Custom payloads

So far, type EC in result<T, EC> has always been a std::error_code. But it need in fact to merely satisfy trait::has_error_code_v<EC> for EC to be treated as a std::error_code.

In turn, trait::has_error_code_v<EC> is true if there exists some ADL discovered free function:

  1. make_error_code(EC) returning a std::error_code.
  2. make_error_code(get<0>(EC)) returning a std::error_code (i.e. EC might be pair<std::error_code, U> or tuple<std::error_code, ...>).

… or if EC is implicitly convertible into a std::error_code.

Thus, we can in fact use any custom EC type we like, including one carrying additional information, or payload. This payload can carry anything you like, and you can tell Outcome to do various things with that payload under various circumstances. For example:

  1. If the user tries to observe an unsuccessful result, throw a custom exception containing the cause of failure with accompanying context from the payload.
  2. If the user ever constructs an outcome from a payload carrying result, set the exception ptr in the constructed outcome to a custom exception containing the cause of the failure with accompanying context from the payload.
  3. Transmit a stack backtrace specifying the exact point at which failure occurred, symbolising that into human readable text at the moment of conversion into human readable text.
  4. Upon a namespace-localised result from library A being copy/moved into a namespace-localised result from C bindings library B, set the C errno if the error code and category map onto the errno domain.

There are many, many other options of course. This tutorial can only cover a reasonable subset. This section covers Example 1 above, throwing custom exceptions with payload upon observation of an unsuccessful result.