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Overloading the Assignment Operator (=)

We know that if we want objects of a class to be operated by common operators then we need to overload them. But there is one operator whose operation is automatically crested by C++ for every class we define, it is the assignment operator ‘=’.

Actually we have been using similar statements like the one below previously

  ob1=ob2;

where ob1 and ob2 are objects of a class.

This is because even if we don’t overload the ‘=’ operator, the above statement is valid.

As I said C++ automatically creates a default assignment operator. The default operator created, does a member-by-member copy, but if we want to do something specific we may overload it.

The simple program below illustrates how it can be done. Here we are defining two similar classes, one with the default assignment operator (created automatically) and the other with the overloaded one. Notice how we could control the way assignments are done in that case.


  // Program to illustrate the
  // overloading of assignment
  // operator '='
  #include <iostream.h>

  // class not overloading the
  // assignment operator
  class myclass
  {
    int a;
    int b;

  public:
    myclass(int, int);
    void show();
  };

  myclass::myclass(int x,int y)
  {
    a=x;
    b=y;
  }

  void myclass::show()
  {
    cout<<a<<endl<<b<<endl;
  }

  // class having overloaded
  // assignment operator
  class myclass2
  {
    int a;
    int b;

  public:
    myclass2(int, int);
    void show();

    myclass2 operator=(myclass2);
  };

  myclass2 myclass2::operator=(myclass2 ob)
  {
    // -- do something specific --
    // this is just to illustrate
    // that when overloading '='
    // we can define our own way
    // of assignment
    b=ob.b;

    return *this;
  };

  myclass2::myclass2(int x,int y)
  {
    a=x;
    b=y;
  }

  void myclass2::show()
  {
    cout<<a<<endl<<b<<endl;
  }

  // main
  void main()
  {
    myclass ob(10,11);
    myclass ob2(20,21);

    myclass2 ob3(100,110);
    myclass2 ob4(200,210);

    // does a member-by-member copy
    // '=' operator is not overloaded
    ob=ob2;
    ob.show();


    // does specific assignment as 
    // defined in the overloaded
    // operator definition
    ob3=ob4;
    ob3.show();
  }

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