OVERLOADING OPERATORS = error

Question

I am trying to use overloading concept to equate 3 objects c1, c2, c3. But it is giving me an error

error: no match for 'operator=' in 'c3 = c2. circle::operator=(((circle&)(& c1)))'

What's the reason behind it how do I rectify it??

#include<iostream>
using namespace std;

class circle
{
  private:
    int radius;
    float x,y;
  public:
    circle()
    {}
    circle(int rr,float xx,float yy)
    {
      radius=rr;
      x=xx;
      y=yy;
    }
    circle& operator=(const circle& c)
    {
     cout<<endl<<"assignment operator invoked";  
     radius=c.radius;
     x=c.x;
     y=c.y;
     return *this;
     }
    void showdata()
    {
      cout<<endl<<"\n radius="<<radius;
      cout<<endl<<"x coordinate="<<x;
      cout<<endl<<"y coordinate="<<y<<endl;
    }
};
int main()
{
  circle c1 (10,2.5,2.5);
  circle c2,c3;
  c3=c2=c1;
  c1.showdata();
  c2.showdata();
  c3.showdata();
  return 0;
}

so this overloaded operator will be called two times.. First for c2=c1 and then for c3=c2 but how will compiler compare it with overloaded operator definition??

Armen Tsirunyan · Accepted Answer · 2012-07-13 08:21:26Z

6

In order to chain operator= calls, you must make sure that it returns a reference

circle& operator=(const circle& c)
{
   cout<<endl<<"assignment operator invoked";  
   radius=c.radius;
   x=c.x;
   y=c.y;
   return *this;
}

c1=c2=c3 is parsed as c1 = (c2 = c3). If operator = doesn't return a reference, c2 = c3 is an rvalue, and it cannot bind to the reference argument of c1.operator = (In case if the argument is a reference to const, it can bind to rvalues, but that doesn't mean you shouldn't return a reference).

Also note that it makes sense to take the parameter by const reference, because you don't want to change the argument that you assign.

Also remember the rule of three, that is, if you really need to do any of the following:

overload operator =
explicitly provide a copy constructor
explicitly provide a destructor

then you probably want to do the other two too. In your particular case, you don't seem to need to overload operator = at all.

edited Jul 13, 2012 at 8:21

answered Jul 13, 2012 at 8:15

Armen Tsirunyan

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12 Comments

Nathan White Over a year ago

@ArmenTsirunyan Normally, constructive comments aren't followed by "duh". That is fine, and as it turns out, I am in the wrong anyway. I must admit (and this makes me sound like an incompetent programmer) that I didn't know that was possible.

Armen Tsirunyan Over a year ago

@amadeus: It might work, but it makes absolutely no sense. Why would you want to return a new object, identical to *this if you already have *this? The idiomatic way of implementing opertator= is to return *this

Armen Tsirunyan Over a year ago

@amadeus: Then I'd recommend to study with a good book. I personally recommend Lippman's C++ Primer.

Max Lybbert Over a year ago

Don't feel too bad, @ArmenTsirunyan; I was in a discussion a month or so ago with very experienced programmers, and the fact that any object can access the guts of all other objects of that same class surprised a couple of them. It's something they just never thought about. But without that feature, you couldn't implement copying constructors. Or just about anything interesting (operator+ that adds private data from both objects, for instance).

Armen Tsirunyan Over a year ago

@MaxLybbert: You probably meant to tag nathanwhite, not me :)

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