
 it has been called), you can assign it to an action variable and use that
 variable repeatedly. For example:
 
 ```cpp
 using ::testing::Action;
 using ::testing::DoAll;
 using ::testing::Return;
 using ::testing::SetArgPointee;
 ...
   Action<bool(int*)> set_flag = DoAll(SetArgPointee<0>(5),
                                       Return(true));
   ... use set_flag in .WillOnce() and .WillRepeatedly() ...
 ```
 
 However, if the action has its own state, you may be surprised if you share the
 action object. Suppose you have an action factory `IncrementCounter(init)` which
 creates an action that increments and returns a counter whose initial value is
 `init`, using two actions created from the same expression and using a shared
 action will exhibit different behaviors. Example:
 
 ```cpp
   EXPECT_CALL(foo, DoThis())
       .WillRepeatedly(IncrementCounter(0));
   EXPECT_CALL(foo, DoThat())
       .WillRepeatedly(IncrementCounter(0));
   foo.DoThis();  // Returns 1.
   foo.DoThis();  // Returns 2.
   foo.DoThat();  // Returns 1 - Blah() uses a different
                  // counter than Bar()'s.
 ```
 
 versus
 
 ```cpp
 using ::testing::Action;
 ...
   Action<int()> increment = IncrementCounter(0);
   EXPECT_CALL(foo, DoThis())
       .WillRepeatedly(increment);
   EXPECT_CALL(foo, DoThat())
       .WillRepeatedly(increment);
   foo.DoThis();  // Returns 1.
   foo.DoThis();  // Returns 2.
   foo.DoThat();  // Returns 3 - the counter is shared.
 ```
 
 ### Testing Asynchronous Behavior
 
 One oft-encountered problem with gMock is that it can be hard to test
 asynchronous behavior. Suppose you had a `EventQueue` class that you wanted to
 test, and you created a separate `EventDispatcher` interface so that you could
 easily mock it out. However, the implementation of the class fired all the
 events on a background thread, which made test timings difficult. You could just
 insert `sleep()` statements and hope for the best, but that makes your test
 behavior nondeterministic. A better way is to use gMock actions and
 `Notification` objects to force your asynchronous test to behave synchronously.
 
 ```cpp
 using ::testing::DoAll;
 using ::testing::InvokeWithoutArgs;
 using ::testing::Return;
 
 class MockEventDispatcher : public EventDispatcher {
   MOCK_METHOD(bool, DispatchEvent, (int32), (override));
 };
 
 ACTION_P(Notify, notification) {
   notification->Notify();
 }
 
 TEST(EventQueueTest, EnqueueEventTest) {
   MockEventDispatcher mock_event_dispatcher;
   EventQueue event_queue(&mock_event_dispatcher);
 
   const int32 kEventId = 321;
   Notification done;
   EXPECT_CALL(mock_event_dispatcher, DispatchEvent(kEventId))
       .WillOnce(Notify(&done));
 
   event_queue.EnqueueEvent(kEventId);
   done.WaitForNotification();
 }
 ```
 
 In the example above, we set our normal gMock expectations, but then add an
 additional action to notify the `Notification` object. Now we can just call
 `Notification::WaitForNotification()` in the main thread to wait for the
 asynchronous call to finish. After that, our test suite is complete and we can
 safely exit.
 
 Note: this example has a downside: namely, if the expectation is not satisfied,
 our test will run forever. It will eventually time-out and fail, but it will
 take longer and be slightly harder to debug. To alleviate this problem, you can
 use `WaitForNotificationWithTimeout(ms)` instead of `WaitForNotification()`.
 
 ## Misc Recipes on Using gMock
 
 ### Mocking Methods That Use Move-Only Types
 
 C++11 introduced *move-only types*. A move-only-typed value can be moved from
 one object to another, but cannot be copied. `std::unique_ptr<T>` is probably
 the most commonly used move-only type.
 
 Mocking a method that takes and/or returns move-only types presents some
 challenges, but nothing insurmountable. This recipe shows you how you can do it.
 Note that the support for move-only method arguments was only introduced to
 gMock in April 2017; in older code, you may find more complex
 [workarounds](#LegacyMoveOnly) for lack of this feature.
 
 Let’s say we are working on a fictional project that lets one post and share
 snippets called “buzzes”. Your code uses these types:
 
 ```cpp
 enum class AccessLevel { kInternal, kPublic };
 
 class Buzz {
  public:
   explicit Buzz(AccessLevel access) { ... }
   ...
 };
 
 class Buzzer {
  public:
   virtual ~Buzzer() {}
   virtual std::unique_ptr<Buzz> MakeBuzz(StringPiece text) = 0;
   virtual bool ShareBuzz(std::unique_ptr<Buzz> buzz, int64_t timestamp) = 0;
   ...
 };
 ```
 
 A `Buzz` object represents a snippet being posted. A class that implements the
 `Buzzer` interface is capable of creating and sharing `Buzz`es. Methods in
 `Buzzer` may return a `unique_ptr<Buzz>` or take a `unique_ptr<Buzz>`. Now we
 need to mock `Buzzer` in our tests.
 
 To mock a method that accepts or returns move-only types, you just use the
 familiar `MOCK_METHOD` syntax as usual:
 
 ```cpp
 class MockBuzzer : public Buzzer {
  public:
   MOCK_METHOD(std::unique_ptr<Buzz>, MakeBuzz, (StringPiece text), (override));
   MOCK_METHOD(bool, ShareBuzz, (std::unique_ptr<Buzz> buzz, int64_t timestamp),
               (override));
 };
 ```
 
 Now that we have the mock class defined, we can use it in tests. In the
 following code examples, we assume that we have defined a `MockBuzzer` object
 named `mock_buzzer_`:
 
 ```cpp
   MockBuzzer mock_buzzer_;
 ```
 
 First let’s see how we can set expectations on the `MakeBuzz()` method, which
 returns a `unique_ptr<Buzz>`.
 
 As usual, if you set an expectation without an action (i.e. the `.WillOnce()` or
 `.WillRepeatedly()` clause), when that expectation fires, the default action for
 that method will be taken. Since `unique_ptr<>` has a default constructor that
 returns a null `unique_ptr`, that’s what you’ll get if you don’t specify an
 action:
 
 ```cpp
   // Use the default action.
   EXPECT_CALL(mock_buzzer_, MakeBuzz("hello"));
 
   // Triggers the previous EXPECT_CALL.
   EXPECT_EQ(nullptr, mock_buzzer_.MakeBuzz("hello"));
 ```
 
 If you are not happy with the default action, you can tweak it as usual; see
 [Setting Default Actions](#OnCall).
 
 If you just need to return a pre-defined move-only value, you can use the
 `Return(ByMove(...))` action:
 
 ```cpp
   // When this fires, the unique_ptr<> specified by ByMove(...) will
   // be returned.
   EXPECT_CALL(mock_buzzer_, MakeBuzz("world"))
       .WillOnce(Return(ByMove(MakeUnique<Buzz>(AccessLevel::kInternal))));
 
   EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("world"));
 ```
 
 Note that `ByMove()` is essential here - if you drop it, the code won’t compile.
 
 Quiz time! What do you think will happen if a `Return(ByMove(...))` action is
 performed more than once (e.g. you write `...
 .WillRepeatedly(Return(ByMove(...)));`)? Come think of it, after the first time
 the action runs, the source value will be consumed (since it’s a move-only
 value), so the next time around, there’s no value to move from -- you’ll get a
 run-time error that `Return(ByMove(...))` can only be run once.
 
