19.6. union: A Space-Saving Class

A union is a special kind of class. A union may have multiple data members, but at any point in time, only one of the members may have a value. When a value is assigned to one member of the union, all other members become undefined. The amount of storage allocated for a union is at least as much as is needed to contain its largest data member. Like any class, a union defines a new type.

Some, but not all, class features apply equally to unions. A union cannot have a member that is a reference, but it can have members of most other types, including, under the new standard, class types that have constructors or destructors. A union can specify protection labels to make members public, private, or protected. By default, like structs, members of a union are public.

A union may define member functions, including constructors and destructors. However, a union may not inherit from another class, nor may a union be used as a base class. As a result, a union may not have virtual functions.