doctest

stringification.md (5354B)

---

 ## String conversions
 
 **doctest** needs to be able to convert types you use in assertions and logging expressions into strings (for logging and reporting purposes).
 Most built-in types are supported out of the box but there are three ways that you can tell **doctest** how to convert your own types (or other, third-party types) into strings.
 
 For stringifying enums checkout [this issue](https://github.com/doctest/doctest/issues/121).
 
 ## ```operator<<``` overload for ```std::ostream```
 
 This is the standard way of providing string conversions in C++ - and the chances are you may already provide this for your own purposes. If you're not familiar with this idiom it involves writing a free function of the form:
 
 ```c++
 std::ostream& operator<< (std::ostream& os, const T& value) {
     os << convertMyTypeToString(value);
     return os;
 }
 ```
 
 (where ```T``` is your type and ```convertMyTypeToString``` is where you'll write whatever code is necessary to make your type printable - it doesn't have to be in another function).
 
 You should put this function in the same namespace as your type.
 
 Alternatively you may prefer to write it as a member function:
 
 ```c++
 std::ostream& T::operator<<(std::ostream& os) const {
     os << convertMyTypeToString(*this);
     return os;
 }
 ```
 
 ## ```doctest::toString``` overload
 
 If you don't want to provide an ```operator<<``` overload, or you want to convert your type differently for testing purposes, you can provide an overload for ```toString()``` for your type which returns ```doctest::String```.
 
 ```c++
 namespace user {
     struct udt {};
     
     doctest::String toString(const udt& value) {
         return convertMyTypeToString(value);
     }
 }
 ```
 
 Note that the function must be in the same namespace as your type. If the type is not in any namespace - then the overload should be in the global namespace as well. ```convertMyTypeToString``` is where you'll write whatever code is necessary to make your type printable.
 
 ## ```doctest::StringMaker<T>``` specialisation
 
 There are some cases where overloading ```toString``` does not work as expected. Specialising ```StringMaker<T>``` gives you more precise and reliable control - but at the cost of slightly more code and complexity:
 
 ```c++
 namespace doctest {
     template<> struct StringMaker<T> {
         static String convert(const T& value) {
             return convertMyTypeToString(value);
         }
     };
 }
 ```
 
 ## Translating exceptions
 
 By default all exceptions deriving from ```std::exception``` will be translated to strings by calling the ```what()``` method (also C strings). For exception types that do not derive from ```std::exception``` - or if ```what()``` does not return a suitable string - use ```REGISTER_EXCEPTION_TRANSLATOR```. This defines a function that takes your exception type and returns a ```doctest::String```. It can appear anywhere in the code - it doesn't have to be in the same translation unit. For example:
 
 ```c++
 REGISTER_EXCEPTION_TRANSLATOR(MyType& ex) {
     return doctest::String(ex.message());
 }
 ```
 
 Note that the exception may be accepted without a reference but it is considered bad practice in C++.
 
 An alternative way to register an exception translator is to do the following in some function - before executing any tests:
 
 ```c++
     // adding a lambda - the signature required is `doctest::String(exception_type)`
     doctest::registerExceptionTranslator<int>([](int in){ return doctest::toString(in); });
 ```
 
 The order of registering exception translators can be controlled - simply call the explicit function in the required order or list the exception translators with the macro in a top-to-bottom fashion in a single translation unit - everything that auto-registers in doctest works in a top-to-bottom way for a single translation unit (source file).
 
 You could also [override the translation mechanism](https://github.com/catchorg/Catch2/issues/539#issuecomment-454549904) for exceptions deriving from ```std::exception```.
 
 ------
 
 - Check out the [**example**](../../examples/all_features/stringification.cpp) which shows how to stringify ```std::vector<T>``` and other types/exceptions.
 - Note that the type ```String``` is used when specializing ```StringMaker<T>``` or overloading ```toString()``` - it is the string type **doctest** works with. ```std::string``` is not an option because doctest would have to include the ```<string>``` header.
 - To support the ```operator<<(std::ostream&...``` stringification the library has to offer a forward declaration of ```std::ostream``` and that is what the library does - but it is forbidden by the standard. It currently works everywhere - on all tested compilers - but if the user wishes to be 100% standards compliant - then the [**```DOCTEST_CONFIG_USE_STD_HEADERS```**](configuration.md#doctest_config_use_std_headers) identifier can be used to force the inclusion of ```<iosfwd>```. The reason the header is not included by default is that on MSVC (for example) it drags a whole bunch of stuff with it - and after the preprocessor is finished the translation unit has grown to 42k lines of C++ code - while Clang and the libc++ are so well implemented that including ```<iosfwd>``` there results in 400 lines of code. 
 
 ---
 
 [Home](readme.md#reference)
 
 <p align="center"><img src="../../scripts/data/logo/icon_2.svg"></p>