Overview

This implements bitwise operation for enums (scoped and unscoped).
For any enum, like

enum class EButtons
{
	Left = 1,
	Right = 2,
	Middle = 4,
};

you can enable bitwise operations by declaring

enable_bitset_enum(EButtons);

A definition is not required: the function is never called, the declaration acs as a tag that enables bitwise operations.

Acknowledgement

I found the idea ... elsewhere. It wasn't the full set of operations, just a demonstration of the general concept.

Operations

The operations defined are | |= & &= ^ ^= - -=

a-b and a-=b take the place of a &~ b and a &= ~b, respectively, i.e., clearing the bits set in b. Negation is not implemented, because that would set bits that are not defined in the enum.

Type Safety

It's recommended to use scoped enums.

Note that these operations are defined for an unscoped enum, but they yield the underlying type (usually int) instead of the enum. Unscoped enums don't provide diagnostics when you mix values of different enums:

enum EFoo { a = 1, b = 2, c = 4 };
enum EBar { d = 1, e = 2, f = 4 };

// ....

void Test()
{
    auto mix = a | d;       // allowed, no diagnostic
}

c++ 17 vs. c++ 20

There are two implementations: one for C++ 20 using concepts to detect the "tag" function declaration, the other using std::enable_if, compatible wiht C++17. Except for the tag function detection, they are only marginally different.