Using enum class with std::bitset

Question

First of all I want a normal enumeration instead of a bit-based enumeration, because the amount of different enums will be beyond any integral type. I also want to take advantage of the type safety of C++11 enum class. To do so, the natural choice would be std::bitset, however I have no idea how to bind those two together.

A custom bitset would be needed? How to go around the implementation of such a class?

Answer 1

Since enum classes are wrappers for enums, you can cast them to underlying type. And using some private inheritance you can selectively import some functionalities from C++ stdlib classes without worrying about Liskov's principle. Composition resulted in clearer code. Using these functionalities, we can wrap std::bitset. Following code contains only subset of functionalites, but it can be expanded further.

There's a problem with max value - that you can't get maximum value of enum class (or am I wrong?). So I added EnumTraits. Now users are required to specialize EnumTraits with const value max equal to the max value of enum before class can be used.

#include <bitset>
#include <type_traits>

template<typename T>
struct EnumTraits;

template<typename T>
class EnumClassBitset
{
private:
    std::bitset<static_cast<typename std::underlying_type<T>::type>(EnumTraits<T>::max)> c;

    typename std::underlying_type<T>::type get_value(T v) const
    {
        return static_cast<typename std::underlying_type<T>::type>(v);
    }

public:
    EnumClassBitset() : c()
    {

    }

    bool test(T pos) const
    {
        return c.test(get_value(pos));
    }

    EnumClassBitset& reset(T pos)
    {
        c.reset(get_value(pos));
        return *this;
    }

    EnumClassBitset& flip(T pos)
    {
        c.flip(get_value(pos));
        return *this;
    }
};

enum class BitFlags
{
    False,
    True, 
    FileNotFound,
    Write,
    Read,
    MaxVal
};

template<>
struct EnumTraits<BitFlags>
{
    static const BitFlags max = BitFlags::MaxVal;
};

#include <iostream>

int main()
{
    EnumClassBitset<BitFlags> f;
    f.flip(BitFlags::True);
    f.flip(BitFlags::FileNotFound);
    //f.flip(2); //fails to compile
    std::cout << "Is False? " << f.test(BitFlags::False) << "\n";
    std::cout << "Is True? " << f.test(BitFlags::True) << "\n";
    std::cout << "Is FileNotFound? " << f.test(BitFlags::FileNotFound) << "\n";
    std::cout << "Is Write? " << f.test(BitFlags::Write) << "\n";
    std::cout << "Is Read? " << f.test(BitFlags::Read) << "\n";
}

Since enums don't have much functionality unfortunately, and what is more, C++11 with enum classes don't improve the situation, some programmers use static map wrapped in a class. Definitely a good read.