 If you need your mock method to do more than just moving a pre-defined value,
 remember that you can always use a lambda or a callable object, which can do
 pretty much anything you want:
 
 ```cpp
   EXPECT_CALL(mock_buzzer_, MakeBuzz("x"))
       .WillRepeatedly([](StringPiece text) {
         return MakeUnique<Buzz>(AccessLevel::kInternal);
       });
 
   EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x"));
   EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x"));
 ```
 
 Every time this `EXPECT_CALL` fires, a new `unique_ptr<Buzz>` will be created
 and returned. You cannot do this with `Return(ByMove(...))`.
 
 That covers returning move-only values; but how do we work with methods
 accepting move-only arguments? The answer is that they work normally, although
 some actions will not compile when any of method's arguments are move-only. You
 can always use `Return`, or a [lambda or functor](#FunctionsAsActions):
 
 ```cpp
   using ::testing::Unused;
 
   EXPECT_CALL(mock_buzzer_, ShareBuzz(NotNull(), _)).WillOnce(Return(true));
   EXPECT_TRUE(mock_buzzer_.ShareBuzz(MakeUnique<Buzz>(AccessLevel::kInternal)),
               0);
 
   EXPECT_CALL(mock_buzzer_, ShareBuzz(_, _)).WillOnce(
       [](std::unique_ptr<Buzz> buzz, Unused) { return buzz != nullptr; });
   EXPECT_FALSE(mock_buzzer_.ShareBuzz(nullptr, 0));
 ```
 
 Many built-in actions (`WithArgs`, `WithoutArgs`,`DeleteArg`, `SaveArg`, ...)
 could in principle support move-only arguments, but the support for this is not
 implemented yet. If this is blocking you, please file a bug.
 
 A few actions (e.g. `DoAll`) copy their arguments internally, so they can never
 work with non-copyable objects; you'll have to use functors instead.
 
 #### Legacy workarounds for move-only types {#LegacyMoveOnly}
 
 Support for move-only function arguments was only introduced to gMock in April
 2017. In older code, you may encounter the following workaround for the lack of
 this feature (it is no longer necessary - we're including it just for
 reference):
 
 ```cpp
 class MockBuzzer : public Buzzer {
  public:
   MOCK_METHOD(bool, DoShareBuzz, (Buzz* buzz, Time timestamp));
   bool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) override {
     return DoShareBuzz(buzz.get(), timestamp);
   }
 };
 ```
 
 The trick is to delegate the `ShareBuzz()` method to a mock method (let’s call
 it `DoShareBuzz()`) that does not take move-only parameters. Then, instead of
 setting expectations on `ShareBuzz()`, you set them on the `DoShareBuzz()` mock
 method:
 
 ```cpp
   MockBuzzer mock_buzzer_;
   EXPECT_CALL(mock_buzzer_, DoShareBuzz(NotNull(), _));
 
   // When one calls ShareBuzz() on the MockBuzzer like this, the call is
   // forwarded to DoShareBuzz(), which is mocked.  Therefore this statement
   // will trigger the above EXPECT_CALL.
   mock_buzzer_.ShareBuzz(MakeUnique<Buzz>(AccessLevel::kInternal), 0);
 ```
 
 ### Making the Compilation Faster
 
 Believe it or not, the *vast majority* of the time spent on compiling a mock
 class is in generating its constructor and destructor, as they perform
 non-trivial tasks (e.g. verification of the expectations). What's more, mock
 methods with different signatures have different types and thus their
 constructors/destructors need to be generated by the compiler separately. As a
 result, if you mock many different types of methods, compiling your mock class
 can get really slow.
 
 If you are experiencing slow compilation, you can move the definition of your
 mock class' constructor and destructor out of the class body and into a `.cc`
 file. This way, even if you `#include` your mock class in N files, the compiler
 only needs to generate its constructor and destructor once, resulting in a much
 faster compilation.
 
 Let's illustrate the idea using an example. Here's the definition of a mock
 class before applying this recipe:
 
 ```cpp
 // File mock_foo.h.
 ...
 class MockFoo : public Foo {
  public:
   // Since we don't declare the constructor or the destructor,
   // the compiler will generate them in every translation unit
   // where this mock class is used.
 
   MOCK_METHOD(int, DoThis, (), (override));
   MOCK_METHOD(bool, DoThat, (const char* str), (override));
   ... more mock methods ...
 };
 ```
 
 After the change, it would look like:
 
 ```cpp
 // File mock_foo.h.
 ...
 class MockFoo : public Foo {
  public:
   // The constructor and destructor are declared, but not defined, here.
   MockFoo();
   virtual ~MockFoo();
 
   MOCK_METHOD(int, DoThis, (), (override));
   MOCK_METHOD(bool, DoThat, (const char* str), (override));
   ... more mock methods ...
 };
 ```
 
 and
 
 ```cpp
 // File mock_foo.cc.
 #include "path/to/mock_foo.h"
 
 // The definitions may appear trivial, but the functions actually do a
 // lot of things through the constructors/destructors of the member
 // variables used to implement the mock methods.
 MockFoo::MockFoo() {}
 MockFoo::~MockFoo() {}
 ```
 
 ### Forcing a Verification
 
 When it's being destroyed, your friendly mock object will automatically verify
 that all expectations on it have been satisfied, and will generate googletest
 failures if not. This is convenient as it leaves you with one less thing to
 worry about. That is, unless you are not sure if your mock object will be
 destroyed.
 
 How could it be that your mock object won't eventually be destroyed? Well, it
 might be created on the heap and owned by the code you are testing. Suppose
 there's a bug in that code and it doesn't delete the mock object properly - you
 could end up with a passing test when there's actually a bug.
 
 Using a heap checker is a good idea and can alleviate the concern, but its
 implementation is not 100% reliable. So, sometimes you do want to *force* gMock
 to verify a mock object before it is (hopefully) destructed. You can do this
 with `Mock::VerifyAndClearExpectations(&mock_object)`:
 
 ```cpp
 TEST(MyServerTest, ProcessesRequest) {
   using ::testing::Mock;
 
   MockFoo* const foo = new MockFoo;
   EXPECT_CALL(*foo, ...)...;
   // ... other expectations ...
 
   // server now owns foo.
   MyServer server(foo);
   server.ProcessRequest(...);
 
   // In case that server's destructor will forget to delete foo,
   // this will verify the expectations anyway.
   Mock::VerifyAndClearExpectations(foo);
 }  // server is destroyed when it goes out of scope here.
 ```
 
 **Tip:** The `Mock::VerifyAndClearExpectations()` function returns a `bool` to
 indicate whether the verification was successful (`true` for yes), so you can
 wrap that function call inside a `ASSERT_TRUE()` if there is no point going
 further when the verification has failed.
 
 ### Using Check Points {#UsingCheckPoints}
 
 Sometimes you may want to "reset" a mock object at various check points in your
 test: at each check point, you verify that all existing expectations on the mock
 object have been satisfied, and then you set some new expectations on it as if
 it's newly created. This allows you to work with a mock object in "phases" whose
 sizes are each manageable.
 
 One such scenario is that in your test's `SetUp()` function, you may want to put
 the object you are testing into a certain state, with the help from a mock
 object. Once in the desired state, you want to clear all expectations on the
 mock, such that in the `TEST_F` body you can set fresh expectations on it.
 
