Inspecting outcome<T, EC, EP>

Continuing with the previous example, in Layer3 we have function z which again reports failures via exceptions. It will call function h from Layer2_old which returns outcome<int> (which may store a double or an std::error_code or an std::exception_ptr). The goal is to unpack it to either the successful return value int or to throw an appropriate exception: if we are storing an std::exception_ptr just rethrow it. If we are storing an std::error_code throw it as std::system_error, which is designed to store std::error_code’s:

auto z() -> int // throws on failure
  if (outcome::outcome<int> o = old::h())
    return o.value();
  else if (o.has_exception())
    throw std::system_error{o.error()};
View this code on Github

Function has_exception checks if it is EP (std::exception_ptr) that is stored, function exception accesses it. Similarly, function error accesses the EC (std::error_code) if it is stored. outcome<> has also function has_failure to check if either EC or EP is being stored. In fact, the above pattern of “unpacking” outcome<> is so common that it has been implemented inside function .value(), so function z could be written as:

return old::h().value();