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 member
Token last_token;          // uninitialized Token object
Token *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 = 42;

Assigning a value to a data member of a union object makes the other data members undefined. As a result, when we use a union, we must always know what type of value is currently stored in the union. Depending on the types of the members, retrieving or assigning to the value stored in the union through the wrong data member can lead to a crash or other incorrect program behavior.