C++ Primer, Fifth Edition

Using a Class to Manage union Members

Because of the complexities involved in constructing and destroying members of class type, unions with class-type members ordinarily are embedded inside another class. That way the class can manage the state transitions to and from the member of class type. As an example, we’ll add a string member to our union. We’ll define our union as an anonymous union and make it a member of a class named Token. The Token class will manage the union’s members.

To keep track of what type of value the union holds, we usually define a separate object known as a discriminant. A discriminant lets us discriminate among the values that the union can hold. In order to keep the union and its discriminant in sync, we’ll make the ...