如何将所有枚举常量引入作用域?

// This already works, of course.
using E = Foo;
Foo::Bar v = E::A | E::B | E::C | E::D;

嗯,不是真的,因为 E::A | E::B | E::C | E::D 是一个 int you can't implicitly convert an int to an enum .

但这并不能阻止你使用 c++20 的 using enum

struct Foo {
    enum Bar {
        A = 1, B = 2, C = 4, D = 8
    };
};

int main() {
    using enum Foo::Bar; // (real C++20!)
    Foo::Bar v = D; 
}