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71 lines
2.7 KiB
Markdown
71 lines
2.7 KiB
Markdown
# String conversions
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Catch needs to be able to convert types you use in assertions and logging expressions into strings (for logging and reporting purposes).
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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.
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## operator << overload for std::ostream
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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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```
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std::ostream& operator << ( std::ostream& os, T const& value ) {
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os << convertMyTypeToString( value );
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return os;
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}
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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).
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You should put this function in the same namespace as your type.
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Alternatively you may prefer to write it as a member function:
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```
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std::ostream& T::operator << ( std::ostream& os ) const {
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os << convertMyTypeToString( *this );
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return os;
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}
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```
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## Catch::toString overload
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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.
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```
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namespace Catch {
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std::string toString( T const& value ) {
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return convertMyTypeToString( value );
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}
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}
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```
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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.
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## Catch::StringMaker<T> specialisation
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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:
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```
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namespace Catch {
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template<> struct StringMaker<T> {
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static std::string convert( T const& value ) {
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return convertMyTypeToString( value );
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}
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};
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}
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```
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## Exceptions
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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:
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```
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CATCH_TRANSLATE_EXCEPTION( MyType& ex ) {
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return ex.message();
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}
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```
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---
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[Home](Readme.md)
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