On events and templates
Event Class
Testing the assertions is not nice when the code call ExitProcess during the test execution. Assertions must raise events to allow for custom behavior.
A first version of an event class was made and posted here C++ Event class
Also the following events are now exposed from the assertion namespace:
extern Event<LPCTSTR, LPCTSTR, int, LPCTSTR, bool&> GlobalOnError;
extern Event<tstring, bool&> GlobalOnErrorOutput;
extern Event<bool&> GlobalOnExitProcess;
Clearly a key feature is missing; To allow the listener to supply his own object, so that the listener’s function can receive userdata.
This goal is answered with the UserDataEvent class, which offer a templated TUserData which is constrained to be a pointer, using a custom c++20 concept.
Yet it’s still broken, because the event template parameters are defined in the event source, the assertions, and the various TUserData are known only to the listeners.
Which is why we often see void* userdata.
In the C language, this solution is nice because void* can be implicitly cast to another type. So a function such as
void OnClick(int x, int y, SomeClass* userdata)
can respond to an event defined as
void (*)(int x, int y, void* userdata)
But in C++, void* cannot be implicitly cast to a different pointer type, and templates cannot be used because the type is not known at the time of the event definition.
Obscure solutions might exists within the knowledge of the universe; if so hopes are that they will be shared.
This reasoning strenghen the case for a templated event class having a hardcoded void* userdata as the last parameter. Less modern but very efficient;
Ref 02a9ad0af467ece2f151dc3477d133fe1f56375e
New Event Class
RIP UserDataEvent
Changed Event class to always have a void* userdata argument.
This is what it looks like now:
template<typename ...TArguments>
class Event {
using TFunc = void (*)(TArguments..., void* userdata);
TFunc _func;
void* _userdata;
public:
Event();
Event(TFunc func, void* userdata = nullptr);
Event& operator=(const Event& rhs);
~Event();
void operator()(TArguments... args) const;
operator bool() const;
};