catch2/docs/tostring.md

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 two 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 and have it declared before including Catch's header.

Catch::StringMaker specialisation

If you don't want to provide an operator << overload, or you want to convert your type differently for testing purposes, you can provide a specialization for Catch::StringMaker<T>:

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();
}

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