 As you may have figured out, the `Mock::VerifyAndClearExpectations()` function
 we saw in the previous recipe can help you here. Or, if you are using
 `ON_CALL()` to set default actions on the mock object and want to clear the
 default actions as well, use `Mock::VerifyAndClear(&mock_object)` instead. This
 function does what `Mock::VerifyAndClearExpectations(&mock_object)` does and
 returns the same `bool`, **plus** it clears the `ON_CALL()` statements on
 `mock_object` too.
 
 Another trick you can use to achieve the same effect is to put the expectations
 in sequences and insert calls to a dummy "check-point" function at specific
 places. Then you can verify that the mock function calls do happen at the right
 time. For example, if you are exercising code:
 
 ```cpp
   Foo(1);
   Foo(2);
   Foo(3);
 ```
 
 and want to verify that `Foo(1)` and `Foo(3)` both invoke `mock.Bar("a")`, but
 `Foo(2)` doesn't invoke anything. You can write:
 
 ```cpp
 using ::testing::MockFunction;
 
 TEST(FooTest, InvokesBarCorrectly) {
   MyMock mock;
   // Class MockFunction<F> has exactly one mock method.  It is named
   // Call() and has type F.
   MockFunction<void(string check_point_name)> check;
   {
     InSequence s;
 
     EXPECT_CALL(mock, Bar("a"));
     EXPECT_CALL(check, Call("1"));
     EXPECT_CALL(check, Call("2"));
     EXPECT_CALL(mock, Bar("a"));
   }
   Foo(1);
   check.Call("1");
   Foo(2);
   check.Call("2");
   Foo(3);
 }
 ```
 
 The expectation spec says that the first `Bar("a")` must happen before check
 point "1", the second `Bar("a")` must happen after check point "2", and nothing
 should happen between the two check points. The explicit check points make it
 easy to tell which `Bar("a")` is called by which call to `Foo()`.
 
 ### Mocking Destructors
 
 Sometimes you want to make sure a mock object is destructed at the right time,
 e.g. after `bar->A()` is called but before `bar->B()` is called. We already know
 that you can specify constraints on the [order](#OrderedCalls) of mock function
 calls, so all we need to do is to mock the destructor of the mock function.
 
 This sounds simple, except for one problem: a destructor is a special function
 with special syntax and special semantics, and the `MOCK_METHOD` macro doesn't
 work for it:
 
 ```cpp
 MOCK_METHOD(void, ~MockFoo, ());  // Won't compile!
 ```
 
 The good news is that you can use a simple pattern to achieve the same effect.
 First, add a mock function `Die()` to your mock class and call it in the
 destructor, like this:
 
 ```cpp
 class MockFoo : public Foo {
   ...
   // Add the following two lines to the mock class.
   MOCK_METHOD(void, Die, ());
   virtual ~MockFoo() { Die(); }
 };
 ```
 
 (If the name `Die()` clashes with an existing symbol, choose another name.) Now,
 we have translated the problem of testing when a `MockFoo` object dies to
 testing when its `Die()` method is called:
 
 ```cpp
   MockFoo* foo = new MockFoo;
   MockBar* bar = new MockBar;
   ...
   {
     InSequence s;
 
     // Expects *foo to die after bar->A() and before bar->B().
     EXPECT_CALL(*bar, A());
     EXPECT_CALL(*foo, Die());
     EXPECT_CALL(*bar, B());
   }
 ```
 
 And that's that.
 
 ### Using gMock and Threads {#UsingThreads}
 
 In a **unit** test, it's best if you could isolate and test a piece of code in a
 single-threaded context. That avoids race conditions and dead locks, and makes
 debugging your test much easier.
 
 Yet most programs are multi-threaded, and sometimes to test something we need to
 pound on it from more than one thread. gMock works for this purpose too.
 
 Remember the steps for using a mock:
 
 1.  Create a mock object `foo`.
 2.  Set its default actions and expectations using `ON_CALL()` and
     `EXPECT_CALL()`.
 3.  The code under test calls methods of `foo`.
 4.  Optionally, verify and reset the mock.
 5.  Destroy the mock yourself, or let the code under test destroy it. The
     destructor will automatically verify it.
 
 If you follow the following simple rules, your mocks and threads can live
 happily together:
 
 *   Execute your *test code* (as opposed to the code being tested) in *one*
     thread. This makes your test easy to follow.
 *   Obviously, you can do step #1 without locking.
 *   When doing step #2 and #5, make sure no other thread is accessing `foo`.
     Obvious too, huh?
 *   #3 and #4 can be done either in one thread or in multiple threads - anyway
     you want. gMock takes care of the locking, so you don't have to do any -
     unless required by your test logic.
 
 If you violate the rules (for example, if you set expectations on a mock while
 another thread is calling its methods), you get undefined behavior. That's not
 fun, so don't do it.
 
 gMock guarantees that the action for a mock function is done in the same thread
 that called the mock function. For example, in
 
 ```cpp
   EXPECT_CALL(mock, Foo(1))
       .WillOnce(action1);
   EXPECT_CALL(mock, Foo(2))
       .WillOnce(action2);
 ```
 
 if `Foo(1)` is called in thread 1 and `Foo(2)` is called in thread 2, gMock will
 execute `action1` in thread 1 and `action2` in thread 2.
 
 gMock does *not* impose a sequence on actions performed in different threads
 (doing so may create deadlocks as the actions may need to cooperate). This means
 that the execution of `action1` and `action2` in the above example *may*
 interleave. If this is a problem, you should add proper synchronization logic to
 `action1` and `action2` to make the test thread-safe.
 
 Also, remember that `DefaultValue<T>` is a global resource that potentially
 affects *all* living mock objects in your program. Naturally, you won't want to
 mess with it from multiple threads or when there still are mocks in action.
 
 ### Controlling How Much Information gMock Prints
 
 When gMock sees something that has the potential of being an error (e.g. a mock
 function with no expectation is called, a.k.a. an uninteresting call, which is
 allowed but perhaps you forgot to explicitly ban the call), it prints some
 warning messages, including the arguments of the function, the return value, and
 the stack trace. Hopefully this will remind you to take a look and see if there
 is indeed a problem.
 
 Sometimes you are confident that your tests are correct and may not appreciate
 such friendly messages. Some other times, you are debugging your tests or
 learning about the behavior of the code you are testing, and wish you could
 observe every mock call that happens (including argument values, the return
 value, and the stack trace). Clearly, one size doesn't fit all.
 
 You can control how much gMock tells you using the `--gmock_verbose=LEVEL`
 command-line flag, where `LEVEL` is a string with three possible values:
 
 *   `info`: gMock will print all informational messages, warnings, and errors
     (most verbose). At this setting, gMock will also log any calls to the
     `ON_CALL/EXPECT_CALL` macros. It will include a stack trace in
     "uninteresting call" warnings.
 *   `warning`: gMock will print both warnings and errors (less verbose); it will
     omit the stack traces in "uninteresting call" warnings. This is the default.
 *   `error`: gMock will print errors only (least verbose).
 
 Alternatively, you can adjust the value of that flag from within your tests like
 so:
 
 ```cpp
   ::testing::FLAGS_gmock_verbose = "error";
 ```
 
 If you find gMock printing too many stack frames with its informational or
 warning messages, remember that you can control their amount with the
 `--gtest_stack_trace_depth=max_depth` flag.
 
 Now, judiciously use the right flag to enable gMock serve you better!
 
