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Make THISFN simpler and more powerful [message #59037]

Tue, 18 October 2022 21:08

Lance
Messages: 527
Registered: March 2007

Contributor

in Core/Function.h, we have THISFN defined like this

template <class Ptr, class Class, class Res, class... ArgTypes>
Function<Res (ArgTypes...)> MemFn(Ptr object, Res (Class::*method)(ArgTypes...))
{
	return [=](ArgTypes... args) { return (object->*method)(args...); };
}

#define THISFN(x)   MemFn(this, &CLASSNAME::x)

This can be done in C++20 with the following

#define THISFN(memfun) std::bind_front(&std::remove_cvref_t<decltype(*this)>::memfun, this)

Of course this involves only library feature (no core language features), so it should be able to be rewritten to make available in current versons of c++ compilers/libraries that UPP aims to conform to atm.

An additional benefit of above macro definition is that it relieves the library user of the burden of an otherwise useless typedef

class MyClass
{
    typedef MyClass CLASSNAME;
//....
}

to be able to use thisfn

BTW, thisfn is provided in the UPP library for a good reason: there are occasions where using thisfn is so much easier than using an lambda.

eg.

	menu.Set( THISFN(MenuMain) );

	menu.Set ( [=, this] ( Bar & bar )
		{
			MenuMain ( bar );
		}
	);

And imagine when the callback accept a few more parameters.
[/code]

U++ is currently aiming at compliance to C++14. What if you want to use it in your project and you know your compiler/c++library are modern enough?

One way is to put the following in a header file and make sure you include it after <Core/Core.h> is included.

#ifdef THISFN
#    undef THISFN
#    define THISFN(memfun) ....
#endif

Reference:
Why use `std::bind_front` over lambdas in C++20?

[Updated on: Tue, 18 October 2022 21:19]

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