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 or destroy that member, respectively: When we switch the union to a member of class type, we must run a constructor for that member’s type; when we switch from that member, we must run its destructor.

When a union has members of built-in type, the compiler will synthesize the memberwise versions of the default constructor or copy-control members. The same is not true for unions that have members of a class type that defines its own default constructor or one or more of the copy-control members. If a union member’s type defines one of these members, the compiler synthesizes the corresponding member of the union as deleted (§ 13.1.6, p. 508).

For example, the string class defines all five copy-control members and the default constructor. If a union contains a string and does not define its own default constructor or one of the copy-control members, then the compiler will synthesize that missing member as deleted. If a class has a union member that has a deleted copy-control member, then that corresponding copy-control operation(s) of the class itself will be deleted as well.