yaml-cpp

advanced.md (96214B)

---

 # Advanced googletest Topics
 
 <!-- GOOGLETEST_CM0016 DO NOT DELETE -->
 
 ## Introduction
 
 Now that you have read the [googletest Primer](primer.md) and learned how to
 write tests using googletest, it's time to learn some new tricks. This document
 will show you more assertions as well as how to construct complex failure
 messages, propagate fatal failures, reuse and speed up your test fixtures, and
 use various flags with your tests.
 
 ## More Assertions
 
 This section covers some less frequently used, but still significant,
 assertions.
 
 ### Explicit Success and Failure
 
 These three assertions do not actually test a value or expression. Instead, they
 generate a success or failure directly. Like the macros that actually perform a
 test, you may stream a custom failure message into them.
 
 ```c++
 SUCCEED();
 ```
 
 Generates a success. This does **NOT** make the overall test succeed. A test is
 considered successful only if none of its assertions fail during its execution.
 
 NOTE: `SUCCEED()` is purely documentary and currently doesn't generate any
 user-visible output. However, we may add `SUCCEED()` messages to googletest's
 output in the future.
 
 ```c++
 FAIL();
 ADD_FAILURE();
 ADD_FAILURE_AT("file_path", line_number);
 ```
 
 `FAIL()` generates a fatal failure, while `ADD_FAILURE()` and `ADD_FAILURE_AT()`
 generate a nonfatal failure. These are useful when control flow, rather than a
 Boolean expression, determines the test's success or failure. For example, you
 might want to write something like:
 
 ```c++
 switch(expression) {
   case 1:
      ... some checks ...
   case 2:
      ... some other checks ...
   default:
      FAIL() << "We shouldn't get here.";
 }
 ```
 
 NOTE: you can only use `FAIL()` in functions that return `void`. See the
 [Assertion Placement section](#assertion-placement) for more information.
 
 ### Exception Assertions
 
 These are for verifying that a piece of code throws (or does not throw) an
 exception of the given type:
 
 Fatal assertion                            | Nonfatal assertion                         | Verifies
 ------------------------------------------ | ------------------------------------------ | --------
 `ASSERT_THROW(statement, exception_type);` | `EXPECT_THROW(statement, exception_type);` | `statement` throws an exception of the given type
 `ASSERT_ANY_THROW(statement);`             | `EXPECT_ANY_THROW(statement);`             | `statement` throws an exception of any type
 `ASSERT_NO_THROW(statement);`              | `EXPECT_NO_THROW(statement);`              | `statement` doesn't throw any exception
 
 Examples:
 
 ```c++
 ASSERT_THROW(Foo(5), bar_exception);
 
 EXPECT_NO_THROW({
   int n = 5;
   Bar(&n);
 });
 ```
 
 **Availability**: requires exceptions to be enabled in the build environment
 
 ### Predicate Assertions for Better Error Messages
 
 Even though googletest has a rich set of assertions, they can never be complete,
 as it's impossible (nor a good idea) to anticipate all scenarios a user might
 run into. Therefore, sometimes a user has to use `EXPECT_TRUE()` to check a
 complex expression, for lack of a better macro. This has the problem of not
 showing you the values of the parts of the expression, making it hard to
 understand what went wrong. As a workaround, some users choose to construct the
 failure message by themselves, streaming it into `EXPECT_TRUE()`. However, this
 is awkward especially when the expression has side-effects or is expensive to
 evaluate.
 
 googletest gives you three different options to solve this problem:
 
 #### Using an Existing Boolean Function
 
 If you already have a function or functor that returns `bool` (or a type that
 can be implicitly converted to `bool`), you can use it in a *predicate
 assertion* to get the function arguments printed for free:
 
 <!-- mdformat off(github rendering does not support multiline tables) -->
 
 | Fatal assertion                   | Nonfatal assertion                | Verifies                    |
 | --------------------------------- | --------------------------------- | --------------------------- |
 | `ASSERT_PRED1(pred1, val1)`       | `EXPECT_PRED1(pred1, val1)`       | `pred1(val1)` is true       |
 | `ASSERT_PRED2(pred2, val1, val2)` | `EXPECT_PRED2(pred2, val1, val2)` | `pred1(val1, val2)` is true |
 | `...`                             | `...`                             | `...`                       |
 
 <!-- mdformat on-->
 In the above, `predn` is an `n`-ary predicate function or functor, where `val1`,
 `val2`, ..., and `valn` are its arguments. The assertion succeeds if the
 predicate returns `true` when applied to the given arguments, and fails
 otherwise. When the assertion fails, it prints the value of each argument. In
 either case, the arguments are evaluated exactly once.
 
 Here's an example. Given
 
 ```c++
 // Returns true if m and n have no common divisors except 1.
 bool MutuallyPrime(int m, int n) { ... }
 
 const int a = 3;
 const int b = 4;
 const int c = 10;
 ```
 
 the assertion
 
 ```c++
   EXPECT_PRED2(MutuallyPrime, a, b);
 ```
 
 will succeed, while the assertion
 
 ```c++
   EXPECT_PRED2(MutuallyPrime, b, c);
 ```
 
 will fail with the message
 
 ```none
 MutuallyPrime(b, c) is false, where
 b is 4
 c is 10
 ```
 
 > NOTE:
 >
 > 1.  If you see a compiler error "no matching function to call" when using
 >     `ASSERT_PRED*` or `EXPECT_PRED*`, please see
 >     [this](faq.md#the-compiler-complains-no-matching-function-to-call-when-i-use-assert-pred-how-do-i-fix-it)
 >     for how to resolve it.
 
 #### Using a Function That Returns an AssertionResult
 
 While `EXPECT_PRED*()` and friends are handy for a quick job, the syntax is not
 satisfactory: you have to use different macros for different arities, and it
 feels more like Lisp than C++. The `::testing::AssertionResult` class solves
 this problem.
 
 An `AssertionResult` object represents the result of an assertion (whether it's
 a success or a failure, and an associated message). You can create an
 `AssertionResult` using one of these factory functions:
 
 ```c++
 namespace testing {
 
 // Returns an AssertionResult object to indicate that an assertion has
 // succeeded.
 AssertionResult AssertionSuccess();
 
 // Returns an AssertionResult object to indicate that an assertion has
 // failed.
 AssertionResult AssertionFailure();
 
 }
 ```
 
 You can then use the `<<` operator to stream messages to the `AssertionResult`
 object.
 
 To provide more readable messages in Boolean assertions (e.g. `EXPECT_TRUE()`),
 write a predicate function that returns `AssertionResult` instead of `bool`. For
 example, if you define `IsEven()` as:
 
 ```c++
 ::testing::AssertionResult IsEven(int n) {
   if ((n % 2) == 0)
      return ::testing::AssertionSuccess();
   else
      return ::testing::AssertionFailure() << n << " is odd";
 }
 ```
 
 instead of:
 
 ```c++
 bool IsEven(int n) {
   return (n % 2) == 0;
 }
 ```
 
 the failed assertion `EXPECT_TRUE(IsEven(Fib(4)))` will print:
 
 ```none
 Value of: IsEven(Fib(4))
   Actual: false (3 is odd)
 Expected: true
 ```
 
 instead of a more opaque
 
 ```none
 Value of: IsEven(Fib(4))
   Actual: false
 Expected: true
 ```
 
 If you want informative messages in `EXPECT_FALSE` and `ASSERT_FALSE` as well
 (one third of Boolean assertions in the Google code base are negative ones), and
 are fine with making the predicate slower in the success case, you can supply a
 success message:
 
 ```c++
 ::testing::AssertionResult IsEven(int n) {
   if ((n % 2) == 0)
      return ::testing::AssertionSuccess() << n << " is even";
   else
      return ::testing::AssertionFailure() << n << " is odd";
 }
 ```
 
 Then the statement `EXPECT_FALSE(IsEven(Fib(6)))` will print
 
 ```none
   Value of: IsEven(Fib(6))
      Actual: true (8 is even)
   Expected: false
 ```
 
 #### Using a Predicate-Formatter
 
 If you find the default message generated by `(ASSERT|EXPECT)_PRED*` and
 `(ASSERT|EXPECT)_(TRUE|FALSE)` unsatisfactory, or some arguments to your
 predicate do not support streaming to `ostream`, you can instead use the
 following *predicate-formatter assertions* to *fully* customize how the message
 is formatted:
 
 Fatal assertion                                  | Nonfatal assertion                               | Verifies
 ------------------------------------------------ | ------------------------------------------------ | --------
 `ASSERT_PRED_FORMAT1(pred_format1, val1);`       | `EXPECT_PRED_FORMAT1(pred_format1, val1);`       | `pred_format1(val1)` is successful
 `ASSERT_PRED_FORMAT2(pred_format2, val1, val2);` | `EXPECT_PRED_FORMAT2(pred_format2, val1, val2);` | `pred_format2(val1, val2)` is successful
 `...`                                            | `...`                                            | ...
 
 The difference between this and the previous group of macros is that instead of
 a predicate, `(ASSERT|EXPECT)_PRED_FORMAT*` take a *predicate-formatter*
 (`pred_formatn`), which is a function or functor with the signature:
 
 ```c++
 ::testing::AssertionResult PredicateFormattern(const char* expr1,
                                                const char* expr2,
                                                ...
                                                const char* exprn,
                                                T1 val1,
                                                T2 val2,
                                                ...
                                                Tn valn);
 ```
 
 where `val1`, `val2`, ..., and `valn` are the values of the predicate arguments,
 and `expr1`, `expr2`, ..., and `exprn` are the corresponding expressions as they
 appear in the source code. The types `T1`, `T2`, ..., and `Tn` can be either
 value types or reference types. For example, if an argument has type `Foo`, you
 can declare it as either `Foo` or `const Foo&`, whichever is appropriate.
 
 As an example, let's improve the failure message in `MutuallyPrime()`, which was
 used with `EXPECT_PRED2()`:
 
 ```c++
 // Returns the smallest prime common divisor of m and n,
 // or 1 when m and n are mutually prime.
 int SmallestPrimeCommonDivisor(int m, int n) { ... }
 
 // A predicate-formatter for asserting that two integers are mutually prime.
 ::testing::AssertionResult AssertMutuallyPrime(const char* m_expr,
                                                const char* n_expr,
                                                int m,
                                                int n) {
   if (MutuallyPrime(m, n)) return ::testing::AssertionSuccess();
 
   return ::testing::AssertionFailure() << m_expr << " and " << n_expr
       << " (" << m << " and " << n << ") are not mutually prime, "
       << "as they have a common divisor " << SmallestPrimeCommonDivisor(m, n);
 }
 ```
 
 With this predicate-formatter, we can use
 
 ```c++
   EXPECT_PRED_FORMAT2(AssertMutuallyPrime, b, c);
 ```
 
 to generate the message
 
 ```none
 b and c (4 and 10) are not mutually prime, as they have a common divisor 2.
 ```
 
 As you may have realized, many of the built-in assertions we introduced earlier
 are special cases of `(EXPECT|ASSERT)_PRED_FORMAT*`. In fact, most of them are
 indeed defined using `(EXPECT|ASSERT)_PRED_FORMAT*`.
 
 ### Floating-Point Comparison
 
 Comparing floating-point numbers is tricky. Due to round-off errors, it is very
 unlikely that two floating-points will match exactly. Therefore, `ASSERT_EQ` 's
 naive comparison usually doesn't work. And since floating-points can have a wide
 value range, no single fixed error bound works. It's better to compare by a
 fixed relative error bound, except for values close to 0 due to the loss of
 precision there.
 
 In general, for floating-point comparison to make sense, the user needs to
 carefully choose the error bound. If they don't want or care to, comparing in
 terms of Units in the Last Place (ULPs) is a good default, and googletest
 provides assertions to do this. Full details about ULPs are quite long; if you
 want to learn more, see
 [here](https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/).
 