 ### Gaining Super Vision into Mock Calls
 
 You have a test using gMock. It fails: gMock tells you some expectations aren't
 satisfied. However, you aren't sure why: Is there a typo somewhere in the
 matchers? Did you mess up the order of the `EXPECT_CALL`s? Or is the code under
 test doing something wrong? How can you find out the cause?
 
 Won't it be nice if you have X-ray vision and can actually see the trace of all
 `EXPECT_CALL`s and mock method calls as they are made? For each call, would you
 like to see its actual argument values and which `EXPECT_CALL` gMock thinks it
 matches? If you still need some help to figure out who made these calls, how
 about being able to see the complete stack trace at each mock call?
 
 You can unlock this power by running your test with the `--gmock_verbose=info`
 flag. For example, given the test program:
 
 ```cpp
 #include "gmock/gmock.h"
 
 using testing::_;
 using testing::HasSubstr;
 using testing::Return;
 
 class MockFoo {
  public:
   MOCK_METHOD(void, F, (const string& x, const string& y));
 };
 
 TEST(Foo, Bar) {
   MockFoo mock;
   EXPECT_CALL(mock, F(_, _)).WillRepeatedly(Return());
   EXPECT_CALL(mock, F("a", "b"));
   EXPECT_CALL(mock, F("c", HasSubstr("d")));
 
   mock.F("a", "good");
   mock.F("a", "b");
 }
 ```
 
 if you run it with `--gmock_verbose=info`, you will see this output:
 
 ```shell
 [ RUN       ] Foo.Bar
 
 foo_test.cc:14: EXPECT_CALL(mock, F(_, _)) invoked
 Stack trace: ...
 
 foo_test.cc:15: EXPECT_CALL(mock, F("a", "b")) invoked
 Stack trace: ...
 
 foo_test.cc:16: EXPECT_CALL(mock, F("c", HasSubstr("d"))) invoked
 Stack trace: ...
 
 foo_test.cc:14: Mock function call matches EXPECT_CALL(mock, F(_, _))...
     Function call: F(@0x7fff7c8dad40"a",@0x7fff7c8dad10"good")
 Stack trace: ...
 
 foo_test.cc:15: Mock function call matches EXPECT_CALL(mock, F("a", "b"))...
     Function call: F(@0x7fff7c8dada0"a",@0x7fff7c8dad70"b")
 Stack trace: ...
 
 foo_test.cc:16: Failure
 Actual function call count doesn't match EXPECT_CALL(mock, F("c", HasSubstr("d")))...
          Expected: to be called once
            Actual: never called - unsatisfied and active
 [  FAILED  ] Foo.Bar
 ```
 
 Suppose the bug is that the `"c"` in the third `EXPECT_CALL` is a typo and
 should actually be `"a"`. With the above message, you should see that the actual
 `F("a", "good")` call is matched by the first `EXPECT_CALL`, not the third as
 you thought. From that it should be obvious that the third `EXPECT_CALL` is
 written wrong. Case solved.
 
 If you are interested in the mock call trace but not the stack traces, you can
 combine `--gmock_verbose=info` with `--gtest_stack_trace_depth=0` on the test
 command line.
 
 <!-- GOOGLETEST_CM0025 DO NOT DELETE -->
 
 ### Running Tests in Emacs
 
 If you build and run your tests in Emacs using the `M-x google-compile` command
 (as many googletest users do), the source file locations of gMock and googletest
 errors will be highlighted. Just press `<Enter>` on one of them and you'll be
 taken to the offending line. Or, you can just type `C-x`` to jump to the next
 error.
 
 To make it even easier, you can add the following lines to your `~/.emacs` file:
 
 ```text
 (global-set-key "\M-m"  'google-compile)  ; m is for make
 (global-set-key [M-down] 'next-error)
 (global-set-key [M-up]  '(lambda () (interactive) (next-error -1)))
 ```
 
 Then you can type `M-m` to start a build (if you want to run the test as well,
 just make sure `foo_test.run` or `runtests` is in the build command you supply
 after typing `M-m`), or `M-up`/`M-down` to move back and forth between errors.
 
 ## Extending gMock
 
 ### Writing New Matchers Quickly {#NewMatchers}
 
 WARNING: gMock does not guarantee when or how many times a matcher will be
 invoked. Therefore, all matchers must be functionally pure. See
 [this section](#PureMatchers) for more details.
 
 The `MATCHER*` family of macros can be used to define custom matchers easily.
 The syntax:
 
 ```cpp
 MATCHER(name, description_string_expression) { statements; }
 ```
 
 will define a matcher with the given name that executes the statements, which
 must return a `bool` to indicate if the match succeeds. Inside the statements,
 you can refer to the value being matched by `arg`, and refer to its type by
 `arg_type`.
 
 The *description string* is a `string`-typed expression that documents what the
 matcher does, and is used to generate the failure message when the match fails.
 It can (and should) reference the special `bool` variable `negation`, and should
 evaluate to the description of the matcher when `negation` is `false`, or that
 of the matcher's negation when `negation` is `true`.
 
 For convenience, we allow the description string to be empty (`""`), in which
 case gMock will use the sequence of words in the matcher name as the
 description.
 
 For example:
 
 ```cpp
 MATCHER(IsDivisibleBy7, "") { return (arg % 7) == 0; }
 ```
 
 allows you to write
 
 ```cpp
   // Expects mock_foo.Bar(n) to be called where n is divisible by 7.
   EXPECT_CALL(mock_foo, Bar(IsDivisibleBy7()));
 ```
 
 or,
 
 ```cpp
   using ::testing::Not;
   ...
   // Verifies that two values are divisible by 7.
   EXPECT_THAT(some_expression, IsDivisibleBy7());
   EXPECT_THAT(some_other_expression, Not(IsDivisibleBy7()));
 ```
 
 If the above assertions fail, they will print something like:
 
 ```shell
   Value of: some_expression
   Expected: is divisible by 7
     Actual: 27
   ...
   Value of: some_other_expression
   Expected: not (is divisible by 7)
     Actual: 21
 ```
 
 where the descriptions `"is divisible by 7"` and `"not (is divisible by 7)"` are
 automatically calculated from the matcher name `IsDivisibleBy7`.
 
 As you may have noticed, the auto-generated descriptions (especially those for
 the negation) may not be so great. You can always override them with a `string`
 expression of your own:
 
 ```cpp
 MATCHER(IsDivisibleBy7,
         absl::StrCat(negation ? "isn't" : "is", " divisible by 7")) {
   return (arg % 7) == 0;
 }
 ```
 
 Optionally, you can stream additional information to a hidden argument named
 `result_listener` to explain the match result. For example, a better definition
 of `IsDivisibleBy7` is:
 
 ```cpp
 MATCHER(IsDivisibleBy7, "") {
   if ((arg % 7) == 0)
     return true;
 
   *result_listener << "the remainder is " << (arg % 7);
   return false;
 }
 ```
 
 With this definition, the above assertion will give a better message:
 
 ```shell
   Value of: some_expression
   Expected: is divisible by 7
     Actual: 27 (the remainder is 6)
 ```
 
 You should let `MatchAndExplain()` print *any additional information* that can
 help a user understand the match result. Note that it should explain why the
 match succeeds in case of a success (unless it's obvious) - this is useful when
 the matcher is used inside `Not()`. There is no need to print the argument value
 itself, as gMock already prints it for you.
 
