Hooking events
Outcome provides multiple methods for user code to intercept various lifecycle 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
(see preceding section).
From Outcome v2.2 onwards, intercepting construction, copies and moves requires
a custom NoValuePolicy.
Before Outcome v2.2, there was an ADL discovered event hook mechanism for intercepting
construction, copies and moves (it was found to be brittle, error prone and surprising
in empirical use, which is why it was replaced). The ADL discovered event hooks still
function in Outcome v2.2 and later if OUTCOME_ENABLE_LEGACY_SUPPORT_FOR
is set to less than 220 to enable emulation.
You will note that the naming is simply one of hook_* => on_*, the parameters remain
identical. This eases porting code from Outcome v2.1 to v2.2, it’s usually just a case
of copy-pasting the ADL hook code into a custom NoValuePolicy.
Policy set event hooks (Outcome v2.2 onwards):
- Constructed
- In-place constructed
- Copied
- Moved
ADL discovered event hooks (before Outcome v2.2):
- Constructed
- In-place constructed
- Copied
- Moved
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 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.