Chapter Summary

In addition to defining the operations on objects of its type, a class also defines what it means to copy, assign, or destroy objects of the type. Special member functionsthe copy constructor, the assignment operator, and the destructor define these operations. Collectively these operations are referred to as the "copy control" functions.

If a class does not define one or more of these operations, the compiler will define them automatically. The synthesized operations perform memberwise initialization, assignment, or destruction: Taking each member in turn, the synthesized operation does whatever is appropriate to the member's type to copy, assign, or destroy that member. If the member is a class type, the synthesized operation calls the corresponding operation for that class (e.g., the copy constructor calls the member's copy constructor, the destructor calls its destructor, etc.). If the member is a built-in type or a pointer, the member is copied or assigned directly; the destructor does nothing to destroy members of built-in or pointer type. If the member is an array, the elements in the array are copied, assigned, or destroyed in a manner appropriate to the element type.

Unlike the copy constructor and assignment operator, the synthesized destructor is created and run, regardless of whether the class defines its own destructor. The synthesized destructor is run after the class-defined destructor, if there is one, completes.

The hardest part of defining the copy-control functions is often simply recognizing that they are necessary.

Classes that allocate memory or other resources almost always require that the class define the copy-control members to manage the allocated resource. If a class needs a destructor, then it almost surely needs to define the copy constructor and assignment operator as well.