How to use C++11 enum class for flags

Question

Say I have such a class:

enum class Flags : char
{
    FLAG_1 = 1;
    FLAG_2 = 2;
    FLAG_3 = 4;
    FLAG_4 = 8;
};

Now can I have a vari

Answer 1

Please don't do this. If you need to do this, enum classs probably aren't what you need.

@T.C. showed you how to do it so long as you specify underlying type, but you will run into places where your program does things it just shouldn't.

An example is where you use a switch and have a case for every defined enum value.

e.g.

enum class my_enum: unsigned int{
    first = 1,
    second = 2,
    third = 4,
    fourth = 8
};

int main(){
    auto e = static_cast(static_cast(my_enum::first) | static_cast(my_enum::second));

    switch(e){
        case my_enum::first:
        case my_enum::second:
        case my_enum::third:
        case my_enum::fourth:
            return 0;
    }

    std::cout << "Oh, no! You reached a part of the program you weren't meant to!\n";
    return 1;
}

Will output:

Oh, no! You reached a part of the program you weren't meant to!

then return the error code 1.

Which is also an example of why you should always have a default case, of course, but that isn't my point.

Of course, you could argue that so long as the user of the enum class never directly uses the value other than passing to a function; it would be a good way of restricting the values of a bitset. But I've always been a little too trustworthy and find std::uint[n]_t and some constexpr variables the best way (if a user sets an invalid bit it simply does nothing).

What you're doing just isn't really suitable for enum class, because it defeats the purpose of having a scoped enumeration. You can no longer enumerate the values if you set it to an undefined one.