 #### Floating-Point Macros
 
 <!-- mdformat off(github rendering does not support multiline tables) -->
 
 | Fatal assertion                 | Nonfatal assertion              | Verifies                                 |
 | ------------------------------- | ------------------------------- | ---------------------------------------- |
 | `ASSERT_FLOAT_EQ(val1, val2);`  | `EXPECT_FLOAT_EQ(val1, val2);`  | the two `float` values are almost equal  |
 | `ASSERT_DOUBLE_EQ(val1, val2);` | `EXPECT_DOUBLE_EQ(val1, val2);` | the two `double` values are almost equal |
 
 <!-- mdformat on-->
 
 By "almost equal" we mean the values are within 4 ULP's from each other.
 
 The following assertions allow you to choose the acceptable error bound:
 
 <!-- mdformat off(github rendering does not support multiline tables) -->
 
 | Fatal assertion                       | Nonfatal assertion                    | Verifies                                                                         |
 | ------------------------------------- | ------------------------------------- | -------------------------------------------------------------------------------- |
 | `ASSERT_NEAR(val1, val2, abs_error);` | `EXPECT_NEAR(val1, val2, abs_error);` | the difference between `val1` and `val2` doesn't exceed the given absolute error |
 
 <!-- mdformat on-->
 
 #### Floating-Point Predicate-Format Functions
 
 Some floating-point operations are useful, but not that often used. In order to
 avoid an explosion of new macros, we provide them as predicate-format functions
 that can be used in predicate assertion macros (e.g. `EXPECT_PRED_FORMAT2`,
 etc).
 
 ```c++
 EXPECT_PRED_FORMAT2(::testing::FloatLE, val1, val2);
 EXPECT_PRED_FORMAT2(::testing::DoubleLE, val1, val2);
 ```
 
 Verifies that `val1` is less than, or almost equal to, `val2`. You can replace
 `EXPECT_PRED_FORMAT2` in the above table with `ASSERT_PRED_FORMAT2`.
 
 ### Asserting Using gMock Matchers
 
 [gMock](../../googlemock) comes with a library of matchers for validating
 arguments passed to mock objects. A gMock *matcher* is basically a predicate
 that knows how to describe itself. It can be used in these assertion macros:
 
 <!-- mdformat off(github rendering does not support multiline tables) -->
 
 | Fatal assertion                | Nonfatal assertion             | Verifies              |
 | ------------------------------ | ------------------------------ | --------------------- |
 | `ASSERT_THAT(value, matcher);` | `EXPECT_THAT(value, matcher);` | value matches matcher |
 
 <!-- mdformat on-->
 
 For example, `StartsWith(prefix)` is a matcher that matches a string starting
 with `prefix`, and you can write:
 
 ```c++
 using ::testing::StartsWith;
 ...
     // Verifies that Foo() returns a string starting with "Hello".
     EXPECT_THAT(Foo(), StartsWith("Hello"));
 ```
 
 Read this
 [recipe](../../googlemock/docs/cook_book.md#using-matchers-in-googletest-assertions)
 in the gMock Cookbook for more details.
 
 gMock has a rich set of matchers. You can do many things googletest cannot do
 alone with them. For a list of matchers gMock provides, read
 [this](../../googlemock/docs/cook_book.md##using-matchers). It's easy to write
 your [own matchers](../../googlemock/docs/cook_book.md#NewMatchers) too.
 
 gMock is bundled with googletest, so you don't need to add any build dependency
 in order to take advantage of this. Just include `"testing/base/public/gmock.h"`
 and you're ready to go.
 
 ### More String Assertions
 
 (Please read the [previous](#asserting-using-gmock-matchers) section first if
 you haven't.)
 
 You can use the gMock
 [string matchers](../../googlemock/docs/cheat_sheet.md#string-matchers) with
 `EXPECT_THAT()` or `ASSERT_THAT()` to do more string comparison tricks
 (sub-string, prefix, suffix, regular expression, and etc). For example,
 
 ```c++
 using ::testing::HasSubstr;
 using ::testing::MatchesRegex;
 ...
   ASSERT_THAT(foo_string, HasSubstr("needle"));
   EXPECT_THAT(bar_string, MatchesRegex("\\w*\\d+"));
 ```
 
 If the string contains a well-formed HTML or XML document, you can check whether
 its DOM tree matches an
 [XPath expression](http://www.w3.org/TR/xpath/#contents):
 
 ```c++
 // Currently still in //template/prototemplate/testing:xpath_matcher
 #include "template/prototemplate/testing/xpath_matcher.h"
 using prototemplate::testing::MatchesXPath;
 EXPECT_THAT(html_string, MatchesXPath("//a[text()='click here']"));
 ```
 
 ### Windows HRESULT assertions
 
 These assertions test for `HRESULT` success or failure.
 
 Fatal assertion                        | Nonfatal assertion                     | Verifies
 -------------------------------------- | -------------------------------------- | --------
 `ASSERT_HRESULT_SUCCEEDED(expression)` | `EXPECT_HRESULT_SUCCEEDED(expression)` | `expression` is a success `HRESULT`
 `ASSERT_HRESULT_FAILED(expression)`    | `EXPECT_HRESULT_FAILED(expression)`    | `expression` is a failure `HRESULT`
 
 The generated output contains the human-readable error message associated with
 the `HRESULT` code returned by `expression`.
 
 You might use them like this:
 
 ```c++
 CComPtr<IShellDispatch2> shell;
 ASSERT_HRESULT_SUCCEEDED(shell.CoCreateInstance(L"Shell.Application"));
 CComVariant empty;
 ASSERT_HRESULT_SUCCEEDED(shell->ShellExecute(CComBSTR(url), empty, empty, empty, empty));
 ```
 
 ### Type Assertions
 
 You can call the function
 
 ```c++
 ::testing::StaticAssertTypeEq<T1, T2>();
 ```
 
 to assert that types `T1` and `T2` are the same. The function does nothing if
 the assertion is satisfied. If the types are different, the function call will
 fail to compile, the compiler error message will say that
 `type1 and type2 are not the same type` and most likely (depending on the compiler)
 show you the actual values of `T1` and `T2`. This is mainly useful inside
 template code.
 
 **Caveat**: When used inside a member function of a class template or a function
 template, `StaticAssertTypeEq<T1, T2>()` is effective only if the function is
 instantiated. For example, given:
 
 ```c++
 template <typename T> class Foo {
  public:
   void Bar() { ::testing::StaticAssertTypeEq<int, T>(); }
 };
 ```
 
 the code:
 
 ```c++
 void Test1() { Foo<bool> foo; }
 ```
 
 will not generate a compiler error, as `Foo<bool>::Bar()` is never actually
 instantiated. Instead, you need:
 
 ```c++
 void Test2() { Foo<bool> foo; foo.Bar(); }
 ```
 
 to cause a compiler error.
 
 ### Assertion Placement
 
 You can use assertions in any C++ function. In particular, it doesn't have to be
 a method of the test fixture class. The one constraint is that assertions that
 generate a fatal failure (`FAIL*` and `ASSERT_*`) can only be used in
 void-returning functions. This is a consequence of Google's not using
 exceptions. By placing it in a non-void function you'll get a confusing compile
 error like `"error: void value not ignored as it ought to be"` or `"cannot
 initialize return object of type 'bool' with an rvalue of type 'void'"` or
 `"error: no viable conversion from 'void' to 'string'"`.
 
 If you need to use fatal assertions in a function that returns non-void, one
 option is to make the function return the value in an out parameter instead. For
 example, you can rewrite `T2 Foo(T1 x)` to `void Foo(T1 x, T2* result)`. You
 need to make sure that `*result` contains some sensible value even when the
 function returns prematurely. As the function now returns `void`, you can use
 any assertion inside of it.
 
 If changing the function's type is not an option, you should just use assertions
 that generate non-fatal failures, such as `ADD_FAILURE*` and `EXPECT_*`.
 
 NOTE: Constructors and destructors are not considered void-returning functions,
 according to the C++ language specification, and so you may not use fatal
 assertions in them; you'll get a compilation error if you try. Instead, either
 call `abort` and crash the entire test executable, or put the fatal assertion in
 a `SetUp`/`TearDown` function; see
 [constructor/destructor vs. `SetUp`/`TearDown`](faq.md#CtorVsSetUp)
 
 WARNING: A fatal assertion in a helper function (private void-returning method)
 called from a constructor or destructor does not does not terminate the current
 test, as your intuition might suggest: it merely returns from the constructor or
 destructor early, possibly leaving your object in a partially-constructed or
 partially-destructed state! You almost certainly want to `abort` or use
 `SetUp`/`TearDown` instead.
 
 ## Teaching googletest How to Print Your Values
 
 When a test assertion such as `EXPECT_EQ` fails, googletest prints the argument
 values to help you debug. It does this using a 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.
 
 As mentioned earlier, the printer is *extensible*. That means you can teach it
 to do a better job at printing your particular type than to dump the bytes. To
 do that, define `<<` for your type:
 
 ```c++
 #include <ostream>
 
 namespace foo {
 
 class Bar {  // We want googletest to be able to print instances of this.
 ...
   // Create a free inline friend function.
   friend std::ostream& operator<<(std::ostream& os, const Bar& bar) {
     return os << bar.DebugString();  // whatever needed to print bar to os
   }
 };
 
 // If you can't declare the function in the class it's important that the
 // << operator is defined in the SAME namespace that defines Bar.  C++'s look-up
 // rules rely on that.
 std::ostream& operator<<(std::ostream& os, const Bar& bar) {
   return os << bar.DebugString();  // whatever needed to print bar to os
 }
 
 }  // namespace foo
 ```
 
 Sometimes, this might not be an option: your team may consider it bad style to
 have a `<<` operator for `Bar`, or `Bar` may already have a `<<` operator that
 doesn't do what you want (and you cannot change it). If so, you can instead
 define a `PrintTo()` function like this:
 
 ```c++
 #include <ostream>
 
 namespace foo {
 
 class Bar {
   ...
   friend void PrintTo(const Bar& bar, std::ostream* os) {
     *os << bar.DebugString();  // whatever needed to print bar to os
   }
 };
 
 // If you can't declare the function in the class it's important that PrintTo()
 // is defined in the SAME namespace that defines Bar.  C++'s look-up rules rely
 // on that.
 void PrintTo(const Bar& bar, std::ostream* os) {
   *os << bar.DebugString();  // whatever needed to print bar to os
 }
 
 }  // namespace foo
 ```
 
 If you have defined both `<<` and `PrintTo()`, the latter will be used when
 googletest is concerned. This allows you to customize how the value appears in
 googletest's output without affecting code that relies on the behavior of its
 `<<` operator.
 
 If you want to print a value `x` using googletest's value printer yourself, just
 call `::testing::PrintToString(x)`, which returns an `std::string`:
 
 ```c++
 vector<pair<Bar, int> > bar_ints = GetBarIntVector();
 
 EXPECT_TRUE(IsCorrectBarIntVector(bar_ints))
     << "bar_ints = " << ::testing::PrintToString(bar_ints);
 ```
 
 ## Death Tests
 
 In many applications, there are assertions that can cause application failure if
 a condition is not met. These sanity checks, which ensure that the program is in
 a known good state, are there to fail at the earliest possible time after some
 program state is corrupted. If the assertion checks the wrong condition, then
 the program may proceed in an erroneous state, which could lead to memory
 corruption, security holes, or worse. Hence it is vitally important to test that
 such assertion statements work as expected.
 
 Since these precondition checks cause the processes to die, we call such tests
 _death tests_. More generally, any test that checks that a program terminates
 (except by throwing an exception) in an expected fashion is also a death test.
 
