catch2/docs/printabletypes.md

Making Types Printable

Catch can print many standard types out of the box, but it is also capable of using user defined print functions and overloads.

Using ostream and operator<<

By default, catch will attempt to display a variable of type T using an ostream overload if available.

std::ostream& operator<<(std::ostream&, T const&);

Overloading Catch::toString

If you don't want your type to work with ostreams, or Catch is unable to find the overload you provide, you may alternatively overload Catch::toString. The overload must appear above the include of catch.hpp. For example, if I want to provide a print functionality for std::pair<int, char> in my test, the top of my test file would look like this:

#include <string>
#include <sstream>
#include <utility>

namespace Catch {
std::string toString(std::pair<int, char> const& p) {
    std::ostringstream oss;
    oss << '{' << p.first << ", " << p.second << '}';
    return oss.str();
}
}

#include "catch.hpp"

Note that the former approach is preferred, and the latter is more error-prone due to possible typos when typing Catch or toString

Default output

If neither of the two conditions has been met, and catch does not provide a string conversion for the type, catch will output {?} by default.