C++ enum flags vs bitset

Question

What are pros/cons of usage bitsets over enum flags?

namespace Flag {
    enum State {
        Read   = 1 << 0,
        Write  = 1 << 1,
        Binary = 1 << 2,
    };
}

namespace Plain {
    enum State {
        Read,
        Write,
        Binary,
        Count
    };
}

int main()
{
    {
        unsigned int state = Flag::Read | Flag::Binary;
        std::cout << state << std::endl;

        state |= Flag::Write;
        state &= ~(Flag::Read | Flag::Binary);
        std::cout << state << std::endl;
    } {
        std::bitset<Plain::Count> state;
        state.set(Plain::Read);
        state.set(Plain::Binary);
        std::cout << state.to_ulong() << std::endl;

        state.flip();
        std::cout << state.to_ulong() << std::endl;
    }

    return 0;
}

As I can see so far, bitsets have more convinient set/clear/flip functions to deal with, but enum-flags usage is a more wide-spreaded approach.

What are possible downsides of bitsets and what and when should I use in my daily code?

Answer 1

Do you compile with optimization on? It is very unlikely that there is a 24x speed factor.

To me, bitset is superior, because it manages space for you:

• can be extended as much as wanted. If you have a lot of flags, you may run out of space in the int/long long version.
• may take less space, if you only use just several flags (it can fit in an unsigned char/unsigned short - I'm not sure that implementations apply this optimization, though)