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Re: Make THISFN simpler and more powerful [message #59153 is a reply to message #59037]

Thu, 10 November 2022 00:32

Lance
Messages: 656
Registered: March 2007

Contributor

This is kind of C++20 related, I am going to reuse this thread for c++ language specific problems or what not.

Here I got a problem: how to detect if a class is Moveable?

A naive first thought would be

#include <Core/Core.h>
#include <concepts>

template <class T>
inline constexpr bool is_moveable = std::derived_from<T, Upp::Moveable<T>>;

using namespace Upp;

CONSOLE_APP_MAIN
{
	LOG(is_moveable<Upp::Rect>);
	LOG(is_moveable<Upp::String>);
}

Output

true
false

It turns out Moveable has an optional second template parameter which failed the test in [b]String[/]'s case.

What makes Moveable so special is that it introduced a friend AssertMoveable0 in <Core/Topt.h>

template <class T>
inline void AssertMoveable0(T *t) { AssertMoveablePtr(&**t, *t); }
// COMPILATION ERROR HERE MEANS TYPE T WAS NOT MARKED AS Moveable

template <class T, class B = EmptyClass>
struct Moveable : public B {
	friend void AssertMoveable0(T *) {}
};

Trying to use it directly, I come up with something like

#include <Core/Core.h>
#include <concepts>

template <class T>
inline constexpr bool is_moveable = /*std::derived_from<T, Upp::Moveable<T>>;*/
	requires (T t){ Upp::AssertMoveable0(&t); };

using namespace Upp;

CONSOLE_APP_MAIN
{
	LOG(is_moveable<Upp::Rect>);
	LOG(is_moveable<Upp::String>);
}

This time both give correct result, but unfortuately, any class T would give a true result regardless of whether it's derived from Upp::Moveable.

How would you check if a class type is Upp::Moveable ?

BTW, something like this

LOG( std::is_base_of<Ctrl, EditField>);

will fail to compile because of a missing guard in the definition of LOG in <Core/Diag.h>

#define LOG(a) UPP::VppLog() << a << UPP::EOL

I would think here change it to

#define LOG(a) UPP::VppLog() << (a) << UPP::EOL

might be a reasonable thing to do.

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