How Can I Use C Enums as Flags Without External Libraries?

Using Enums as Flags in C without External Libraries

The ability to treat enums as flags, as available in C# through the [Flags] attribute, is a convenient feature for representing bitwise combination of values. Implementing this functionality in C requires a slightly different approach.

To achieve this, one method is to define bitwise operators for the enum, thereby providing a type-safe way to combine flag values. Here's how it can be done:

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));
}

Define additional operators (e.g., &, ^) as needed. If the enum range exceeds the range of int, adjust the casting operations accordingly.

By using this approach, you can now use the enum values as flags, ensuring type safety. For example:

struct Animal
{
    AnimalFlags flags;
    ...
};

int main()
{
    Animal seahawk;
    seahawk.flags = CanFly | EatsFish | Endangered;
    // seahawk.flags = HasMaximizeButton; // Compile error
    ...
}

This allows you to safely combine flag values and prevents incorrect assignments like assigning the window flag HasMaximizeButton to the animal's flags.

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