 Note that if a piece of code throws an exception, we don't consider it "death"
 for the purpose of death tests, as the caller of the code could catch the
 exception and avoid the crash. If you want to verify exceptions thrown by your
 code, see [Exception Assertions](#ExceptionAssertions).
 
 If you want to test `EXPECT_*()/ASSERT_*()` failures in your test code, see
 Catching Failures
 
 ### How to Write a Death Test
 
 googletest has the following macros to support death tests:
 
 Fatal assertion                                  | Nonfatal assertion                               | Verifies
 ------------------------------------------------ | ------------------------------------------------ | --------
 `ASSERT_DEATH(statement, matcher);`              | `EXPECT_DEATH(statement, matcher);`              | `statement` crashes with the given error
 `ASSERT_DEATH_IF_SUPPORTED(statement, matcher);` | `EXPECT_DEATH_IF_SUPPORTED(statement, matcher);` | if death tests are supported, verifies that `statement` crashes with the given error; otherwise verifies nothing
 `ASSERT_EXIT(statement, predicate, matcher);`    | `EXPECT_EXIT(statement, predicate, matcher);`    | `statement` exits with the given error, and its exit code matches `predicate`
 
 where `statement` is a statement that is expected to cause the process to die,
 `predicate` is a function or function object that evaluates an integer exit
 status, and `matcher` is either a GMock matcher matching a `const std::string&`
 or a (Perl) regular expression - either of which is matched against the stderr
 output of `statement`. For legacy reasons, a bare string (i.e. with no matcher)
 is interpreted as `ContainsRegex(str)`, **not** `Eq(str)`. Note that `statement`
 can be *any valid statement* (including *compound statement*) and doesn't have
 to be an expression.
 
 As usual, the `ASSERT` variants abort the current test function, while the
 `EXPECT` variants do not.
 
 > NOTE: We use the word "crash" here to mean that the process terminates with a
 > *non-zero* exit status code. There are two possibilities: either the process
 > has called `exit()` or `_exit()` with a non-zero value, or it may be killed by
 > a signal.
 >
 > This means that if `*statement*` terminates the process with a 0 exit code, it
 > is *not* considered a crash by `EXPECT_DEATH`. Use `EXPECT_EXIT` instead if
 > this is the case, or if you want to restrict the exit code more precisely.
 
 A predicate here must accept an `int` and return a `bool`. The death test
 succeeds only if the predicate returns `true`. googletest defines a few
 predicates that handle the most common cases:
 
 ```c++
 ::testing::ExitedWithCode(exit_code)
 ```
 
 This expression is `true` if the program exited normally with the given exit
 code.
 
 ```c++
 ::testing::KilledBySignal(signal_number)  // Not available on Windows.
 ```
 
 This expression is `true` if the program was killed by the given signal.
 
 The `*_DEATH` macros are convenient wrappers for `*_EXIT` that use a predicate
 that verifies the process' exit code is non-zero.
 
 Note that a death test only cares about three things:
 
 1.  does `statement` abort or exit the process?
 2.  (in the case of `ASSERT_EXIT` and `EXPECT_EXIT`) does the exit status
     satisfy `predicate`? Or (in the case of `ASSERT_DEATH` and `EXPECT_DEATH`)
     is the exit status non-zero? And
 3.  does the stderr output match `regex`?
 
 In particular, if `statement` generates an `ASSERT_*` or `EXPECT_*` failure, it
 will **not** cause the death test to fail, as googletest assertions don't abort
 the process.
 
 To write a death test, simply use one of the above macros inside your test
 function. For example,
 
 ```c++
 TEST(MyDeathTest, Foo) {
   // This death test uses a compound statement.
   ASSERT_DEATH({
     int n = 5;
     Foo(&n);
   }, "Error on line .* of Foo()");
 }
 
 TEST(MyDeathTest, NormalExit) {
   EXPECT_EXIT(NormalExit(), ::testing::ExitedWithCode(0), "Success");
 }
 
 TEST(MyDeathTest, KillMyself) {
   EXPECT_EXIT(KillMyself(), ::testing::KilledBySignal(SIGKILL),
               "Sending myself unblockable signal");
 }
 ```
 
 verifies that:
 
 *   calling `Foo(5)` causes the process to die with the given error message,
 *   calling `NormalExit()` causes the process to print `"Success"` to stderr and
     exit with exit code 0, and
 *   calling `KillMyself()` kills the process with signal `SIGKILL`.
 
 The test function body may contain other assertions and statements as well, if
 necessary.
 
 ### Death Test Naming
 
 IMPORTANT: We strongly recommend you to follow the convention of naming your
 **test suite** (not test) `*DeathTest` when it contains a death test, as
 demonstrated in the above example. The
 [Death Tests And Threads](#death-tests-and-threads) section below explains why.
 
 If a test fixture class is shared by normal tests and death tests, you can use
 `using` or `typedef` to introduce an alias for the fixture class and avoid
 duplicating its code:
 
 ```c++
 class FooTest : public ::testing::Test { ... };
 
 using FooDeathTest = FooTest;
 
 TEST_F(FooTest, DoesThis) {
   // normal test
 }
 
 TEST_F(FooDeathTest, DoesThat) {
   // death test
 }
 ```
 
 ### Regular Expression Syntax
 
 On POSIX systems (e.g. Linux, Cygwin, and Mac), googletest uses the
 [POSIX extended regular expression](http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap09.html#tag_09_04)
 syntax. To learn about this syntax, you may want to read this
 [Wikipedia entry](http://en.wikipedia.org/wiki/Regular_expression#POSIX_Extended_Regular_Expressions).
 
 On Windows, googletest uses its own simple regular expression implementation. It
 lacks many features. For example, we don't support union (`"x|y"`), grouping
 (`"(xy)"`), brackets (`"[xy]"`), and repetition count (`"x{5,7}"`), among
 others. Below is what we do support (`A` denotes a literal character, period
 (`.`), or a single `\\ ` escape sequence; `x` and `y` denote regular
 expressions.):
 
 Expression | Meaning
 ---------- | --------------------------------------------------------------
 `c`        | matches any literal character `c`
 `\\d`      | matches any decimal digit
 `\\D`      | matches any character that's not a decimal digit
 `\\f`      | matches `\f`
 `\\n`      | matches `\n`
 `\\r`      | matches `\r`
 `\\s`      | matches any ASCII whitespace, including `\n`
 `\\S`      | matches any character that's not a whitespace
 `\\t`      | matches `\t`
 `\\v`      | matches `\v`
 `\\w`      | matches any letter, `_`, or decimal digit
 `\\W`      | matches any character that `\\w` doesn't match
 `\\c`      | matches any literal character `c`, which must be a punctuation
 `.`        | matches any single character except `\n`
 `A?`       | matches 0 or 1 occurrences of `A`
 `A*`       | matches 0 or many occurrences of `A`
 `A+`       | matches 1 or many occurrences of `A`
 `^`        | matches the beginning of a string (not that of each line)
 `$`        | matches the end of a string (not that of each line)
 `xy`       | matches `x` followed by `y`
 
 To help you determine which capability is available on your system, googletest
 defines macros to govern which regular expression it is using. The macros are:
 `GTEST_USES_SIMPLE_RE=1` or `GTEST_USES_POSIX_RE=1`. If you want your death
 tests to work in all cases, you can either `#if` on these macros or use the more
 limited syntax only.
 
 ### How It Works
 
 Under the hood, `ASSERT_EXIT()` spawns a new process and executes the death test
 statement in that process. The details of how precisely that happens depend on
 the platform and the variable ::testing::GTEST_FLAG(death_test_style) (which is
 initialized from the command-line flag `--gtest_death_test_style`).
 
 *   On POSIX systems, `fork()` (or `clone()` on Linux) is used to spawn the
     child, after which:
     *   If the variable's value is `"fast"`, the death test statement is
         immediately executed.
     *   If the variable's value is `"threadsafe"`, the child process re-executes
         the unit test binary just as it was originally invoked, but with some
         extra flags to cause just the single death test under consideration to
         be run.
 *   On Windows, the child is spawned using the `CreateProcess()` API, and
     re-executes the binary to cause just the single death test under
     consideration to be run - much like the `threadsafe` mode on POSIX.
 
 Other values for the variable are illegal and will cause the death test to fail.
 Currently, the flag's default value is **"fast"**
 
 1.  the child's exit status satisfies the predicate, and
 2.  the child's stderr matches the regular expression.
 
 If the death test statement runs to completion without dying, the child process
 will nonetheless terminate, and the assertion fails.
 
 ### Death Tests And Threads
 
 The reason for the two death test styles has to do with thread safety. Due to
 well-known problems with forking in the presence of threads, death tests should
 be run in a single-threaded context. Sometimes, however, it isn't feasible to
 arrange that kind of environment. For example, statically-initialized modules
 may start threads before main is ever reached. Once threads have been created,
 it may be difficult or impossible to clean them up.
 
 googletest has three features intended to raise awareness of threading issues.
 
 1.  A warning is emitted if multiple threads are running when a death test is
     encountered.
 2.  Test suites with a name ending in "DeathTest" are run before all other
     tests.
 3.  It uses `clone()` instead of `fork()` to spawn the child process on Linux
     (`clone()` is not available on Cygwin and Mac), as `fork()` is more likely
     to cause the child to hang when the parent process has multiple threads.
 
 It's perfectly fine to create threads inside a death test statement; they are
 executed in a separate process and cannot affect the parent.
 
 ### Death Test Styles
 
 The "threadsafe" death test style was introduced in order to help mitigate the
 risks of testing in a possibly multithreaded environment. It trades increased
 test execution time (potentially dramatically so) for improved thread safety.
 
 The automated testing framework does not set the style flag. You can choose a
 particular style of death tests by setting the flag programmatically:
 
 ```c++
 testing::FLAGS_gtest_death_test_style="threadsafe"
 ```
 
 You can do this in `main()` to set the style for all death tests in the binary,
 or in individual tests. Recall that flags are saved before running each test and
 restored afterwards, so you need not do that yourself. For example:
 
 ```c++
 int main(int argc, char** argv) {
   InitGoogle(argv[0], &argc, &argv, true);
   ::testing::FLAGS_gtest_death_test_style = "fast";
   return RUN_ALL_TESTS();
 }
 
 TEST(MyDeathTest, TestOne) {
   ::testing::FLAGS_gtest_death_test_style = "threadsafe";
   // This test is run in the "threadsafe" style:
   ASSERT_DEATH(ThisShouldDie(), "");
 }
 
 TEST(MyDeathTest, TestTwo) {
   // This test is run in the "fast" style:
   ASSERT_DEATH(ThisShouldDie(), "");
 }
 ```
 
 ### Caveats
 
 The `statement` argument of `ASSERT_EXIT()` can be any valid C++ statement. If
 it leaves the current function via a `return` statement or by throwing an
 exception, the death test is considered to have failed. Some googletest macros
 may return from the current function (e.g. `ASSERT_TRUE()`), so be sure to avoid
 them in `statement`.
 
 Since `statement` runs in the child process, any in-memory side effect (e.g.
 modifying a variable, releasing memory, etc) it causes will *not* be observable
 in the parent process. In particular, if you release memory in a death test,
 your program will fail the heap check as the parent process will never see the
 memory reclaimed. To solve this problem, you can
 
 1.  try not to free memory in a death test;
 2.  free the memory again in the parent process; or
 3.  do not use the heap checker in your program.
 
 Due to an implementation detail, you cannot place multiple death test assertions
 on the same line; otherwise, compilation will fail with an unobvious error
 message.
 
 Despite the improved thread safety afforded by the "threadsafe" style of death
 test, thread problems such as deadlock are still possible in the presence of
 handlers registered with `pthread_atfork(3)`.
 
