C++ Primer, Fifth Edition

Defining a union

unions offer a convenient way to represent a set of mutually exclusive values of different types. As an example, we might have a process that handles different kinds of numeric or character data. That process might define a union to hold these values:

// objects of type Token have a single member, which could be of any of the listed typesunion Token {// members are public by default    char   cval;    int    ival;    double dval;};

A union is defined starting with the keyword union, followed by an (optional) name for the union and a set of member declarations enclosed in curly braces. This code defines a union named Token that can hold a value that is either a char, an int, or a double.