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:
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.
#include <iostream>
using namespace std;
struct A {
A& operator=(const A&) {
cout << "A::operator=(const A&)" << endl;
return *this;
}
A& operator=(A&) {
cout << "A::operator=(A&)" << endl;
return *this;
}
};
class B {
A a;
};
struct C {
C& operator=(C&) {
cout << "C::operator=(C&)" << endl;
return *this;
}
C() { }
};
int main() {
B x, y;
x = y;
A w, z;
w = z;
C i;
const C j();
// i = j;
}
A::operator=(const A&)
A::operator=(A&)
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:
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 an lvalue 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:
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.