 NOTE: The type of the value being matched (`arg_type`) is determined by the
 context in which you use the matcher and is supplied to you by the compiler, so
 you don't need to worry about declaring it (nor can you). This allows the
 matcher to be polymorphic. For example, `IsDivisibleBy7()` can be used to match
 any type where the value of `(arg % 7) == 0` can be implicitly converted to a
 `bool`. In the `Bar(IsDivisibleBy7())` example above, if method `Bar()` takes an
 `int`, `arg_type` will be `int`; if it takes an `unsigned long`, `arg_type` will
 be `unsigned long`; and so on.
 
 ### Writing New Parameterized Matchers Quickly
 
 Sometimes you'll want to define a matcher that has parameters. For that you can
 use the macro:
 
 ```cpp
 MATCHER_P(name, param_name, description_string) { statements; }
 ```
 
 where the description string can be either `""` or a `string` expression that
 references `negation` and `param_name`.
 
 For example:
 
 ```cpp
 MATCHER_P(HasAbsoluteValue, value, "") { return abs(arg) == value; }
 ```
 
 will allow you to write:
 
 ```cpp
   EXPECT_THAT(Blah("a"), HasAbsoluteValue(n));
 ```
 
 which may lead to this message (assuming `n` is 10):
 
 ```shell
   Value of: Blah("a")
   Expected: has absolute value 10
     Actual: -9
 ```
 
 Note that both the matcher description and its parameter are printed, making the
 message human-friendly.
 
 In the matcher definition body, you can write `foo_type` to reference the type
 of a parameter named `foo`. For example, in the body of
 `MATCHER_P(HasAbsoluteValue, value)` above, you can write `value_type` to refer
 to the type of `value`.
 
 gMock also provides `MATCHER_P2`, `MATCHER_P3`, ..., up to `MATCHER_P10` to
 support multi-parameter matchers:
 
 ```cpp
 MATCHER_Pk(name, param_1, ..., param_k, description_string) { statements; }
 ```
 
 Please note that the custom description string is for a particular *instance* of
 the matcher, where the parameters have been bound to actual values. Therefore
 usually you'll want the parameter values to be part of the description. gMock
 lets you do that by referencing the matcher parameters in the description string
 expression.
 
 For example,
 
 ```cpp
 using ::testing::PrintToString;
 MATCHER_P2(InClosedRange, low, hi,
            absl::StrFormat("%s in range [%s, %s]", negation ? "isn't" : "is",
                            PrintToString(low), PrintToString(hi))) {
   return low <= arg && arg <= hi;
 }
 ...
 EXPECT_THAT(3, InClosedRange(4, 6));
 ```
 
 would generate a failure that contains the message:
 
 ```shell
   Expected: is in range [4, 6]
 ```
 
 If you specify `""` as the description, the failure message will contain the
 sequence of words in the matcher name followed by the parameter values printed
 as a tuple. For example,
 
 ```cpp
   MATCHER_P2(InClosedRange, low, hi, "") { ... }
   ...
   EXPECT_THAT(3, InClosedRange(4, 6));
 ```
 
 would generate a failure that contains the text:
 
 ```shell
   Expected: in closed range (4, 6)
 ```
 
 For the purpose of typing, you can view
 
 ```cpp
 MATCHER_Pk(Foo, p1, ..., pk, description_string) { ... }
 ```
 
 as shorthand for
 
 ```cpp
 template <typename p1_type, ..., typename pk_type>
 FooMatcherPk<p1_type, ..., pk_type>
 Foo(p1_type p1, ..., pk_type pk) { ... }
 ```
 
 When you write `Foo(v1, ..., vk)`, the compiler infers the types of the
 parameters `v1`, ..., and `vk` for you. If you are not happy with the result of
 the type inference, you can specify the types by explicitly instantiating the
 template, as in `Foo<long, bool>(5, false)`. As said earlier, you don't get to
 (or need to) specify `arg_type` as that's determined by the context in which the
 matcher is used.
 
 You can assign the result of expression `Foo(p1, ..., pk)` to a variable of type
 `FooMatcherPk<p1_type, ..., pk_type>`. This can be useful when composing
 matchers. Matchers that don't have a parameter or have only one parameter have
 special types: you can assign `Foo()` to a `FooMatcher`-typed variable, and
 assign `Foo(p)` to a `FooMatcherP<p_type>`-typed variable.
 
 While you can instantiate a matcher template with reference types, passing the
 parameters by pointer usually makes your code more readable. If, however, you
 still want to pass a parameter by reference, be aware that in the failure
 message generated by the matcher you will see the value of the referenced object
 but not its address.
 
 You can overload matchers with different numbers of parameters:
 
 ```cpp
 MATCHER_P(Blah, a, description_string_1) { ... }
 MATCHER_P2(Blah, a, b, description_string_2) { ... }
 ```
 
 While it's tempting to always use the `MATCHER*` macros when defining a new
 matcher, you should also consider implementing `MatcherInterface` or using
 `MakePolymorphicMatcher()` instead (see the recipes that follow), especially if
 you need to use the matcher a lot. While these approaches require more work,
 they give you more control on the types of the value being matched and the
 matcher parameters, which in general leads to better compiler error messages
 that pay off in the long run. They also allow overloading matchers based on
 parameter types (as opposed to just based on the number of parameters).
 
 ### Writing New Monomorphic Matchers
 
 A matcher of argument type `T` implements `::testing::MatcherInterface<T>` and
 does two things: it tests whether a value of type `T` matches the matcher, and
 can describe what kind of values it matches. The latter ability is used for
 generating readable error messages when expectations are violated.
 
 The interface looks like this:
 
 ```cpp
 class MatchResultListener {
  public:
   ...
   // Streams x to the underlying ostream; does nothing if the ostream
   // is NULL.
   template <typename T>
   MatchResultListener& operator<<(const T& x);
 
   // Returns the underlying ostream.
   ::std::ostream* stream();
 };
 
 template <typename T>
 class MatcherInterface {
  public:
   virtual ~MatcherInterface();
 
   // Returns true if and only if the matcher matches x; also explains the match
   // result to 'listener'.
   virtual bool MatchAndExplain(T x, MatchResultListener* listener) const = 0;
 
   // Describes this matcher to an ostream.
   virtual void DescribeTo(::std::ostream* os) const = 0;
 
   // Describes the negation of this matcher to an ostream.
   virtual void DescribeNegationTo(::std::ostream* os) const;
 };
 ```
 
 If you need a custom matcher but `Truly()` is not a good option (for example,
 you may not be happy with the way `Truly(predicate)` describes itself, or you
 may want your matcher to be polymorphic as `Eq(value)` is), you can define a
 matcher to do whatever you want in two steps: first implement the matcher
 interface, and then define a factory function to create a matcher instance. The
 second step is not strictly needed but it makes the syntax of using the matcher
 nicer.
 
