c++ - 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 variable that has type flags and assign a value 7 for example? Can I do this:

Flags f = Flags::FLAG_1 | Flags::FLAG_2 | Flags::FLAG_3;

or

Flags f = 7;

This question arises because in the enum I have not defined value for 7.

Answer 1

You need to write your own overloaded operator| (and presumably operator& etc.).

Flags operator|(Flags lhs, Flags rhs) 
{
    return static_cast<Flags>(static_cast<char>(lhs) | static_cast<char>(rhs));
}

Conversion of an integer to an enumeration type (scoped or not) is well-defined as long as the value is within the range of enumeration values (and UB otherwise; [expr.static.cast]/p10). For enums with fixed underlying types (this includes all scoped enums; [dcl.enum]/p5), the range of enumeration values is the same as the range of values of the underlying type ([dcl.enum]/p8). The rules are trickier if the underlying type is not fixed - so don't do it :)