Outcome provides multiple methods for user code to intercept various events which occur. The deepest method is simply to inherit from basic_result or basic_outcome, and override member functions, for which you will need to study the source code as that form of customisation is out of scope for this tutorial.

Another option is to supply a custom NoValuePolicy which can get you surprisingly far into customisation (see preceding section).

The final option, which this section covers, is to use the ADL discovered event hooks which tell you when a namespace-localised basic_outcome or basic_result has been:

• Constructed
  • void hook_result_construction(T *, U &&) noexcept
  • void hook_outcome_construction(T *, U &&) noexcept
• In-place constructed
  • void hook_result_in_place_construction(T *, in_place_type_t<U>, Args &&...) noexcept
  • void hook_outcome_in_place_construction(T *, in_place_type_t<U>, Args &&...) noexcept
• Copied
  • void hook_result_copy_construction(T *, U &&) noexcept
  • void hook_outcome_copy_construction(T *, U &&) noexcept
• Moved
  • void hook_result_move_construction(T *, U &&) noexcept
  • void hook_outcome_move_construction(T *, U &&) noexcept

One criticism often levelled against library-based exception throw alternatives is that they do not provide as rich a set of facilities as C++ exception throws. This section shows you how to configure Outcome, using the ADL event hooks, to take a stack backtrace on construction of an errored result<T, error_code>, and if that result<T, error_code> should ever be converted into an outcome<T, error_code, std::exception_ptr>, a custom std::exception_ptr will be just-in-time synthesised consisting of the std::system_error for the error code, plus an expanded message string containing the stack backtrace of where the error originally occurred.

One can see the use case for such a configuration where low-level, deterministic, fixed latency code is built with result, and it dovetails into higher-level application code built with outcome where execution time guarantees are not important, and thus where a malloc is okay. One effectively has constructed a “lazy indeterminism”, or “just-in-time indeterminism” mechanism for handling failure, but with all the rich information of throwing C++ exceptions.