 
 ## Using Assertions in Sub-routines
 
 ### Adding Traces to Assertions
 
 If a test sub-routine is called from several places, when an assertion inside it
 fails, it can be hard to tell which invocation of the sub-routine the failure is
 from. You can alleviate this problem using extra logging or custom failure
 messages, but that usually clutters up your tests. A better solution is to use
 the `SCOPED_TRACE` macro or the `ScopedTrace` utility:
 
 ```c++
 SCOPED_TRACE(message);
 ScopedTrace trace("file_path", line_number, message);
 ```
 
 where `message` can be anything streamable to `std::ostream`. `SCOPED_TRACE`
 macro will cause the current file name, line number, and the given message to be
 added in every failure message. `ScopedTrace` accepts explicit file name and
 line number in arguments, which is useful for writing test helpers. The effect
 will be undone when the control leaves the current lexical scope.
 
 For example,
 
 ```c++
 10: void Sub1(int n) {
 11:   EXPECT_EQ(Bar(n), 1);
 12:   EXPECT_EQ(Bar(n + 1), 2);
 13: }
 14:
 15: TEST(FooTest, Bar) {
 16:   {
 17:     SCOPED_TRACE("A");  // This trace point will be included in
 18:                         // every failure in this scope.
 19:     Sub1(1);
 20:   }
 21:   // Now it won't.
 22:   Sub1(9);
 23: }
 ```
 
 could result in messages like these:
 
 ```none
 path/to/foo_test.cc:11: Failure
 Value of: Bar(n)
 Expected: 1
   Actual: 2
    Trace:
 path/to/foo_test.cc:17: A
 
 path/to/foo_test.cc:12: Failure
 Value of: Bar(n + 1)
 Expected: 2
   Actual: 3
 ```
 
 Without the trace, it would've been difficult to know which invocation of
 `Sub1()` the two failures come from respectively. (You could add an extra
 message to each assertion in `Sub1()` to indicate the value of `n`, but that's
 tedious.)
 
 Some tips on using `SCOPED_TRACE`:
 
 1.  With a suitable message, it's often enough to use `SCOPED_TRACE` at the
     beginning of a sub-routine, instead of at each call site.
 2.  When calling sub-routines inside a loop, make the loop iterator part of the
     message in `SCOPED_TRACE` such that you can know which iteration the failure
     is from.
 3.  Sometimes the line number of the trace point is enough for identifying the
     particular invocation of a sub-routine. In this case, you don't have to
     choose a unique message for `SCOPED_TRACE`. You can simply use `""`.
 4.  You can use `SCOPED_TRACE` in an inner scope when there is one in the outer
     scope. In this case, all active trace points will be included in the failure
     messages, in reverse order they are encountered.
 5.  The trace dump is clickable in Emacs - hit `return` on a line number and
     you'll be taken to that line in the source file!
 
 ### Propagating Fatal Failures
 
 A common pitfall when using `ASSERT_*` and `FAIL*` is not understanding that
 when they fail they only abort the _current function_, not the entire test. For
 example, the following test will segfault:
 
 ```c++
 void Subroutine() {
   // Generates a fatal failure and aborts the current function.
   ASSERT_EQ(1, 2);
 
   // The following won't be executed.
   ...
 }
 
 TEST(FooTest, Bar) {
   Subroutine();  // The intended behavior is for the fatal failure
                  // in Subroutine() to abort the entire test.
 
   // The actual behavior: the function goes on after Subroutine() returns.
   int* p = NULL;
   *p = 3;  // Segfault!
 }
 ```
 
 To alleviate this, googletest provides three different solutions. You could use
 either exceptions, the `(ASSERT|EXPECT)_NO_FATAL_FAILURE` assertions or the
 `HasFatalFailure()` function. They are described in the following two
 subsections.
 
 #### Asserting on Subroutines with an exception
 
 The following code can turn ASSERT-failure into an exception:
 
 ```c++
 class ThrowListener : public testing::EmptyTestEventListener {
   void OnTestPartResult(const testing::TestPartResult& result) override {
     if (result.type() == testing::TestPartResult::kFatalFailure) {
       throw testing::AssertionException(result);
     }
   }
 };
 int main(int argc, char** argv) {
   ...
   testing::UnitTest::GetInstance()->listeners().Append(new ThrowListener);
   return RUN_ALL_TESTS();
 }
 ```
 
 This listener should be added after other listeners if you have any, otherwise
 they won't see failed `OnTestPartResult`.
 
 #### Asserting on Subroutines
 
 As shown above, if your test calls a subroutine that has an `ASSERT_*` failure
 in it, the test will continue after the subroutine returns. This may not be what
 you want.
 
 Often people want fatal failures to propagate like exceptions. For that
 googletest offers the following macros:
 
 Fatal assertion                       | Nonfatal assertion                    | Verifies
 ------------------------------------- | ------------------------------------- | --------
 `ASSERT_NO_FATAL_FAILURE(statement);` | `EXPECT_NO_FATAL_FAILURE(statement);` | `statement` doesn't generate any new fatal failures in the current thread.
 
 Only failures in the thread that executes the assertion are checked to determine
 the result of this type of assertions. If `statement` creates new threads,
 failures in these threads are ignored.
 
 Examples:
 
 ```c++
 ASSERT_NO_FATAL_FAILURE(Foo());
 
 int i;
 EXPECT_NO_FATAL_FAILURE({
   i = Bar();
 });
 ```
 
 Assertions from multiple threads are currently not supported on Windows.
 
 #### Checking for Failures in the Current Test
 
 `HasFatalFailure()` in the `::testing::Test` class returns `true` if an
 assertion in the current test has suffered a fatal failure. This allows
 functions to catch fatal failures in a sub-routine and return early.
 
 ```c++
 class Test {
  public:
   ...
   static bool HasFatalFailure();
 };
 ```
 
 The typical usage, which basically simulates the behavior of a thrown exception,
 is:
 
 ```c++
 TEST(FooTest, Bar) {
   Subroutine();
   // Aborts if Subroutine() had a fatal failure.
   if (HasFatalFailure()) return;
 
   // The following won't be executed.
   ...
 }
 ```
 
 If `HasFatalFailure()` is used outside of `TEST()` , `TEST_F()` , or a test
 fixture, you must add the `::testing::Test::` prefix, as in:
 
 ```c++
 if (::testing::Test::HasFatalFailure()) return;
 ```
 
 Similarly, `HasNonfatalFailure()` returns `true` if the current test has at
 least one non-fatal failure, and `HasFailure()` returns `true` if the current
 test has at least one failure of either kind.
 
 ## Logging Additional Information
 
 In your test code, you can call `RecordProperty("key", value)` to log additional
 information, where `value` can be either a string or an `int`. The *last* value
 recorded for a key will be emitted to the
 [XML output](#generating-an-xml-report) if you specify one. For example, the
 test
 
 ```c++
 TEST_F(WidgetUsageTest, MinAndMaxWidgets) {
   RecordProperty("MaximumWidgets", ComputeMaxUsage());
   RecordProperty("MinimumWidgets", ComputeMinUsage());
 }
 ```
 
 will output XML like this:
 
 ```xml
   ...
     <testcase name="MinAndMaxWidgets" status="run" time="0.006" classname="WidgetUsageTest" MaximumWidgets="12" MinimumWidgets="9" />
   ...
 ```
 
 > NOTE:
 >
 > *   `RecordProperty()` is a static member of the `Test` class. Therefore it
 >     needs to be prefixed with `::testing::Test::` if used outside of the
 >     `TEST` body and the test fixture class.
 > *   `*key*` must be a valid XML attribute name, and cannot conflict with the
 >     ones already used by googletest (`name`, `status`, `time`, `classname`,
 >     `type_param`, and `value_param`).
 > *   Calling `RecordProperty()` outside of the lifespan of a test is allowed.
 >     If it's called outside of a test but between a test suite's
 >     `SetUpTestSuite()` and `TearDownTestSuite()` methods, it will be
 >     attributed to the XML element for the test suite. If it's called outside
 >     of all test suites (e.g. in a test environment), it will be attributed to
 >     the top-level XML element.
 
 ## Sharing Resources Between Tests in the Same Test Suite
 
 googletest creates a new test fixture object for each test in order to make
 tests independent and easier to debug. However, sometimes tests use resources
 that are expensive to set up, making the one-copy-per-test model prohibitively
 expensive.
 
 If the tests don't change the resource, there's no harm in their sharing a
 single resource copy. So, in addition to per-test set-up/tear-down, googletest
 also supports per-test-suite set-up/tear-down. To use it:
 
 1.  In your test fixture class (say `FooTest` ), declare as `static` some member
     variables to hold the shared resources.
 2.  Outside your test fixture class (typically just below it), define those
     member variables, optionally giving them initial values.
 3.  In the same test fixture class, define a `static void SetUpTestSuite()`
     function (remember not to spell it as **`SetupTestSuite`** with a small
     `u`!) to set up the shared resources and a `static void TearDownTestSuite()`
     function to tear them down.
 
 That's it! googletest automatically calls `SetUpTestSuite()` before running the
 *first test* in the `FooTest` test suite (i.e. before creating the first
 `FooTest` object), and calls `TearDownTestSuite()` after running the *last test*
 in it (i.e. after deleting the last `FooTest` object). In between, the tests can
 use the shared resources.
 
 Remember that the test order is undefined, so your code can't depend on a test
 preceding or following another. Also, the tests must either not modify the state
 of any shared resource, or, if they do modify the state, they must restore the
 state to its original value before passing control to the next test.
 
 Here's an example of per-test-suite set-up and tear-down:
 
 ```c++
 class FooTest : public ::testing::Test {
  protected:
   // Per-test-suite set-up.
   // Called before the first test in this test suite.
   // Can be omitted if not needed.
   static void SetUpTestSuite() {
     shared_resource_ = new ...;
   }
 
   // Per-test-suite tear-down.
   // Called after the last test in this test suite.
   // Can be omitted if not needed.
   static void TearDownTestSuite() {
     delete shared_resource_;
     shared_resource_ = NULL;
   }
 
   // You can define per-test set-up logic as usual.
   virtual void SetUp() { ... }
 
   // You can define per-test tear-down logic as usual.
   virtual void TearDown() { ... }
 
   // Some expensive resource shared by all tests.
   static T* shared_resource_;
 };
 
 T* FooTest::shared_resource_ = NULL;
 
 TEST_F(FooTest, Test1) {
   ... you can refer to shared_resource_ here ...
 }
 
 TEST_F(FooTest, Test2) {
   ... you can refer to shared_resource_ here ...
 }
 ```
 
 NOTE: Though the above code declares `SetUpTestSuite()` protected, it may
 sometimes be necessary to declare it public, such as when using it with
 `TEST_P`.
 
 ## Global Set-Up and Tear-Down
 
 Just as you can do set-up and tear-down at the test level and the test suite
 level, you can also do it at the test program level. Here's how.
 
 First, you subclass the `::testing::Environment` class to define a test
 environment, which knows how to set-up and tear-down:
 
 ```c++
 class Environment : public ::testing::Environment {
  public:
   virtual ~Environment() {}
 
   // Override this to define how to set up the environment.
   void SetUp() override {}
 
   // Override this to define how to tear down the environment.
   void TearDown() override {}
 };
 ```
 
 Then, you register an instance of your environment class with googletest by
 calling the `::testing::AddGlobalTestEnvironment()` function:
 
 ```c++
 Environment* AddGlobalTestEnvironment(Environment* env);
 ```
 
 Now, when `RUN_ALL_TESTS()` is called, it first calls the `SetUp()` method of
 each environment object, then runs the tests if none of the environments
 reported fatal failures and `GTEST_SKIP()` was not called. `RUN_ALL_TESTS()`
 always calls `TearDown()` with each environment object, regardless of whether or
 not the tests were run.
 
 It's OK to register multiple environment objects. In this suite, their `SetUp()`
 will be called in the order they are registered, and their `TearDown()` will be
 called in the reverse order.
 
