Is it possible to manually define a conversion for an enum class?

Question

Normally you can define a cast for a class by using the following syntax:

class Test {
public:
  explicit operator bool() { return false; }
};

Answer 1

No, but you can make a normal class type act like an enum class, using constexpr members and constructors. And then you can add all the additional member functions you want.

---

Proof that it can work even with switch:

#include 

struct FakeEnum
{
    int x;

    constexpr FakeEnum(int y = 0) : x(y) {}

    constexpr operator int() const { return x; }

    static const FakeEnum A, B, Z;
};

constexpr const FakeEnum FakeEnum::A{1}, FakeEnum::B{2}, FakeEnum::Z{26};

std::istream& operator>>(std::istream& st, FakeEnum& fe)
{
    int val;
    st >> val;
    fe = FakeEnum{val};
    return st;
}

int main()
{
    std::cout << "Hello, world!\n";
    FakeEnum fe;
    std::cin >> fe;

    switch (fe)
    {
        case FakeEnum::A:
        std::cout << "A\n";
        break;
        case FakeEnum::B:
        std::cout << "B\n";
        break;
        case FakeEnum::Z:
        std::cout << "Z\n";
        break;
    }
}

---

Proof that working with switch does not require implicit interconversion with int:

#include 

/* pseudo-enum compatible with switch and not implicitly convertible to integral type */
struct FakeEnum
{
    enum class Values { A = 1, B = 2, Z = 26 };
    Values x;

    explicit constexpr FakeEnum(int y = 0) : FakeEnum{static_cast(y)} {}
    constexpr FakeEnum(Values y) : x(y) {}

    constexpr operator Values() const { return x; }
    explicit constexpr operator bool() const { return x == Values::Z; }

    static const FakeEnum A, B, Z;
};

constexpr const FakeEnum FakeEnum::A{Values::A}, FakeEnum::B{Values::B}, FakeEnum::Z{Values::Z};

std::istream& operator>>(std::istream& st, FakeEnum& fe)
{
    int val;
    st >> val;
    fe = FakeEnum(val);
    return st;
}

int main()
{
    std::cout << "Hello, world!\n";
    FakeEnum fe;
    std::cin >> fe;

    switch (fe)
    {
        case FakeEnum::A:
        std::cout << "A\n";
        break;
        case FakeEnum::B:
        std::cout << "B\n";
        break;
        case FakeEnum::Z:
        std::cout << "Z\n";
        break;
    }
    // THIS ERRORS: int z = fe;
}