How to use enums as flags in C++?


Question

Treating enums as flags works nicely in C# via the [Flags] attribute, but what's the best way to do this in C++?

For example, I'd like to write:

enum AnimalFlags
{
    HasClaws = 1,
    CanFly =2,
    EatsFish = 4,
    Endangered = 8
};

seahawk.flags = CanFly | EatsFish | Endangered;

However, I get compiler errors regarding int/enum conversions. Is there a nicer way to express this than just blunt casting? Preferably, I don't want to rely on constructs from 3rd party libraries such as boost or Qt.

EDIT: As indicated in the answers, I can avoid the compiler error by declaring seahawk.flags as int. However, I'd like to have some mechanism to enforce type safety, so someone can't write seahawk.flags = HasMaximizeButton.

1
166
9/19/2009 11:57:03 AM

Accepted Answer

The "correct" way is to define bit operators for the enum, as:

enum AnimalFlags
{
    HasClaws   = 1,
    CanFly     = 2,
    EatsFish   = 4,
    Endangered = 8
};

inline AnimalFlags operator|(AnimalFlags a, AnimalFlags b)
{
    return static_cast<AnimalFlags>(static_cast<int>(a) | static_cast<int>(b));
}

Etc. rest of the bit operators. Modify as needed if the enum range exceeds int range.

221
8/28/2019 9:02:19 AM

Note (also a bit off topic): Another way to make unique flags can be done using a bit shift. I, myself, find this easier to read.

enum Flags
{
    A = 1 << 0, // binary 0001
    B = 1 << 1, // binary 0010
    C = 1 << 2, // binary 0100
    D = 1 << 3, // binary 1000
};

It can hold values up to an int so that is, most of the time, 32 flags which is clearly reflected in the shift amount.


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