The final step is to add event hooks for the very specific case of when our localised outcome is copy or move constructed from our localised result.

You ought to be very careful that the noexcept-ness of these matches the noexcept-ness of the types in the outcome. You may have noticed that poke_exception() creates a std::string and appends to it. This can throw an exception. If the copy and/or move constructors of T, EC and EP are noexcept, then so will be outcome’s copy and/or move constructor. Thus if poke_exception() throws, instant program termination would occur, which is bad.

We avoid that problem in this case by wrapping poke_exception() in a try...catch which throws away any exceptions thrown. For Outcome before v2.2, these specially named free functions must be placed into a namespace which is ADL searched:

namespace error_code_extended
{
  // Specialise the outcome copy and move conversion hook for when our localised result
  // is used as the source for copy construction our localised outcome
  template <class T, class U> inline void hook_outcome_copy_construction(outcome<T> *res, const result<U> & /*unused*/) noexcept
  {
    try
    {
      // when copy constructing from a result<T>, poke in an exception
      poke_exception(res);
    }
    catch(...)
    {
      // Do nothing
    }
  }
  template <class T, class U> inline void hook_outcome_move_construction(outcome<T> *res, result<U> && /*unused*/) noexcept
  {
    try
    {
      // when move constructing from a result<T>, poke in an exception
      poke_exception(res);
    }
    catch(...)
    {
      // Do nothing
    }
  }
}

For Outcome v2.2 and later, these functions must be placed into a custom no value policy with the names on_outcome_copy_construction() and on_outcome_move_construction() respectively. As with before, the implementation of the functions is identical, just the name and location has changed.