Enumerations

An enumeration defines a set of named integer constants. It improves readability for states, modes, result kinds, and other finite choices.

enum ConnectionState {
    CONNECTION_DISCONNECTED,
    CONNECTION_CONNECTING,
    CONNECTION_CONNECTED,
    CONNECTION_FAILED
};

Without explicit values, the first enumerator is zero and each following value increases by one. Values may be assigned and later entries continue from them.

enum HttpStatus {
    HTTP_OK = 200,
    HTTP_NOT_FOUND = 404,
    HTTP_SERVER_ERROR = 500
};

Use enums in control flow

const char *state_name(enum ConnectionState state) {
    switch (state) {
        case CONNECTION_DISCONNECTED: return "disconnected";
        case CONNECTION_CONNECTING: return "connecting";
        case CONNECTION_CONNECTED: return "connected";
        case CONNECTION_FAILED: return "failed";
    }
    return "invalid";
}

Compilers can warn about missing enum cases when a switch omits default. Whether to include a default depends on the contract: it can safely handle invalid input, but can also hide a newly added enumerator from warning-based review.

Enums are not automatic validation

An enum object can receive integer values outside its listed enumerators through conversions or corrupted input. Validate values that cross trust boundaries.

Enumeration constants are integer constant expressions, useful for compile-time array sizes and case labels. Traditional C does not guarantee a specific storage width for every enum; do not use raw enum representation in a portable file or network format.

Flags are different

Mutually exclusive states suit enums. Combinable options suit unsigned bit masks:

enum Permission {
    PERMISSION_READ = 1u << 0,
    PERMISSION_WRITE = 1u << 1,
    PERMISSION_EXECUTE = 1u << 2
};

The type system does not prevent mixing unrelated enum values, so use distinctive prefixes and narrow interfaces.