 For example, you can define a matcher to test whether an `int` is divisible by 7
 and then use it like this:
 
 ```cpp
 using ::testing::MakeMatcher;
 using ::testing::Matcher;
 using ::testing::MatcherInterface;
 using ::testing::MatchResultListener;
 
 class DivisibleBy7Matcher : public MatcherInterface<int> {
  public:
   bool MatchAndExplain(int n,
                        MatchResultListener* /* listener */) const override {
     return (n % 7) == 0;
   }
 
   void DescribeTo(::std::ostream* os) const override {
     *os << "is divisible by 7";
   }
 
   void DescribeNegationTo(::std::ostream* os) const override {
     *os << "is not divisible by 7";
   }
 };
 
 Matcher<int> DivisibleBy7() {
   return MakeMatcher(new DivisibleBy7Matcher);
 }
 
 ...
   EXPECT_CALL(foo, Bar(DivisibleBy7()));
 ```
 
 You may improve the matcher message by streaming additional information to the
 `listener` argument in `MatchAndExplain()`:
 
 ```cpp
 class DivisibleBy7Matcher : public MatcherInterface<int> {
  public:
   bool MatchAndExplain(int n,
                        MatchResultListener* listener) const override {
     const int remainder = n % 7;
     if (remainder != 0) {
       *listener << "the remainder is " << remainder;
     }
     return remainder == 0;
   }
   ...
 };
 ```
 
 Then, `EXPECT_THAT(x, DivisibleBy7());` may generate a message like this:
 
 ```shell
 Value of: x
 Expected: is divisible by 7
   Actual: 23 (the remainder is 2)
 ```
 
 ### Writing New Polymorphic Matchers
 
 You've learned how to write your own matchers in the previous recipe. Just one
 problem: a matcher created using `MakeMatcher()` only works for one particular
 type of arguments. If you want a *polymorphic* matcher that works with arguments
 of several types (for instance, `Eq(x)` can be used to match a *`value`* as long
 as `value == x` compiles -- *`value`* and `x` don't have to share the same
 type), you can learn the trick from `testing/base/public/gmock-matchers.h` but
 it's a bit involved.
 
 Fortunately, most of the time you can define a polymorphic matcher easily with
 the help of `MakePolymorphicMatcher()`. Here's how you can define `NotNull()` as
 an example:
 
 ```cpp
 using ::testing::MakePolymorphicMatcher;
 using ::testing::MatchResultListener;
 using ::testing::PolymorphicMatcher;
 
 class NotNullMatcher {
  public:
   // To implement a polymorphic matcher, first define a COPYABLE class
   // that has three members MatchAndExplain(), DescribeTo(), and
   // DescribeNegationTo(), like the following.
 
   // In this example, we want to use NotNull() with any pointer, so
   // MatchAndExplain() accepts a pointer of any type as its first argument.
   // In general, you can define MatchAndExplain() as an ordinary method or
   // a method template, or even overload it.
   template <typename T>
   bool MatchAndExplain(T* p,
                        MatchResultListener* /* listener */) const {
     return p != NULL;
   }
 
   // Describes the property of a value matching this matcher.
   void DescribeTo(std::ostream* os) const { *os << "is not NULL"; }
 
   // Describes the property of a value NOT matching this matcher.
   void DescribeNegationTo(std::ostream* os) const { *os << "is NULL"; }
 };
 
 // To construct a polymorphic matcher, pass an instance of the class
 // to MakePolymorphicMatcher().  Note the return type.
 PolymorphicMatcher<NotNullMatcher> NotNull() {
   return MakePolymorphicMatcher(NotNullMatcher());
 }
 
 ...
 
   EXPECT_CALL(foo, Bar(NotNull()));  // The argument must be a non-NULL pointer.
 ```
 
 **Note:** Your polymorphic matcher class does **not** need to inherit from
 `MatcherInterface` or any other class, and its methods do **not** need to be
 virtual.
 
 Like in a monomorphic matcher, you may explain the match result by streaming
 additional information to the `listener` argument in `MatchAndExplain()`.
 
 ### Writing New Cardinalities
 
 A cardinality is used in `Times()` to tell gMock how many times you expect a
 call to occur. It doesn't have to be exact. For example, you can say
 `AtLeast(5)` or `Between(2, 4)`.
 
 If the [built-in set](cheat_sheet.md#CardinalityList) of cardinalities doesn't
 suit you, you are free to define your own by implementing the following
 interface (in namespace `testing`):
 
 ```cpp
 class CardinalityInterface {
  public:
   virtual ~CardinalityInterface();
 
   // Returns true if and only if call_count calls will satisfy this cardinality.
   virtual bool IsSatisfiedByCallCount(int call_count) const = 0;
 
   // Returns true if and only if call_count calls will saturate this
   // cardinality.
   virtual bool IsSaturatedByCallCount(int call_count) const = 0;
 
   // Describes self to an ostream.
   virtual void DescribeTo(std::ostream* os) const = 0;
 };
 ```
 
 For example, to specify that a call must occur even number of times, you can
 write
 
 ```cpp
 using ::testing::Cardinality;
 using ::testing::CardinalityInterface;
 using ::testing::MakeCardinality;
 
 class EvenNumberCardinality : public CardinalityInterface {
  public:
   bool IsSatisfiedByCallCount(int call_count) const override {
     return (call_count % 2) == 0;
   }
 
   bool IsSaturatedByCallCount(int call_count) const override {
     return false;
   }
 
   void DescribeTo(std::ostream* os) const {
     *os << "called even number of times";
   }
 };
 
 Cardinality EvenNumber() {
   return MakeCardinality(new EvenNumberCardinality);
 }
 
 ...
   EXPECT_CALL(foo, Bar(3))
       .Times(EvenNumber());
 ```
 
 ### Writing New Actions Quickly {#QuickNewActions}
 
 If the built-in actions don't work for you, you can easily define your own one.
 Just define a functor class with a (possibly templated) call operator, matching
 the signature of your action.
 
 ```cpp
 struct Increment {
   template <typename T>
   T operator()(T* arg) {
     return ++(*arg);
   }
 }
 ```
 
 The same approach works with stateful functors (or any callable, really):
 
 ```
 struct MultiplyBy {
   template <typename T>
   T operator()(T arg) { return arg * multiplier; }
 
   int multiplier;
 }
 
 // Then use:
 // EXPECT_CALL(...).WillOnce(MultiplyBy{7});
 ```
 
 #### Legacy macro-based Actions
 
 Before C++11, the functor-based actions were not supported; the old way of
 writing actions was through a set of `ACTION*` macros. We suggest to avoid them
 in new code; they hide a lot of logic behind the macro, potentially leading to
 harder-to-understand compiler errors. Nevertheless, we cover them here for
 completeness.
 
 By writing
 
 ```cpp
 ACTION(name) { statements; }
 ```
 
 in a namespace scope (i.e. not inside a class or function), you will define an
 action with the given name that executes the statements. The value returned by
 `statements` will be used as the return value of the action. Inside the
 statements, you can refer to the K-th (0-based) argument of the mock function as
 `argK`. For example:
 
 ```cpp
 ACTION(IncrementArg1) { return ++(*arg1); }
 ```
 
 allows you to write
 
 ```cpp
 ... WillOnce(IncrementArg1());
 ```
 
 Note that you don't need to specify the types of the mock function arguments.
 Rest assured that your code is type-safe though: you'll get a compiler error if
 `*arg1` doesn't support the `++` operator, or if the type of `++(*arg1)` isn't
 compatible with the mock function's return type.
 
 Another example:
 
 ```cpp
 ACTION(Foo) {
   (*arg2)(5);
   Blah();
   *arg1 = 0;
   return arg0;
 }
 ```
 
 defines an action `Foo()` that invokes argument #2 (a function pointer) with 5,
 calls function `Blah()`, sets the value pointed to by argument #1 to 0, and
 returns argument #0.
 