 Note that googletest takes ownership of the registered environment objects.
 Therefore **do not delete them** by yourself.
 
 You should call `AddGlobalTestEnvironment()` before `RUN_ALL_TESTS()` is called,
 probably in `main()`. If you use `gtest_main`, you need to call this before
 `main()` starts for it to take effect. One way to do this is to define a global
 variable like this:
 
 ```c++
 ::testing::Environment* const foo_env =
     ::testing::AddGlobalTestEnvironment(new FooEnvironment);
 ```
 
 However, we strongly recommend you to write your own `main()` and call
 `AddGlobalTestEnvironment()` there, as relying on initialization of global
 variables makes the code harder to read and may cause problems when you register
 multiple environments from different translation units and the environments have
 dependencies among them (remember that the compiler doesn't guarantee the order
 in which global variables from different translation units are initialized).
 
 ## Value-Parameterized Tests
 
 *Value-parameterized tests* allow you to test your code with different
 parameters without writing multiple copies of the same test. This is useful in a
 number of situations, for example:
 
 *   You have a piece of code whose behavior is affected by one or more
     command-line flags. You want to make sure your code performs correctly for
     various values of those flags.
 *   You want to test different implementations of an OO interface.
 *   You want to test your code over various inputs (a.k.a. data-driven testing).
     This feature is easy to abuse, so please exercise your good sense when doing
     it!
 
 ### How to Write Value-Parameterized Tests
 
 To write value-parameterized tests, first you should define a fixture class. It
 must be derived from both `testing::Test` and `testing::WithParamInterface<T>`
 (the latter is a pure interface), where `T` is the type of your parameter
 values. For convenience, you can just derive the fixture class from
 `testing::TestWithParam<T>`, which itself is derived from both `testing::Test`
 and `testing::WithParamInterface<T>`. `T` can be any copyable type. If it's a
 raw pointer, you are responsible for managing the lifespan of the pointed
 values.
 
 NOTE: If your test fixture defines `SetUpTestSuite()` or `TearDownTestSuite()`
 they must be declared **public** rather than **protected** in order to use
 `TEST_P`.
 
 ```c++
 class FooTest :
     public testing::TestWithParam<const char*> {
   // You can implement all the usual fixture class members here.
   // To access the test parameter, call GetParam() from class
   // TestWithParam<T>.
 };
 
 // Or, when you want to add parameters to a pre-existing fixture class:
 class BaseTest : public testing::Test {
   ...
 };
 class BarTest : public BaseTest,
                 public testing::WithParamInterface<const char*> {
   ...
 };
 ```
 
 Then, use the `TEST_P` macro to define as many test patterns using this fixture
 as you want. The `_P` suffix is for "parameterized" or "pattern", whichever you
 prefer to think.
 
 ```c++
 TEST_P(FooTest, DoesBlah) {
   // Inside a test, access the test parameter with the GetParam() method
   // of the TestWithParam<T> class:
   EXPECT_TRUE(foo.Blah(GetParam()));
   ...
 }
 
 TEST_P(FooTest, HasBlahBlah) {
   ...
 }
 ```
 
 Finally, you can use `INSTANTIATE_TEST_SUITE_P` to instantiate the test suite
 with any set of parameters you want. googletest defines a number of functions
 for generating test parameters. They return what we call (surprise!) *parameter
 generators*. Here is a summary of them, which are all in the `testing`
 namespace:
 
 <!-- mdformat off(github rendering does not support multiline tables) -->
 
 | Parameter Generator                                                                       | Behavior                                                                                                          |
 | ----------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------- |
 | `Range(begin, end [, step])`                                                              | Yields values `{begin, begin+step, begin+step+step, ...}`. The values do not include `end`. `step` defaults to 1. |
 | `Values(v1, v2, ..., vN)`                                                                 | Yields values `{v1, v2, ..., vN}`.                                                                                |
 | `ValuesIn(container)` and  `ValuesIn(begin,end)`                                          | Yields values from a C-style array, an  STL-style container, or an iterator range `[begin, end)`                  |
 | `Bool()`                                                                                  | Yields sequence `{false, true}`.                                                                                  |
 | `Combine(g1, g2, ..., gN)`                                                                | Yields all combinations (Cartesian product) as std\:\:tuples of the values generated by the `N` generators.       |
 
 <!-- mdformat on-->
 
 For more details, see the comments at the definitions of these functions.
 
 The following statement will instantiate tests from the `FooTest` test suite
 each with parameter values `"meeny"`, `"miny"`, and `"moe"`.
 
 ```c++
 INSTANTIATE_TEST_SUITE_P(InstantiationName,
                          FooTest,
                          testing::Values("meeny", "miny", "moe"));
 ```
 
 NOTE: The code above must be placed at global or namespace scope, not at
 function scope.
 
 NOTE: Don't forget this step! If you do your test will silently pass, but none
 of its suites will ever run!
 
 To distinguish different instances of the pattern (yes, you can instantiate it
 more than once), the first argument to `INSTANTIATE_TEST_SUITE_P` is a prefix
 that will be added to the actual test suite name. Remember to pick unique
 prefixes for different instantiations. The tests from the instantiation above
 will have these names:
 
 *   `InstantiationName/FooTest.DoesBlah/0` for `"meeny"`
 *   `InstantiationName/FooTest.DoesBlah/1` for `"miny"`
 *   `InstantiationName/FooTest.DoesBlah/2` for `"moe"`
 *   `InstantiationName/FooTest.HasBlahBlah/0` for `"meeny"`
 *   `InstantiationName/FooTest.HasBlahBlah/1` for `"miny"`
 *   `InstantiationName/FooTest.HasBlahBlah/2` for `"moe"`
 
 You can use these names in [`--gtest_filter`](#running-a-subset-of-the-tests).
 
 This statement will instantiate all tests from `FooTest` again, each with
 parameter values `"cat"` and `"dog"`:
 
 ```c++
 const char* pets[] = {"cat", "dog"};
 INSTANTIATE_TEST_SUITE_P(AnotherInstantiationName, FooTest,
                          testing::ValuesIn(pets));
 ```
 
 The tests from the instantiation above will have these names:
 
 *   `AnotherInstantiationName/FooTest.DoesBlah/0` for `"cat"`
 *   `AnotherInstantiationName/FooTest.DoesBlah/1` for `"dog"`
 *   `AnotherInstantiationName/FooTest.HasBlahBlah/0` for `"cat"`
 *   `AnotherInstantiationName/FooTest.HasBlahBlah/1` for `"dog"`
 
 Please note that `INSTANTIATE_TEST_SUITE_P` will instantiate *all* tests in the
 given test suite, whether their definitions come before or *after* the
 `INSTANTIATE_TEST_SUITE_P` statement.
 
 You can see [sample7_unittest.cc] and [sample8_unittest.cc] for more examples.
 
 [sample7_unittest.cc]: ../samples/sample7_unittest.cc "Parameterized Test example"
 [sample8_unittest.cc]: ../samples/sample8_unittest.cc "Parameterized Test example with multiple parameters"
 
 ### Creating Value-Parameterized Abstract Tests
 
 In the above, we define and instantiate `FooTest` in the *same* source file.
 Sometimes you may want to define value-parameterized tests in a library and let
 other people instantiate them later. This pattern is known as *abstract tests*.
 As an example of its application, when you are designing an interface you can
 write a standard suite of abstract tests (perhaps using a factory function as
 the test parameter) that all implementations of the interface are expected to
 pass. When someone implements the interface, they can instantiate your suite to
 get all the interface-conformance tests for free.
 
 To define abstract tests, you should organize your code like this:
 
 1.  Put the definition of the parameterized test fixture class (e.g. `FooTest`)
     in a header file, say `foo_param_test.h`. Think of this as *declaring* your
     abstract tests.
 2.  Put the `TEST_P` definitions in `foo_param_test.cc`, which includes
     `foo_param_test.h`. Think of this as *implementing* your abstract tests.
 
 Once they are defined, you can instantiate them by including `foo_param_test.h`,
 invoking `INSTANTIATE_TEST_SUITE_P()`, and depending on the library target that
 contains `foo_param_test.cc`. You can instantiate the same abstract test suite
 multiple times, possibly in different source files.
 
 ### Specifying Names for Value-Parameterized Test Parameters
 
 The optional last argument to `INSTANTIATE_TEST_SUITE_P()` allows the user to
 specify a function or functor that generates custom test name suffixes based on
 the test parameters. The function should accept one argument of type
 `testing::TestParamInfo<class ParamType>`, and return `std::string`.
 
 `testing::PrintToStringParamName` is a builtin test suffix generator that
 returns the value of `testing::PrintToString(GetParam())`. It does not work for
 `std::string` or C strings.
 
 NOTE: test names must be non-empty, unique, and may only contain ASCII
 alphanumeric characters. In particular, they
 [should not contain underscores](faq.md#why-should-test-suite-names-and-test-names-not-contain-underscore)
 
 ```c++
 class MyTestSuite : public testing::TestWithParam<int> {};
 
 TEST_P(MyTestSuite, MyTest)
 {
   std::cout << "Example Test Param: " << GetParam() << std::endl;
 }
 
 INSTANTIATE_TEST_SUITE_P(MyGroup, MyTestSuite, testing::Range(0, 10),
                          testing::PrintToStringParamName());
 ```
 
 Providing a custom functor allows for more control over test parameter name
 generation, especially for types where the automatic conversion does not
 generate helpful parameter names (e.g. strings as demonstrated above). The
 following example illustrates this for multiple parameters, an enumeration type
 and a string, and also demonstrates how to combine generators. It uses a lambda
 for conciseness:
 
 ```c++
 enum class MyType { MY_FOO = 0, MY_BAR = 1 };
 
 class MyTestSuite : public testing::TestWithParam<std::tuple<MyType, string>> {
 };
 
 INSTANTIATE_TEST_SUITE_P(
     MyGroup, MyTestSuite,
     testing::Combine(
         testing::Values(MyType::VALUE_0, MyType::VALUE_1),
         testing::ValuesIn("", "")),
     [](const testing::TestParamInfo<MyTestSuite::ParamType>& info) {
       string name = absl::StrCat(
           std::get<0>(info.param) == MY_FOO ? "Foo" : "Bar", "_",
           std::get<1>(info.param));
       absl::c_replace_if(name, [](char c) { return !std::isalnum(c); }, '_');
       return name;
     });
 ```
 
 ## Typed Tests
 
 Suppose you have multiple implementations of the same interface and want to make
 sure that all of them satisfy some common requirements. Or, you may have defined
 several types that are supposed to conform to the same "concept" and you want to
 verify it. In both cases, you want the same test logic repeated for different
 types.
 
 While you can write one `TEST` or `TEST_F` for each type you want to test (and
 you may even factor the test logic into a function template that you invoke from
 the `TEST`), it's tedious and doesn't scale: if you want `m` tests over `n`
 types, you'll end up writing `m*n` `TEST`s.
 
 *Typed tests* allow you to repeat the same test logic over a list of types. You
 only need to write the test logic once, although you must know the type list
 when writing typed tests. Here's how you do it:
 
 First, define a fixture class template. It should be parameterized by a type.
 Remember to derive it from `::testing::Test`:
 
 ```c++
 template <typename T>
 class FooTest : public ::testing::Test {
  public:
   ...
   typedef std::list<T> List;
   static T shared_;
   T value_;
 };
 ```
 
 Next, associate a list of types with the test suite, which will be repeated for
 each type in the list:
 
 ```c++
 using MyTypes = ::testing::Types<char, int, unsigned int>;
 TYPED_TEST_SUITE(FooTest, MyTypes);
 ```
 
 The type alias (`using` or `typedef`) is necessary for the `TYPED_TEST_SUITE`
 macro to parse correctly. Otherwise the compiler will think that each comma in
 the type list introduces a new macro argument.
 
