C++ Primer, Fifth Edition

Using a union Type

The name of a union is a type name. Like the built-in types, by default unions are uninitialized. We can explicitly initialize a union in the same way that we can explicitly initialize aggregate classes (§ 7.5.5, p. 298) by enclosing the initializer in a pair of curly braces:

Token first_token = {'a'}; // initializes the cval memberToken last_token;          // uninitialized Token objectToken *pt = new Token;     // pointer to an uninitialized Token object

If an initializer is present, it is used to initialize the first member. Hence, the initialization of first_token gives a value to its cval member.

The members of an object of union type are accessed using the normal member access operators:

last_token.cval = 'z';pt->ival ...