 For more convenience and flexibility, you can also use the following pre-defined
 symbols in the body of `ACTION`:
 
 `argK_type`     | The type of the K-th (0-based) argument of the mock function
 :-------------- | :-----------------------------------------------------------
 `args`          | All arguments of the mock function as a tuple
 `args_type`     | The type of all arguments of the mock function as a tuple
 `return_type`   | The return type of the mock function
 `function_type` | The type of the mock function
 
 For example, when using an `ACTION` as a stub action for mock function:
 
 ```cpp
 int DoSomething(bool flag, int* ptr);
 ```
 
 we have:
 
 Pre-defined Symbol | Is Bound To
 ------------------ | ---------------------------------
 `arg0`             | the value of `flag`
 `arg0_type`        | the type `bool`
 `arg1`             | the value of `ptr`
 `arg1_type`        | the type `int*`
 `args`             | the tuple `(flag, ptr)`
 `args_type`        | the type `std::tuple<bool, int*>`
 `return_type`      | the type `int`
 `function_type`    | the type `int(bool, int*)`
 
 #### Legacy macro-based parameterized Actions
 
 Sometimes you'll want to parameterize an action you define. For that we have
 another macro
 
 ```cpp
 ACTION_P(name, param) { statements; }
 ```
 
 For example,
 
 ```cpp
 ACTION_P(Add, n) { return arg0 + n; }
 ```
 
 will allow you to write
 
 ```cpp
 // Returns argument #0 + 5.
 ... WillOnce(Add(5));
 ```
 
 For convenience, we use the term *arguments* for the values used to invoke the
 mock function, and the term *parameters* for the values used to instantiate an
 action.
 
 Note that you don't need to provide the type of the parameter either. Suppose
 the parameter is named `param`, you can also use the gMock-defined symbol
 `param_type` to refer to the type of the parameter as inferred by the compiler.
 For example, in the body of `ACTION_P(Add, n)` above, you can write `n_type` for
 the type of `n`.
 
 gMock also provides `ACTION_P2`, `ACTION_P3`, and etc to support multi-parameter
 actions. For example,
 
 ```cpp
 ACTION_P2(ReturnDistanceTo, x, y) {
   double dx = arg0 - x;
   double dy = arg1 - y;
   return sqrt(dx*dx + dy*dy);
 }
 ```
 
 lets you write
 
 ```cpp
 ... WillOnce(ReturnDistanceTo(5.0, 26.5));
 ```
 
 You can view `ACTION` as a degenerated parameterized action where the number of
 parameters is 0.
 
 You can also easily define actions overloaded on the number of parameters:
 
 ```cpp
 ACTION_P(Plus, a) { ... }
 ACTION_P2(Plus, a, b) { ... }
 ```
 
 ### Restricting the Type of an Argument or Parameter in an ACTION
 
 For maximum brevity and reusability, the `ACTION*` macros don't ask you to
 provide the types of the mock function arguments and the action parameters.
 Instead, we let the compiler infer the types for us.
 
 Sometimes, however, we may want to be more explicit about the types. There are
 several tricks to do that. For example:
 
 ```cpp
 ACTION(Foo) {
   // Makes sure arg0 can be converted to int.
   int n = arg0;
   ... use n instead of arg0 here ...
 }
 
 ACTION_P(Bar, param) {
   // Makes sure the type of arg1 is const char*.
   ::testing::StaticAssertTypeEq<const char*, arg1_type>();
 
   // Makes sure param can be converted to bool.
   bool flag = param;
 }
 ```
 
 where `StaticAssertTypeEq` is a compile-time assertion in googletest that
 verifies two types are the same.
 
 ### Writing New Action Templates Quickly
 
 Sometimes you want to give an action explicit template parameters that cannot be
 inferred from its value parameters. `ACTION_TEMPLATE()` supports that and can be
 viewed as an extension to `ACTION()` and `ACTION_P*()`.
 
 The syntax:
 
 ```cpp
 ACTION_TEMPLATE(ActionName,
                 HAS_m_TEMPLATE_PARAMS(kind1, name1, ..., kind_m, name_m),
                 AND_n_VALUE_PARAMS(p1, ..., p_n)) { statements; }
 ```
 
 defines an action template that takes *m* explicit template parameters and *n*
 value parameters, where *m* is in [1, 10] and *n* is in [0, 10]. `name_i` is the
 name of the *i*-th template parameter, and `kind_i` specifies whether it's a
 `typename`, an integral constant, or a template. `p_i` is the name of the *i*-th
 value parameter.
 
 Example:
 
 ```cpp
 // DuplicateArg<k, T>(output) converts the k-th argument of the mock
 // function to type T and copies it to *output.
 ACTION_TEMPLATE(DuplicateArg,
                 // Note the comma between int and k:
                 HAS_2_TEMPLATE_PARAMS(int, k, typename, T),
                 AND_1_VALUE_PARAMS(output)) {
   *output = T(::std::get<k>(args));
 }
 ```
 
 To create an instance of an action template, write:
 
 ```cpp
 ActionName<t1, ..., t_m>(v1, ..., v_n)
 ```
 
 where the `t`s are the template arguments and the `v`s are the value arguments.
 The value argument types are inferred by the compiler. For example:
 
 ```cpp
 using ::testing::_;
 ...
   int n;
   EXPECT_CALL(mock, Foo).WillOnce(DuplicateArg<1, unsigned char>(&n));
 ```
 
 If you want to explicitly specify the value argument types, you can provide
 additional template arguments:
 
 ```cpp
 ActionName<t1, ..., t_m, u1, ..., u_k>(v1, ..., v_n)
 ```
 
 where `u_i` is the desired type of `v_i`.
 
 `ACTION_TEMPLATE` and `ACTION`/`ACTION_P*` can be overloaded on the number of
 value parameters, but not on the number of template parameters. Without the
 restriction, the meaning of the following is unclear:
 
 ```cpp
   OverloadedAction<int, bool>(x);
 ```
 
 Are we using a single-template-parameter action where `bool` refers to the type
 of `x`, or a two-template-parameter action where the compiler is asked to infer
 the type of `x`?
 
 ### Using the ACTION Object's Type
 
 If you are writing a function that returns an `ACTION` object, you'll need to
 know its type. The type depends on the macro used to define the action and the
 parameter types. The rule is relatively simple:
 
 | Given Definition              | Expression          | Has Type              |
 | ----------------------------- | ------------------- | --------------------- |
 | `ACTION(Foo)`                 | `Foo()`             | `FooAction`           |
 | `ACTION_TEMPLATE(Foo,`        | `Foo<t1, ...,       | `FooAction<t1, ...,   |
 : `HAS_m_TEMPLATE_PARAMS(...),` : t_m>()`             : t_m>`                 :
 : `AND_0_VALUE_PARAMS())`       :                     :                       :
 | `ACTION_P(Bar, param)`        | `Bar(int_value)`    | `BarActionP<int>`     |
 | `ACTION_TEMPLATE(Bar,`        | `Bar<t1, ..., t_m>` | `FooActionP<t1, ...,  |
 : `HAS_m_TEMPLATE_PARAMS(...),` : `(int_value)`       : t_m, int>`            :
 : `AND_1_VALUE_PARAMS(p1))`     :                     :                       :
 | `ACTION_P2(Baz, p1, p2)`      | `Baz(bool_value,`   | `BazActionP2<bool,    |
 :                               : `int_value)`        : int>`                 :
 | `ACTION_TEMPLATE(Baz,`        | `Baz<t1, ..., t_m>` | `FooActionP2<t1, ..., |
 : `HAS_m_TEMPLATE_PARAMS(...),` : `(bool_value,`      : t_m,` `bool, int>`    :
 : `AND_2_VALUE_PARAMS(p1, p2))` : `int_value)`        :                       :
 | ...                           | ...                 | ...                   |
 
 Note that we have to pick different suffixes (`Action`, `ActionP`, `ActionP2`,
 and etc) for actions with different numbers of value parameters, or the action
 definitions cannot be overloaded on the number of them.
 
