The copy assignment operator lets you create a new object from an existing one by initialization. A copy assignment operator of a class A is a nonstatic non-template member function that has one of the following forms:
- A::operator=(A)
- A::operator=(A&)
- A::operator=(const A&)
- A::operator=(volatile A&)
- A::operator=(const volatile A&)
If you do not declare a copy assignment operator for a class A, the compiler will implicitly declare one for you which will be inline public.
The following example demonstrates implicitly defined and user-defined copy assignment operators:
The following is the output of the above example:
The assignment x = y calls the implicitly defined copy assignment operator of B, which calls the user-defined copy assignment operator A::operator=(const A&). The assignment w = z calls the user-defined operator A::operator=(A&). The compiler will not allow the assignment i = j because an operator C::operator=(const C&) has not been defined.
The implicitly declared copy assignment operator of a class A will have the form A& A::operator=(const A&) if the following are true:
- A direct or virtual base B of class A has a copy assignment operator whose parameter is of type const B&, const volatile B&, or B.
- A non-static class type data member of type X that belongs to class A has a copy constructor whose parameter is of type const X&, const volatile X&, or X.
If the above are not true for a class A, the compiler will implicitly declare a copy assignment operator with the form A& A::operator=(A&).
The implicitly declared copy assignment operator returns a reference to the operator's argument.
The copy assignment operator of a derived class hides the copy assignment operator of its base class.
The compiler cannot allow a program in which the compiler must implicitly define a copy assignment operator for a class A and one or more of the following are true:
- Class A has a nonstatic data member of a const type or a reference type
- Class A has a nonstatic data member of a type which has an inaccessible copy assignment operator
- Class A is derived from a base class with an inaccessible copy assignment operator.
An implicitly defined copy assignment operator of a class A will first assign the direct base classes of A in the order that they appear in the definition of A. Next, the implicitly defined copy assignment operator will assign the nonstatic data members of A in the order of their declaration in the definition of A.