Tedious brainstorm

Event Class Revisited

Discerning a satisfactorily fair class architecture for a gui is very tedious. But daily contemplation result in enduring evolution.

The need for Events returning bool instead of void is growing more crucial, and even though the previous interation concluded that having the need to have, or not to have, a return statement with a value in the Event class operator() function requires too much obscure complexity, it should be possible to do so nicely with c++20, so let’s take another look.

Given this class definition
accepting a TResult template parameter

template<typename TResult, typename... TArguments>
class Event {
	using TFunc = TResult (*)(TArguments..., void* userdata);
	TFunc _func;
	void* _userdata;

The operator() function should be
using std::is__void_v_ type trait

template<typename TResult, typename... TArguments>
TResult Event<TResult, TArguments...>::operator()(TArguments... args) const
{
	if(_func)
		if (std::is_void_v<TResult>) {
			_func(args..., _userdata);
			return;
		}
		else
			return _func(args..., _userdata);
	else if (std::is_void_v<TResult>)
		return;
	else
		return TResult{};
}

But as expected it does not compile
Error C2561 ‘shoujin::BoolEvent<int,int,int>::operator ()’: function must return a value

if constexpr - static if
Since c++17 constexpr can be applied to conditions.
Now the return statement can be included or excluded at compile time!

And this is how Event can now support TResult :)

template<typename TResult, typename... TArguments>
TResult Event<TResult, TArguments...>::operator()(TArguments... args) const
{
	if(_func)
		if constexpr(std::is_void_v<TResult>)
			_func(args..., _userdata);
		else
			return _func(args..., _userdata);
	else if constexpr(!std::is_void_v<TResult>)
		return TResult{};
}