Discriminated Union

Description

A discriminated union is a type composed of a discriminant followed by a type selected from a set of prearranged types according to the value of the discriminant. The type of discriminant is either int, unsigned int, or an enumerated type, such as bool. The component types are called arms of the union, and are preceded by the value of the discriminant that implies their encoding.

Declaration

Discriminated unions are declared as follows:

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 it is not specified, then 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 its discriminant followed by the encoding of the implied arm.

Encoding

Discriminated Union