Comparison with std::nullopt in Catch2 gives unreadable results

Question

I have the following check in my cpp test, which is written with Catch2:

REQUIRE(foo() == std::nullopt);

The test fails with a meaningless message when the function returns std::optional<int>:

  REQUIRE( foo() == std::nullopt )
with expansion:
  {?} == {?}

Is it because Catch2 cannot convert the values to something meaningful, and that's why it shows {?} == {?}? What would be the best workaround for it to have a proper error message there?

Answer 1

Take a look at https://github.com/catchorg/Catch2/blob/devel/docs/tostring.md#top

Given how std::optional<int> interferes with int I found the Catch::StringMaker specialisation to be easier than overloading operator<< for std::ostream (though you can get both to work).

Try to add this code to your testcases (after the Catch2 header include):

namespace Catch
{
    template <>
    struct StringMaker<std::optional<int>>
    {
        static std::string convert(std::optional<int> const &value)
        {
            if (value)
                return std::to_string(*value);
            else
                return std::string("<empty>");
        }
    };

    template <>
    struct StringMaker<std::nullopt_t>
    {
        static std::string convert(std::nullopt_t const &)
        {
            return std::string("<empty>");
        }
    };
}

The first part handles std::optional<int>, the second handles the cases where you compare to std::nullopt (try them one at a time to see the effect).