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 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 {
	template<> 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();
}
```

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