C++ Primer, Fifth Edition

unions with Members of Class Type

Under earlier versions of C++, unions could not have members of a class type that defined its own constructors or copy-control members. Under the new standard, this restriction is lifted. However, unions with members that define their own constructors and/or copy-control members are more complicated to use than unions that have members of built-in type.

When a union has members of built-in type, we can use ordinary assignment to change the value that the union holds. Not so for unions that have members of nontrivial class types. When we switch the union’s value to and from a member of class type, we must construct ...