[c++] Overloading assignments (C++ only)

struct X {

  int data;

  X& operator=(X& a) { return a; }

  X& operator=(int a) {

    data = a;

    return *this;

  }

};

int main() {

  X x1, x2;

  x1 = x2;      // call x1.operator=(x2)

  x1 = 5;       // call x1.operator=(5)

}

==============================================================

#include <iostream>

using namespace std;

struct A {

  A& operator=(char) {

    cout << "A& A::operator=(char)" << endl;

    return *this;

  }

  virtual A& operator=(const A&) {

    cout << "A& A::operator=(const A&)" << endl;

    return *this;

  }

};

struct B : A {

    B& operator=(char) {

      cout << "B& B::operator=(char)" << endl;

      return *this;

    }

    virtual B& operator=(const A&) {

      cout << "B& B::operator=(const A&)" << endl;

      return *this;

    }

};

struct C : B { };

int main() {

  B b1;

  B b2;

  A* ap1 = &b1;

  A* ap2 = &b1;

  *ap1 = 'z';

  *ap2 = b2;

  C c1;

//  c1 = 'z';

}

A& A::operator=(char)

B& B::operator=(const A&)

The assignment *ap1 = 'z' calls A& A::operator=(char). Because this operator has not been declared virtual, the compiler chooses the function based on the type of the pointer ap1.

The assignment *ap2 = b2 calls B& B::operator=(const &A). Because this operator has been declared virtual, the compiler chooses the function based on the type of the object that the pointer ap1 points to. 

The compiler would not allow the assignment c1 = 'z' because the implicitly declared copy assignment operator declared in class C hides B& B::operator=(char).