Write one function for all combinations of const and volatile

[Frederick Virchanza Gotham]

Recently I wanted to have a class with a method called "Drop" that would return 'this', and so I wrote the following:

class MyClass {

#define DROP(modifiers) \
MyClass modifiers *Drop(void) modifiers \
{ \
return this; \
}

DROP()
DROP(const)
DROP(volatile)
DROP(const volatile)

};

Is there a better way of writing a member function just once and then having it work for all combinations of 'const' and 'volatile'?

[Alf P. Steinbach]

Off the cuff:

// C++ support machinery.

template< class Derived, Base >
constexpr bool is_same_or_derived_and_base_ =
std::is_base_of_v<Base, Derived>; // (sic!)

template< bool condition >
using Enable_if_ = std::enable_if_v<condition, int>;

template< class T > using const_ = const T;

// Problem solution: use a freestanding function.
// Possibly with trivial non-static member function wrappers.

struct My_class {};

template<
class T,
Enable_if_<is_same_or_derived_and_base_<T, My_class>> = 0
>
auto drop( const_<T*> p ) -> T* { return p; }

Disclaimer: not checked by a compiler.

See also: upcoming C++23 support for exactly this problem; <url:
https://www.google.com/search?q=c%2B%2B23+deduced+this>.

- Alf

[Alf P. Steinbach]

oh should be enable_if_t, not v. gah.

- Alf