catch2/docs/tostring.md

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# 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.
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## 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:
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```
std::ostream& operator << ( std::ostream& os, T const& value ) {
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os << convertMyTypeToString( value );
return os;
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}
```
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(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.
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## Catch::StringMaker<T> 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>`:
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```
namespace Catch {
template<>
struct StringMaker<T> {
static std::string convert( T const& value ) {
return convertMyTypeToString( value );
}
};
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}
```
## 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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