catch2/docs/printabletypes.md

Making Types Printable

Catch is capable of using user defined print functions and overloads.

Using ostreams

By default, catch will attempt to display types using an ostream overload is available.

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

using 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, catch will output {?} by default.