catch2/docs/tostring.md

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# String conversions
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**Contents**<br>
[operator << overload for std::ostream](#operator--overload-for-stdostream)<br>
[Catch::StringMaker specialisation](#catchstringmaker-specialisation)<br>
[Catch::is_range specialisation](#catchis_range-specialisation)<br>
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[Exceptions](#exceptions)<br>
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[Enums](#enums)<br>
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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).
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 ) {
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, or the global namespace, and have it declared before including Catch's header.
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## 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>`:
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```
namespace Catch {
template<>
struct StringMaker<T> {
static std::string convert( T const& value ) {
return convertMyTypeToString( value );
}
};
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}
```
## Catch::is_range specialisation
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As a fallback, Catch attempts to detect if the type can be iterated
(`begin(T)` and `end(T)` are valid) and if it can be, it is stringified
as a range. For certain types this can lead to infinite recursion, so
it can be disabled by specializing `Catch::is_range` like so:
```cpp
namespace Catch {
template<>
struct is_range<T> {
static const bool value = false;
};
}
```
## 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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## Enums
Enums that already have a `<<` overload for `std::ostream` will convert to strings as expected.
If you only need to convert enums to strings for test reporting purposes you can provide a `StringMaker` specialisations as any other type.
However, as a convenience, Catch provides the `REGISTER_ENUM` helper macro that will generate the `StringMaker` specialiation for you with minimal code.
Simply provide it the (qualified) enum name, followed by all the enum values, and you're done!
E.g.
```
enum class Fruits { Banana, Apple, Mango };
CATCH_REGISTER_ENUM( Fruits, Fruits::Banana, Fruits::Apple, Fruits::Mango );
TEST_CASE() {
REQUIRE( Fruits::Mango == Fruits::Apple );
}
```
... or if the enum is in a namespace:
```
namespace Bikeshed {
enum class Colours { Red, Green, Blue };
}
// Important!: This macro must appear at top level scope - not inside a namespace
// You can fully qualify the names, or use a using if you prefer
CATCH_REGISTER_ENUM( Bikeshed::Colours,
Bikeshed::Colours::Red,
Bikeshed::Colours::Green,
Bikeshed::Colours::Blue );
TEST_CASE() {
REQUIRE( Bikeshed::Colours::Red == Bikeshed::Colours::Blue );
}
```
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