 Then, use `TYPED_TEST()` instead of `TEST_F()` to define a typed test for this
 test suite. You can repeat this as many times as you want:
 
 ```c++
 TYPED_TEST(FooTest, DoesBlah) {
   // Inside a test, refer to the special name TypeParam to get the type
   // parameter.  Since we are inside a derived class template, C++ requires
   // us to visit the members of FooTest via 'this'.
   TypeParam n = this->value_;
 
   // To visit static members of the fixture, add the 'TestFixture::'
   // prefix.
   n += TestFixture::shared_;
 
   // To refer to typedefs in the fixture, add the 'typename TestFixture::'
   // prefix.  The 'typename' is required to satisfy the compiler.
   typename TestFixture::List values;
 
   values.push_back(n);
   ...
 }
 
 TYPED_TEST(FooTest, HasPropertyA) { ... }
 ```
 
 You can see [sample6_unittest.cc] for a complete example.
 
 [sample6_unittest.cc]: ../samples/sample6_unittest.cc "Typed Test example"
 
 ## Type-Parameterized Tests
 
 *Type-parameterized tests* are like typed tests, except that they don't require
 you to know the list of types ahead of time. Instead, you can define the test
 logic first and instantiate it with different type lists later. You can even
 instantiate it more than once in the same program.
 
 If you are designing an interface or concept, you can define a suite of
 type-parameterized tests to verify properties that any valid implementation of
 the interface/concept should have. Then, the author of each implementation can
 just instantiate the test suite with their type to verify that it conforms to
 the requirements, without having to write similar tests repeatedly. Here's an
 example:
 
 First, define a fixture class template, as we did with typed tests:
 
 ```c++
 template <typename T>
 class FooTest : public ::testing::Test {
   ...
 };
 ```
 
 Next, declare that you will define a type-parameterized test suite:
 
 ```c++
 TYPED_TEST_SUITE_P(FooTest);
 ```
 
 Then, use `TYPED_TEST_P()` to define a type-parameterized test. You can repeat
 this as many times as you want:
 
 ```c++
 TYPED_TEST_P(FooTest, DoesBlah) {
   // Inside a test, refer to TypeParam to get the type parameter.
   TypeParam n = 0;
   ...
 }
 
 TYPED_TEST_P(FooTest, HasPropertyA) { ... }
 ```
 
 Now the tricky part: you need to register all test patterns using the
 `REGISTER_TYPED_TEST_SUITE_P` macro before you can instantiate them. The first
 argument of the macro is the test suite name; the rest are the names of the
 tests in this test suite:
 
 ```c++
 REGISTER_TYPED_TEST_SUITE_P(FooTest,
                             DoesBlah, HasPropertyA);
 ```
 
 Finally, you are free to instantiate the pattern with the types you want. If you
 put the above code in a header file, you can `#include` it in multiple C++
 source files and instantiate it multiple times.
 
 ```c++
 typedef ::testing::Types<char, int, unsigned int> MyTypes;
 INSTANTIATE_TYPED_TEST_SUITE_P(My, FooTest, MyTypes);
 ```
 
 To distinguish different instances of the pattern, the first argument to the
 `INSTANTIATE_TYPED_TEST_SUITE_P` macro is a prefix that will be added to the
 actual test suite name. Remember to pick unique prefixes for different
 instances.
 
 In the special case where the type list contains only one type, you can write
 that type directly without `::testing::Types<...>`, like this:
 
 ```c++
 INSTANTIATE_TYPED_TEST_SUITE_P(My, FooTest, int);
 ```
 
 You can see [sample6_unittest.cc] for a complete example.
 
 ## Testing Private Code
 
 If you change your software's internal implementation, your tests should not
 break as long as the change is not observable by users. Therefore, **per the
 black-box testing principle, most of the time you should test your code through
 its public interfaces.**
 
 **If you still find yourself needing to test internal implementation code,
 consider if there's a better design.** The desire to test internal
 implementation is often a sign that the class is doing too much. Consider
 extracting an implementation class, and testing it. Then use that implementation
 class in the original class.
 
 If you absolutely have to test non-public interface code though, you can. There
 are two cases to consider:
 
 *   Static functions ( *not* the same as static member functions!) or unnamed
     namespaces, and
 *   Private or protected class members
 
 To test them, we use the following special techniques:
 
 *   Both static functions and definitions/declarations in an unnamed namespace
     are only visible within the same translation unit. To test them, you can
     `#include` the entire `.cc` file being tested in your `*_test.cc` file.
     (#including `.cc` files is not a good way to reuse code - you should not do
     this in production code!)
 
     However, a better approach is to move the private code into the
     `foo::internal` namespace, where `foo` is the namespace your project
     normally uses, and put the private declarations in a `*-internal.h` file.
     Your production `.cc` files and your tests are allowed to include this
     internal header, but your clients are not. This way, you can fully test your
     internal implementation without leaking it to your clients.
 
 *   Private class members are only accessible from within the class or by
     friends. To access a class' private members, you can declare your test
     fixture as a friend to the class and define accessors in your fixture. Tests
     using the fixture can then access the private members of your production
     class via the accessors in the fixture. Note that even though your fixture
     is a friend to your production class, your tests are not automatically
     friends to it, as they are technically defined in sub-classes of the
     fixture.
 
     Another way to test private members is to refactor them into an
     implementation class, which is then declared in a `*-internal.h` file. Your
     clients aren't allowed to include this header but your tests can. Such is
     called the
     [Pimpl](https://www.gamedev.net/articles/programming/general-and-gameplay-programming/the-c-pimpl-r1794/)
     (Private Implementation) idiom.
 
     Or, you can declare an individual test as a friend of your class by adding
     this line in the class body:
 
     ```c++
         FRIEND_TEST(TestSuiteName, TestName);
     ```
 
     For example,
 
     ```c++
     // foo.h
     class Foo {
       ...
      private:
       FRIEND_TEST(FooTest, BarReturnsZeroOnNull);
 
       int Bar(void* x);
     };
 
     // foo_test.cc
     ...
     TEST(FooTest, BarReturnsZeroOnNull) {
       Foo foo;
       EXPECT_EQ(foo.Bar(NULL), 0);  // Uses Foo's private member Bar().
     }
     ```
 
     Pay special attention when your class is defined in a namespace, as you
     should define your test fixtures and tests in the same namespace if you want
     them to be friends of your class. For example, if the code to be tested
     looks like:
 
     ```c++
     namespace my_namespace {
 
     class Foo {
       friend class FooTest;
       FRIEND_TEST(FooTest, Bar);
       FRIEND_TEST(FooTest, Baz);
       ... definition of the class Foo ...
     };
 
     }  // namespace my_namespace
     ```
 
     Your test code should be something like:
 
     ```c++
     namespace my_namespace {
 
     class FooTest : public ::testing::Test {
      protected:
       ...
     };
 
     TEST_F(FooTest, Bar) { ... }
     TEST_F(FooTest, Baz) { ... }
 
     }  // namespace my_namespace
     ```
 
 ## "Catching" Failures
 
 If you are building a testing utility on top of googletest, you'll want to test
 your utility. What framework would you use to test it? googletest, of course.
 
 The challenge is to verify that your testing utility reports failures correctly.
 In frameworks that report a failure by throwing an exception, you could catch
 the exception and assert on it. But googletest doesn't use exceptions, so how do
 we test that a piece of code generates an expected failure?
 
 gunit-spi.h contains some constructs to do this. After #including this header,
 you can use
 
 ```c++
   EXPECT_FATAL_FAILURE(statement, substring);
 ```
 
 to assert that `statement` generates a fatal (e.g. `ASSERT_*`) failure in the
 current thread whose message contains the given `substring`, or use
 
 ```c++
   EXPECT_NONFATAL_FAILURE(statement, substring);
 ```
 
 if you are expecting a non-fatal (e.g. `EXPECT_*`) failure.
 
 Only failures in the current thread are checked to determine the result of this
 type of expectations. If `statement` creates new threads, failures in these
 threads are also ignored. If you want to catch failures in other threads as
 well, use one of the following macros instead:
 
 ```c++
   EXPECT_FATAL_FAILURE_ON_ALL_THREADS(statement, substring);
   EXPECT_NONFATAL_FAILURE_ON_ALL_THREADS(statement, substring);
 ```
 
 NOTE: Assertions from multiple threads are currently not supported on Windows.
 
 For technical reasons, there are some caveats:
 
 1.  You cannot stream a failure message to either macro.
 
 2.  `statement` in `EXPECT_FATAL_FAILURE{_ON_ALL_THREADS}()` cannot reference
     local non-static variables or non-static members of `this` object.
 
 3.  `statement` in `EXPECT_FATAL_FAILURE{_ON_ALL_THREADS}()` cannot return a
     value.
 
 ## Registering tests programmatically
 
 The `TEST` macros handle the vast majority of all use cases, but there are few
 were runtime registration logic is required. For those cases, the framework
 provides the `::testing::RegisterTest` that allows callers to register arbitrary
 tests dynamically.
 
 This is an advanced API only to be used when the `TEST` macros are insufficient.
 The macros should be preferred when possible, as they avoid most of the
 complexity of calling this function.
 
 It provides the following signature:
 
 ```c++
 template <typename Factory>
 TestInfo* RegisterTest(const char* test_suite_name, const char* test_name,
                        const char* type_param, const char* value_param,
                        const char* file, int line, Factory factory);
 ```
 
 The `factory` argument is a factory callable (move-constructible) object or
 function pointer that creates a new instance of the Test object. It handles
 ownership to the caller. The signature of the callable is `Fixture*()`, where
 `Fixture` is the test fixture class for the test. All tests registered with the
 same `test_suite_name` must return the same fixture type. This is checked at
 runtime.
 
 The framework will infer the fixture class from the factory and will call the
 `SetUpTestSuite` and `TearDownTestSuite` for it.
 
 Must be called before `RUN_ALL_TESTS()` is invoked, otherwise behavior is
 undefined.
 
 Use case example:
 
 ```c++
 class MyFixture : public ::testing::Test {
  public:
   // All of these optional, just like in regular macro usage.
   static void SetUpTestSuite() { ... }
   static void TearDownTestSuite() { ... }
   void SetUp() override { ... }
   void TearDown() override { ... }
 };
 
 class MyTest : public MyFixture {
  public:
   explicit MyTest(int data) : data_(data) {}
   void TestBody() override { ... }
 
  private:
   int data_;
 };
 
 void RegisterMyTests(const std::vector<int>& values) {
   for (int v : values) {
     ::testing::RegisterTest(
         "MyFixture", ("Test" + std::to_string(v)).c_str(), nullptr,
         std::to_string(v).c_str(),
         __FILE__, __LINE__,
         // Important to use the fixture type as the return type here.
         [=]() -> MyFixture* { return new MyTest(v); });
   }
 }
 ...
 int main(int argc, char** argv) {
   std::vector<int> values_to_test = LoadValuesFromConfig();
   RegisterMyTests(values_to_test);
   ...
   return RUN_ALL_TESTS();
 }
 ```
 ## Getting the Current Test's Name
 
 Sometimes a function may need to know the name of the currently running test.
 For example, you may be using the `SetUp()` method of your test fixture to set
 the golden file name based on which test is running. The `::testing::TestInfo`
 class has this information:
 
 ```c++
 namespace testing {
 
 class TestInfo {
  public:
   // Returns the test suite name and the test name, respectively.
   //
   // Do NOT delete or free the return value - it's managed by the
   // TestInfo class.
   const char* test_suite_name() const;
   const char* name() const;
 };
 
 }
 ```
 
 To obtain a `TestInfo` object for the currently running test, call
 `current_test_info()` on the `UnitTest` singleton object:
 
 ```c++
   // Gets information about the currently running test.
   // Do NOT delete the returned object - it's managed by the UnitTest class.
   const ::testing::TestInfo* const test_info =
     ::testing::UnitTest::GetInstance()->current_test_info();
 
 
 
   printf("We are in test %s of test suite %s.\n",
          test_info->name(),
          test_info->test_suite_name());
 ```
 
 `current_test_info()` returns a null pointer if no test is running. In
 particular, you cannot find the test suite name in `TestSuiteSetUp()`,
 `TestSuiteTearDown()` (where you know the test suite name implicitly), or
 functions called from them.
 
