C++ Primer, Fifth Edition

Managing Union Members That Require Copy Control

Like the type-specific assignment operators, the copy constructor and assignment operators have to test the discriminant to know how to copy the given value. To do this common work, we’ll define a member named copyUnion.

When we call copyUnion from the copy constructor, the union member will have been default-initialized, meaning that the first member of the union will have been initialized. Because our string is not the first member, we know that the union member doesn’t hold a string. In the assignment operator, it is possible that the union already holds a string. We’ll handle that case directly in the assignment operator. That way copyUnion can assume that if its parameter holds a string, ...