 ### Writing New Monomorphic Actions {#NewMonoActions}
 
 While the `ACTION*` macros are very convenient, sometimes they are
 inappropriate. For example, despite the tricks shown in the previous recipes,
 they don't let you directly specify the types of the mock function arguments and
 the action parameters, which in general leads to unoptimized compiler error
 messages that can baffle unfamiliar users. They also don't allow overloading
 actions based on parameter types without jumping through some hoops.
 
 An alternative to the `ACTION*` macros is to implement
 `::testing::ActionInterface<F>`, where `F` is the type of the mock function in
 which the action will be used. For example:
 
 ```cpp
 template <typename F>
 class ActionInterface {
  public:
   virtual ~ActionInterface();
 
   // Performs the action.  Result is the return type of function type
   // F, and ArgumentTuple is the tuple of arguments of F.
   //
 
   // For example, if F is int(bool, const string&), then Result would
   // be int, and ArgumentTuple would be ::std::tuple<bool, const string&>.
   virtual Result Perform(const ArgumentTuple& args) = 0;
 };
 ```
 
 ```cpp
 using ::testing::_;
 using ::testing::Action;
 using ::testing::ActionInterface;
 using ::testing::MakeAction;
 
 typedef int IncrementMethod(int*);
 
 class IncrementArgumentAction : public ActionInterface<IncrementMethod> {
  public:
   int Perform(const ::std::tuple<int*>& args) override {
     int* p = ::std::get<0>(args);  // Grabs the first argument.
     return *p++;
   }
 };
 
 Action<IncrementMethod> IncrementArgument() {
   return MakeAction(new IncrementArgumentAction);
 }
 
 ...
   EXPECT_CALL(foo, Baz(_))
       .WillOnce(IncrementArgument());
 
   int n = 5;
   foo.Baz(&n);  // Should return 5 and change n to 6.
 ```
 
 ### Writing New Polymorphic Actions {#NewPolyActions}
 
 The previous recipe showed you how to define your own action. This is all good,
 except that you need to know the type of the function in which the action will
 be used. Sometimes that can be a problem. For example, if you want to use the
 action in functions with *different* types (e.g. like `Return()` and
 `SetArgPointee()`).
 
 If an action can be used in several types of mock functions, we say it's
 *polymorphic*. The `MakePolymorphicAction()` function template makes it easy to
 define such an action:
 
 ```cpp
 namespace testing {
 template <typename Impl>
 PolymorphicAction<Impl> MakePolymorphicAction(const Impl& impl);
 }  // namespace testing
 ```
 
 As an example, let's define an action that returns the second argument in the
 mock function's argument list. The first step is to define an implementation
 class:
 
 ```cpp
 class ReturnSecondArgumentAction {
  public:
   template <typename Result, typename ArgumentTuple>
   Result Perform(const ArgumentTuple& args) const {
     // To get the i-th (0-based) argument, use ::std::get(args).
     return ::std::get<1>(args);
   }
 };
 ```
 
 This implementation class does *not* need to inherit from any particular class.
 What matters is that it must have a `Perform()` method template. This method
 template takes the mock function's arguments as a tuple in a **single**
 argument, and returns the result of the action. It can be either `const` or not,
 but must be invokable with exactly one template argument, which is the result
 type. In other words, you must be able to call `Perform<R>(args)` where `R` is
 the mock function's return type and `args` is its arguments in a tuple.
 
 Next, we use `MakePolymorphicAction()` to turn an instance of the implementation
 class into the polymorphic action we need. It will be convenient to have a
 wrapper for this:
 
 ```cpp
 using ::testing::MakePolymorphicAction;
 using ::testing::PolymorphicAction;
 
 PolymorphicAction<ReturnSecondArgumentAction> ReturnSecondArgument() {
   return MakePolymorphicAction(ReturnSecondArgumentAction());
 }
 ```
 
 Now, you can use this polymorphic action the same way you use the built-in ones:
 
 ```cpp
 using ::testing::_;
 
 class MockFoo : public Foo {
  public:
   MOCK_METHOD(int, DoThis, (bool flag, int n), (override));
   MOCK_METHOD(string, DoThat, (int x, const char* str1, const char* str2),
               (override));
 };
 
   ...
   MockFoo foo;
   EXPECT_CALL(foo, DoThis).WillOnce(ReturnSecondArgument());
   EXPECT_CALL(foo, DoThat).WillOnce(ReturnSecondArgument());
   ...
   foo.DoThis(true, 5);  // Will return 5.
   foo.DoThat(1, "Hi", "Bye");  // Will return "Hi".
 ```
 
 ### Teaching gMock How to Print Your Values
 
 When an uninteresting or unexpected call occurs, gMock prints the argument
 values and the stack trace to help you debug. Assertion macros like
 `EXPECT_THAT` and `EXPECT_EQ` also print the values in question when the
 assertion fails. gMock and googletest do this using googletest's user-extensible
 value printer.
 
 This printer knows how to print built-in C++ types, native arrays, STL
 containers, and any type that supports the `<<` operator. For other types, it
 prints the raw bytes in the value and hopes that you the user can figure it out.
 [googletest's advanced guide](../../googletest/docs/advanced.md#teaching-googletest-how-to-print-your-values)
 explains how to extend the printer to do a better job at printing your
 particular type than to dump the bytes.
 
 ## Useful Mocks Created Using gMock
 
 <!--#include file="includes/g3_testing_LOGs.md"-->
 <!--#include file="includes/g3_mock_callbacks.md"-->
 
 ### Mock std::function {#MockFunction}
 
 `std::function` is a general function type introduced in C++11. It is a
 preferred way of passing callbacks to new interfaces. Functions are copiable,
 and are not usually passed around by pointer, which makes them tricky to mock.
 But fear not - `MockFunction` can help you with that.
 
 `MockFunction<R(T1, ..., Tn)>` has a mock method `Call()` with the signature:
 
 ```cpp
   R Call(T1, ..., Tn);
 ```
 
 It also has a `AsStdFunction()` method, which creates a `std::function` proxy
 forwarding to Call:
 
 ```cpp
   std::function<R(T1, ..., Tn)> AsStdFunction();
 ```
 
 To use `MockFunction`, first create `MockFunction` object and set up
 expectations on its `Call` method. Then pass proxy obtained from
 `AsStdFunction()` to the code you are testing. For example:
 
 ```cpp
 TEST(FooTest, RunsCallbackWithBarArgument) {
   // 1. Create a mock object.
   MockFunction<int(string)> mock_function;
 
   // 2. Set expectations on Call() method.
   EXPECT_CALL(mock_function, Call("bar")).WillOnce(Return(1));
 
   // 3. Exercise code that uses std::function.
   Foo(mock_function.AsStdFunction());
   // Foo's signature can be either of:
   // void Foo(const std::function<int(string)>& fun);
   // void Foo(std::function<int(string)> fun);
 
   // 4. All expectations will be verified when mock_function
   //     goes out of scope and is destroyed.
 }
 ```
 
 Remember that function objects created with `AsStdFunction()` are just
 forwarders. If you create multiple of them, they will share the same set of
 expectations.
 
 Although `std::function` supports unlimited number of arguments, `MockFunction`
 implementation is limited to ten. If you ever hit that limit... well, your
 callback has bigger problems than being mockable. :-)
 
 <!-- GOOGLETEST_CM0034 DO NOT DELETE -->