 ## Extending googletest by Handling Test Events
 
 googletest provides an **event listener API** to let you receive notifications
 about the progress of a test program and test failures. The events you can
 listen to include the start and end of the test program, a test suite, or a test
 method, among others. You may use this API to augment or replace the standard
 console output, replace the XML output, or provide a completely different form
 of output, such as a GUI or a database. You can also use test events as
 checkpoints to implement a resource leak checker, for example.
 
 ### Defining Event Listeners
 
 To define a event listener, you subclass either testing::TestEventListener or
 testing::EmptyTestEventListener The former is an (abstract) interface, where
 *each pure virtual method can be overridden to handle a test event* (For
 example, when a test starts, the `OnTestStart()` method will be called.). The
 latter provides an empty implementation of all methods in the interface, such
 that a subclass only needs to override the methods it cares about.
 
 When an event is fired, its context is passed to the handler function as an
 argument. The following argument types are used:
 
 *   UnitTest reflects the state of the entire test program,
 *   TestSuite has information about a test suite, which can contain one or more
     tests,
 *   TestInfo contains the state of a test, and
 *   TestPartResult represents the result of a test assertion.
 
 An event handler function can examine the argument it receives to find out
 interesting information about the event and the test program's state.
 
 Here's an example:
 
 ```c++
   class MinimalistPrinter : public ::testing::EmptyTestEventListener {
     // Called before a test starts.
     virtual void OnTestStart(const ::testing::TestInfo& test_info) {
       printf("*** Test %s.%s starting.\n",
              test_info.test_suite_name(), test_info.name());
     }
 
     // Called after a failed assertion or a SUCCESS().
     virtual void OnTestPartResult(const ::testing::TestPartResult& test_part_result) {
       printf("%s in %s:%d\n%s\n",
              test_part_result.failed() ? "*** Failure" : "Success",
              test_part_result.file_name(),
              test_part_result.line_number(),
              test_part_result.summary());
     }
 
     // Called after a test ends.
     virtual void OnTestEnd(const ::testing::TestInfo& test_info) {
       printf("*** Test %s.%s ending.\n",
              test_info.test_suite_name(), test_info.name());
     }
   };
 ```
 
 ### Using Event Listeners
 
 To use the event listener you have defined, add an instance of it to the
 googletest event listener list (represented by class TestEventListeners - note
 the "s" at the end of the name) in your `main()` function, before calling
 `RUN_ALL_TESTS()`:
 
 ```c++
 int main(int argc, char** argv) {
   ::testing::InitGoogleTest(&argc, argv);
   // Gets hold of the event listener list.
   ::testing::TestEventListeners& listeners =
         ::testing::UnitTest::GetInstance()->listeners();
   // Adds a listener to the end.  googletest takes the ownership.
   listeners.Append(new MinimalistPrinter);
   return RUN_ALL_TESTS();
 }
 ```
 
 There's only one problem: the default test result printer is still in effect, so
 its output will mingle with the output from your minimalist printer. To suppress
 the default printer, just release it from the event listener list and delete it.
 You can do so by adding one line:
 
 ```c++
   ...
   delete listeners.Release(listeners.default_result_printer());
   listeners.Append(new MinimalistPrinter);
   return RUN_ALL_TESTS();
 ```
 
 Now, sit back and enjoy a completely different output from your tests. For more
 details, see [sample9_unittest.cc].
 
 [sample9_unittest.cc]: ../samples/sample9_unittest.cc "Event listener example"
 
 You may append more than one listener to the list. When an `On*Start()` or
 `OnTestPartResult()` event is fired, the listeners will receive it in the order
 they appear in the list (since new listeners are added to the end of the list,
 the default text printer and the default XML generator will receive the event
 first). An `On*End()` event will be received by the listeners in the *reverse*
 order. This allows output by listeners added later to be framed by output from
 listeners added earlier.
 
 ### Generating Failures in Listeners
 
 You may use failure-raising macros (`EXPECT_*()`, `ASSERT_*()`, `FAIL()`, etc)
 when processing an event. There are some restrictions:
 
 1.  You cannot generate any failure in `OnTestPartResult()` (otherwise it will
     cause `OnTestPartResult()` to be called recursively).
 2.  A listener that handles `OnTestPartResult()` is not allowed to generate any
     failure.
 
 When you add listeners to the listener list, you should put listeners that
 handle `OnTestPartResult()` *before* listeners that can generate failures. This
 ensures that failures generated by the latter are attributed to the right test
 by the former.
 
 See [sample10_unittest.cc] for an example of a failure-raising listener.
 
 [sample10_unittest.cc]: ../samples/sample10_unittest.cc "Failure-raising listener example"
 
 ## Running Test Programs: Advanced Options
 
 googletest test programs are ordinary executables. Once built, you can run them
 directly and affect their behavior via the following environment variables
 and/or command line flags. For the flags to work, your programs must call
 `::testing::InitGoogleTest()` before calling `RUN_ALL_TESTS()`.
 
 To see a list of supported flags and their usage, please run your test program
 with the `--help` flag. You can also use `-h`, `-?`, or `/?` for short.
 
 If an option is specified both by an environment variable and by a flag, the
 latter takes precedence.
 
 ### Selecting Tests
 
 #### Listing Test Names
 
 Sometimes it is necessary to list the available tests in a program before
 running them so that a filter may be applied if needed. Including the flag
 `--gtest_list_tests` overrides all other flags and lists tests in the following
 format:
 
 ```none
 TestSuite1.
   TestName1
   TestName2
 TestSuite2.
   TestName
 ```
 
 None of the tests listed are actually run if the flag is provided. There is no
 corresponding environment variable for this flag.
 
 #### Running a Subset of the Tests
 
 By default, a googletest program runs all tests the user has defined. Sometimes,
 you want to run only a subset of the tests (e.g. for debugging or quickly
 verifying a change). If you set the `GTEST_FILTER` environment variable or the
 `--gtest_filter` flag to a filter string, googletest will only run the tests
 whose full names (in the form of `TestSuiteName.TestName`) match the filter.
 
 The format of a filter is a '`:`'-separated list of wildcard patterns (called
 the *positive patterns*) optionally followed by a '`-`' and another
 '`:`'-separated pattern list (called the *negative patterns*). A test matches
 the filter if and only if it matches any of the positive patterns but does not
 match any of the negative patterns.
 
 A pattern may contain `'*'` (matches any string) or `'?'` (matches any single
 character). For convenience, the filter `'*-NegativePatterns'` can be also
 written as `'-NegativePatterns'`.
 
 For example:
 
 *   `./foo_test` Has no flag, and thus runs all its tests.
 *   `./foo_test --gtest_filter=*` Also runs everything, due to the single
     match-everything `*` value.
 *   `./foo_test --gtest_filter=FooTest.*` Runs everything in test suite
     `FooTest` .
 *   `./foo_test --gtest_filter=*Null*:*Constructor*` Runs any test whose full
     name contains either `"Null"` or `"Constructor"` .
 *   `./foo_test --gtest_filter=-*DeathTest.*` Runs all non-death tests.
 *   `./foo_test --gtest_filter=FooTest.*-FooTest.Bar` Runs everything in test
     suite `FooTest` except `FooTest.Bar`.
 *   `./foo_test --gtest_filter=FooTest.*:BarTest.*-FooTest.Bar:BarTest.Foo` Runs
     everything in test suite `FooTest` except `FooTest.Bar` and everything in
     test suite `BarTest` except `BarTest.Foo`.
 
 #### Temporarily Disabling Tests
 
 If you have a broken test that you cannot fix right away, you can add the
 `DISABLED_` prefix to its name. This will exclude it from execution. This is
 better than commenting out the code or using `#if 0`, as disabled tests are
 still compiled (and thus won't rot).
 
 If you need to disable all tests in a test suite, you can either add `DISABLED_`
 to the front of the name of each test, or alternatively add it to the front of
 the test suite name.
 
 For example, the following tests won't be run by googletest, even though they
 will still be compiled:
 
 ```c++
 // Tests that Foo does Abc.
 TEST(FooTest, DISABLED_DoesAbc) { ... }
 
 class DISABLED_BarTest : public ::testing::Test { ... };
 
 // Tests that Bar does Xyz.
 TEST_F(DISABLED_BarTest, DoesXyz) { ... }
 ```
 
 NOTE: This feature should only be used for temporary pain-relief. You still have
 to fix the disabled tests at a later date. As a reminder, googletest will print
 a banner warning you if a test program contains any disabled tests.
 
 TIP: You can easily count the number of disabled tests you have using `gsearch`
 and/or `grep`. This number can be used as a metric for improving your test
 quality.
 
 #### Temporarily Enabling Disabled Tests
 
 To include disabled tests in test execution, just invoke the test program with
 the `--gtest_also_run_disabled_tests` flag or set the
 `GTEST_ALSO_RUN_DISABLED_TESTS` environment variable to a value other than `0`.
 You can combine this with the `--gtest_filter` flag to further select which
 disabled tests to run.
 
 ### Repeating the Tests
 
 Once in a while you'll run into a test whose result is hit-or-miss. Perhaps it
 will fail only 1% of the time, making it rather hard to reproduce the bug under
 a debugger. This can be a major source of frustration.
 
 The `--gtest_repeat` flag allows you to repeat all (or selected) test methods in
 a program many times. Hopefully, a flaky test will eventually fail and give you
 a chance to debug. Here's how to use it:
 
 ```none
 $ foo_test --gtest_repeat=1000
 Repeat foo_test 1000 times and don't stop at failures.
 
 $ foo_test --gtest_repeat=-1
 A negative count means repeating forever.
 
 $ foo_test --gtest_repeat=1000 --gtest_break_on_failure
 Repeat foo_test 1000 times, stopping at the first failure.  This
 is especially useful when running under a debugger: when the test
 fails, it will drop into the debugger and you can then inspect
 variables and stacks.
 
 $ foo_test --gtest_repeat=1000 --gtest_filter=FooBar.*
 Repeat the tests whose name matches the filter 1000 times.
 ```
 
 If your test program contains
 [global set-up/tear-down](#global-set-up-and-tear-down) code, it will be
 repeated in each iteration as well, as the flakiness may be in it. You can also
 specify the repeat count by setting the `GTEST_REPEAT` environment variable.
 
 ### Shuffling the Tests
 
 You can specify the `--gtest_shuffle` flag (or set the `GTEST_SHUFFLE`
 environment variable to `1`) to run the tests in a program in a random order.
 This helps to reveal bad dependencies between tests.
 
 By default, googletest uses a random seed calculated from the current time.
 Therefore you'll get a different order every time. The console output includes
 the random seed value, such that you can reproduce an order-related test failure
 later. To specify the random seed explicitly, use the `--gtest_random_seed=SEED`
 flag (or set the `GTEST_RANDOM_SEED` environment variable), where `SEED` is an
 integer in the range [0, 99999]. The seed value 0 is special: it tells
 googletest to do the default behavior of calculating the seed from the current
 time.
 
 If you combine this with `--gtest_repeat=N`, googletest will pick a different
 random seed and re-shuffle the tests in each iteration.
 
