Discriminated Unions

A discriminated union is a union data structure that holds various objects, with one of the objects identified directly by a discriminant. The discriminant is the first item to be serialized or deserialized.

A discriminated union includes both a discriminant and a component. The type of discriminant is either integer, unsigned integer, or an enumerated type, such as bool. The component is selected from a set of types that are prearranged according to the value of the discriminant. The component types are called arms of the union. The arms of a discriminated union are preceded by the value of the discriminant that implies their encoding. See Using an XDR Discriminated Union Example.

union switch (discriminant-declaration) {
    case discriminant-value-A:
    arm-declaration-A;
    case discriminant-value-B:
    arm-declaration-B;
    ...
    default: default-declaration;
} identifier;

Each case keyword is followed by a legal value of the discriminant. The default arm is optional. If an arm is not specified, a valid encoding of the union cannot take on unspecified discriminant values. The size of the implied arm is always a multiple of four bytes.

The discriminated union is encoded as the discriminant, followed by the encoding of the implied arm.

See the Discriminated Union figure (Figure 1).