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diff --git a/docs/tostring.md b/docs/tostring.md new file mode 100644 index 0000000..dbb6cb8 --- /dev/null +++ b/docs/tostring.md @@ -0,0 +1,70 @@ +# String conversions + +Catch needs to be able to convert types you use in assertions and logging expressions into strings (for logging and reporting purposes). +Most built-in or std types are supported out of the box but there are three ways that you can tell Catch how to convert your own types (or other, third-party types) into strings. + +## 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: + +``` +std::ostream& operator << ( std::ostream& os, T const& 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: + +``` +std::ostream& T::operator << ( std::ostream& os ) const { + os << convertMyTypeToString( *this ); + return os; +} +``` + +## Catch::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 ```Catch::toString()``` for your type. + +``` +namespace Catch { + std::string toString( T const& value ) { + return convertMyTypeToString( value ); + } +} +``` + +Again ```T``` is your type and ```convertMyTypeToString``` is where you'll write whatever code is necessary to make your type printable. Note that the function must be in the Catch namespace, which itself must be in the global namespace. + +## Catch::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: + +``` +namespace Catch { + template<> struct StringMaker<T> { + static std::string convert( T const& value ) { + return convertMyTypeToString( value ); + } + }; +} +``` + +## Exceptions + +By default all exceptions deriving from `std::exception` will be translated to strings by calling the `what()` method. For exception types that do not derive from `std::exception` - or if `what()` does not return a suitable string - use `CATCH_TRANSLATE_EXCEPTION`. This defines a function that takes your exception type, by reference, and returns a string. It can appear anywhere in the code - it doesn't have to be in the same translation unit. For example: + +``` +CATCH_TRANSLATE_EXCEPTION( MyType& ex ) { + return ex.message(); +} +``` + +--- + +[Home](Readme.md) |