 ### Controlling Test Output
 
 #### Colored Terminal Output
 
 googletest can use colors in its terminal output to make it easier to spot the
 important information:
 
 <code>
 ...<br/>
   <font color="green">[----------]</font><font color="black"> 1 test from
   FooTest</font><br/>
   <font color="green">[ RUN &nbsp; &nbsp; &nbsp;]</font><font color="black">
   FooTest.DoesAbc</font><br/>
   <font color="green">[ &nbsp; &nbsp; &nbsp; OK ]</font><font color="black">
   FooTest.DoesAbc </font><br/>
   <font color="green">[----------]</font><font color="black">
   2 tests from BarTest</font><br/>
   <font color="green">[ RUN &nbsp; &nbsp; &nbsp;]</font><font color="black">
   BarTest.HasXyzProperty </font><br/>
   <font color="green">[ &nbsp; &nbsp; &nbsp; OK ]</font><font color="black">
   BarTest.HasXyzProperty</font><br/>
   <font color="green">[ RUN &nbsp; &nbsp; &nbsp;]</font><font color="black">
   BarTest.ReturnsTrueOnSuccess ... some error messages ...</font><br/>
   <font color="red">[ &nbsp; FAILED ]</font><font color="black">
   BarTest.ReturnsTrueOnSuccess ...</font><br/>
   <font color="green">[==========]</font><font color="black">
   30 tests from 14 test suites ran.</font><br/>
   <font color="green">[ &nbsp; PASSED ]</font><font color="black">
   28 tests.</font><br/>
   <font color="red">[ &nbsp; FAILED ]</font><font color="black">
   2 tests, listed below:</font><br/>
   <font color="red">[ &nbsp; FAILED ]</font><font color="black">
   BarTest.ReturnsTrueOnSuccess</font><br/>
   <font color="red">[ &nbsp; FAILED ]</font><font color="black">
   AnotherTest.DoesXyz<br/>
 <br/>
   2 FAILED TESTS
   </font>
 </code>
 
 You can set the `GTEST_COLOR` environment variable or the `--gtest_color`
 command line flag to `yes`, `no`, or `auto` (the default) to enable colors,
 disable colors, or let googletest decide. When the value is `auto`, googletest
 will use colors if and only if the output goes to a terminal and (on non-Windows
 platforms) the `TERM` environment variable is set to `xterm` or `xterm-color`.
 
 #### Suppressing the Elapsed Time
 
 By default, googletest prints the time it takes to run each test. To disable
 that, run the test program with the `--gtest_print_time=0` command line flag, or
 set the GTEST_PRINT_TIME environment variable to `0`.
 
 #### Suppressing UTF-8 Text Output
 
 In case of assertion failures, googletest prints expected and actual values of
 type `string` both as hex-encoded strings as well as in readable UTF-8 text if
 they contain valid non-ASCII UTF-8 characters. If you want to suppress the UTF-8
 text because, for example, you don't have an UTF-8 compatible output medium, run
 the test program with `--gtest_print_utf8=0` or set the `GTEST_PRINT_UTF8`
 environment variable to `0`.
 
 
 
 #### Generating an XML Report
 
 googletest can emit a detailed XML report to a file in addition to its normal
 textual output. The report contains the duration of each test, and thus can help
 you identify slow tests. The report is also used by the http://unittest
 dashboard to show per-test-method error messages.
 
 To generate the XML report, set the `GTEST_OUTPUT` environment variable or the
 `--gtest_output` flag to the string `"xml:path_to_output_file"`, which will
 create the file at the given location. You can also just use the string `"xml"`,
 in which case the output can be found in the `test_detail.xml` file in the
 current directory.
 
 If you specify a directory (for example, `"xml:output/directory/"` on Linux or
 `"xml:output\directory\"` on Windows), googletest will create the XML file in
 that directory, named after the test executable (e.g. `foo_test.xml` for test
 program `foo_test` or `foo_test.exe`). If the file already exists (perhaps left
 over from a previous run), googletest will pick a different name (e.g.
 `foo_test_1.xml`) to avoid overwriting it.
 
 The report is based on the `junitreport` Ant task. Since that format was
 originally intended for Java, a little interpretation is required to make it
 apply to googletest tests, as shown here:
 
 ```xml
 <testsuites name="AllTests" ...>
   <testsuite name="test_case_name" ...>
     <testcase    name="test_name" ...>
       <failure message="..."/>
       <failure message="..."/>
       <failure message="..."/>
     </testcase>
   </testsuite>
 </testsuites>
 ```
 
 *   The root `<testsuites>` element corresponds to the entire test program.
 *   `<testsuite>` elements correspond to googletest test suites.
 *   `<testcase>` elements correspond to googletest test functions.
 
 For instance, the following program
 
 ```c++
 TEST(MathTest, Addition) { ... }
 TEST(MathTest, Subtraction) { ... }
 TEST(LogicTest, NonContradiction) { ... }
 ```
 
 could generate this report:
 
 ```xml
 <?xml version="1.0" encoding="UTF-8"?>
 <testsuites tests="3" failures="1" errors="0" time="0.035" timestamp="2011-10-31T18:52:42" name="AllTests">
   <testsuite name="MathTest" tests="2" failures="1" errors="0" time="0.015">
     <testcase name="Addition" status="run" time="0.007" classname="">
       <failure message="Value of: add(1, 1)&#x0A;  Actual: 3&#x0A;Expected: 2" type="">...</failure>
       <failure message="Value of: add(1, -1)&#x0A;  Actual: 1&#x0A;Expected: 0" type="">...</failure>
     </testcase>
     <testcase name="Subtraction" status="run" time="0.005" classname="">
     </testcase>
   </testsuite>
   <testsuite name="LogicTest" tests="1" failures="0" errors="0" time="0.005">
     <testcase name="NonContradiction" status="run" time="0.005" classname="">
     </testcase>
   </testsuite>
 </testsuites>
 ```
 
 Things to note:
 
 *   The `tests` attribute of a `<testsuites>` or `<testsuite>` element tells how
     many test functions the googletest program or test suite contains, while the
     `failures` attribute tells how many of them failed.
 
 *   The `time` attribute expresses the duration of the test, test suite, or
     entire test program in seconds.
 
 *   The `timestamp` attribute records the local date and time of the test
     execution.
 
 *   Each `<failure>` element corresponds to a single failed googletest
     assertion.
 
 #### Generating a JSON Report
 
 googletest can also emit a JSON report as an alternative format to XML. To
 generate the JSON report, set the `GTEST_OUTPUT` environment variable or the
 `--gtest_output` flag to the string `"json:path_to_output_file"`, which will
 create the file at the given location. You can also just use the string
 `"json"`, in which case the output can be found in the `test_detail.json` file
 in the current directory.
 
 The report format conforms to the following JSON Schema:
 
 ```json
 {
   "$schema": "http://json-schema.org/schema#",
   "type": "object",
   "definitions": {
     "TestCase": {
       "type": "object",
       "properties": {
         "name": { "type": "string" },
         "tests": { "type": "integer" },
         "failures": { "type": "integer" },
         "disabled": { "type": "integer" },
         "time": { "type": "string" },
         "testsuite": {
           "type": "array",
           "items": {
             "$ref": "#/definitions/TestInfo"
           }
         }
       }
     },
     "TestInfo": {
       "type": "object",
       "properties": {
         "name": { "type": "string" },
         "status": {
           "type": "string",
           "enum": ["RUN", "NOTRUN"]
         },
         "time": { "type": "string" },
         "classname": { "type": "string" },
         "failures": {
           "type": "array",
           "items": {
             "$ref": "#/definitions/Failure"
           }
         }
       }
     },
     "Failure": {
       "type": "object",
       "properties": {
         "failures": { "type": "string" },
         "type": { "type": "string" }
       }
     }
   },
   "properties": {
     "tests": { "type": "integer" },
     "failures": { "type": "integer" },
     "disabled": { "type": "integer" },
     "errors": { "type": "integer" },
     "timestamp": {
       "type": "string",
       "format": "date-time"
     },
     "time": { "type": "string" },
     "name": { "type": "string" },
     "testsuites": {
       "type": "array",
       "items": {
         "$ref": "#/definitions/TestCase"
       }
     }
   }
 }
 ```
 
 The report uses the format that conforms to the following Proto3 using the
 [JSON encoding](https://developers.google.com/protocol-buffers/docs/proto3#json):
 
 ```proto
 syntax = "proto3";
 
 package googletest;
 
 import "google/protobuf/timestamp.proto";
 import "google/protobuf/duration.proto";
 
 message UnitTest {
   int32 tests = 1;
   int32 failures = 2;
   int32 disabled = 3;
   int32 errors = 4;
   google.protobuf.Timestamp timestamp = 5;
   google.protobuf.Duration time = 6;
   string name = 7;
   repeated TestCase testsuites = 8;
 }
 
 message TestCase {
   string name = 1;
   int32 tests = 2;
   int32 failures = 3;
   int32 disabled = 4;
   int32 errors = 5;
   google.protobuf.Duration time = 6;
   repeated TestInfo testsuite = 7;
 }
 
 message TestInfo {
   string name = 1;
   enum Status {
     RUN = 0;
     NOTRUN = 1;
   }
   Status status = 2;
   google.protobuf.Duration time = 3;
   string classname = 4;
   message Failure {
     string failures = 1;
     string type = 2;
   }
   repeated Failure failures = 5;
 }
 ```
 
 For instance, the following program
 
 ```c++
 TEST(MathTest, Addition) { ... }
 TEST(MathTest, Subtraction) { ... }
 TEST(LogicTest, NonContradiction) { ... }
 ```
 
 could generate this report:
 
 ```json
 {
   "tests": 3,
   "failures": 1,
   "errors": 0,
   "time": "0.035s",
   "timestamp": "2011-10-31T18:52:42Z",
   "name": "AllTests",
   "testsuites": [
     {
       "name": "MathTest",
       "tests": 2,
       "failures": 1,
       "errors": 0,
       "time": "0.015s",
       "testsuite": [
         {
           "name": "Addition",
           "status": "RUN",
           "time": "0.007s",
           "classname": "",
           "failures": [
             {
               "message": "Value of: add(1, 1)\n  Actual: 3\nExpected: 2",
               "type": ""
             },
             {
               "message": "Value of: add(1, -1)\n  Actual: 1\nExpected: 0",
               "type": ""
             }
           ]
         },
         {
           "name": "Subtraction",
           "status": "RUN",
           "time": "0.005s",
           "classname": ""
         }
       ]
     },
     {
       "name": "LogicTest",
       "tests": 1,
       "failures": 0,
       "errors": 0,
       "time": "0.005s",
       "testsuite": [
         {
           "name": "NonContradiction",
           "status": "RUN",
           "time": "0.005s",
           "classname": ""
         }
       ]
     }
   ]
 }
 ```
 
 IMPORTANT: The exact format of the JSON document is subject to change.
 
 ### Controlling How Failures Are Reported
 
 #### Turning Assertion Failures into Break-Points
 
 When running test programs under a debugger, it's very convenient if the
 debugger can catch an assertion failure and automatically drop into interactive
 mode. googletest's *break-on-failure* mode supports this behavior.
 
 To enable it, set the `GTEST_BREAK_ON_FAILURE` environment variable to a value
 other than `0`. Alternatively, you can use the `--gtest_break_on_failure`
 command line flag.
 
 #### Disabling Catching Test-Thrown Exceptions
 
 googletest can be used either with or without exceptions enabled. If a test
 throws a C++ exception or (on Windows) a structured exception (SEH), by default
 googletest catches it, reports it as a test failure, and continues with the next
 test method. This maximizes the coverage of a test run. Also, on Windows an
 uncaught exception will cause a pop-up window, so catching the exceptions allows
 you to run the tests automatically.
 
 When debugging the test failures, however, you may instead want the exceptions
 to be handled by the debugger, such that you can examine the call stack when an
 exception is thrown. To achieve that, set the `GTEST_CATCH_EXCEPTIONS`
 environment variable to `0`, or use the `--gtest_catch_exceptions=0` flag when
 running the tests.