How Can I Safely Use Enums as Flags in C ?

Overcoming Type Safety Issues When Treating Enums as Flags in C

In C#, applying the [Flags] attribute to enums allows them to be treated as boolean flags. However, replicating this behavior in C requires a different approach.

One method is to define bitwise operators for the enum:

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));
}

This allows for bitwise operations on the enums, such as:

seahwk.flags = CanFly | EatsFish | Endangered;

However, potential type safety issues arise when assigning non-enum values to the enum variable. To address this, consider the following:

struct AnimalFlagsGuard
{
    enum : AnimalFlags m_flags;
};

AnimalFlagsGuard seahawk;
seahwak.m_flags = CanFly | EatsFish | Endangered;

By enclosing the enum within a struct, the assignment of non-enum values is prevented at the type level. This approach maintains type safety and allows for a more structured and encapsulated handling of enums as flags.

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