ADL bridging

note

In Outcome v2.2 the ADL-based event hooks were replaced with policy-based event hooks (next page). The code in this section is still valid in v2.2 onwards, it’s just that ADL is no longer used to find the hooks.

In a previous section, we used the failure_info type to create the ADL bridge into the namespace where the ADL discovered outcome_throw_as_system_error_with_payload() function was to be found.

Here we do the same, but more directly by creating a thin clone of std::error_code into the local namespace. This ensures that this namespace will be searched by the compiler when discovering the event hooks (Outcome v2.1 and earlier only).

namespace error_code_extended
{
  // Use the error_code type as the ADL bridge for the hooks by creating a type here
  // It can be any type that your localised result uses, including the value type but
  // by localising the error code type here you prevent nasty surprises later when the
  // value type you use doesn't trigger the ADL bridge.
  struct error_code : public std::error_code
  {
    // literally passthrough
    using std::error_code::error_code;
    error_code() = default;
    error_code(std::error_code ec)
        : std::error_code(ec)
    {
    }
  };

  // Localise result and outcome to using the local error_code so this namespace gets looked up for the hooks
  template <class R> using result = BOOST_OUTCOME_V2_NAMESPACE::result<R, error_code>;
  template <class R> using outcome = BOOST_OUTCOME_V2_NAMESPACE::outcome<R, error_code /*, std::exception_ptr */>;
}
View this code on Github

For convenience, we template alias local copies of result and outcome in this namespace bound to the ADL bridging error_code.

Boost C++ Libraries

ADL bridging