Copy constructor issue

[yepMe]

Hi all,

We know copy constructor is required either of the following scenario :
1) when we try to intialize one object using another object.
2) When we try to pass an object by value to a function or
3) when we return an object by value from a function.
I have written the following code just to test all the scenarios the first one worked well. Second one also didn't throw any error but it didn't give the desired result

  #include<iostream>

using namespace std;

class c_const
{
      int x;
      int y;
      public :
             c_const(int a, int b)
             {
                          cout <<"in constructor!!"<<endl;
                          x = a;
                          y = b;
                          
             }
             void print()
             {
              cout << "x  " << x << endl;
              cout << "y  " << y << endl;
             }
             ~c_const()
             {
                       cout << "detructed " << endl;
                       cout << "x_d " << x <<endl;;
                       cout << "y_d  " << y << endl;;
             }
             c_const( c_const & pp)
             {
                      cout <<"in copy constructor!!!"<< endl;
                      x = pp.x;
                      y = pp.y;
                      
             }
};

void func(c_const & cp)
{
     cout <<"hey ya "<<endl;
     cp.print();
     
 }
 
 c_const & func1  (int x , int y)
{
     c_const pq(5,6);
     cout << x << endl;
     cout << y << endl;
     return pq;    
}
int main()
{
    {c_const cc(10,20);
   // c_const cc1(cc);
    //c_const cc2 = cc1;
   // cc2 = cc1;
    cc.print();
   // cc1.print();
    func(cc);
    c_const rr(7,8);
    rr = func1(2,3);
     }
    system("pause");
    return 0;
}

}

When I am calling the function from main and passing an object by value to it copy constructor is not getting called same thing is happening when I am trying to return an object by value from a function as in the code.. please tell me where am I wrong?

[ResidentBiscuit]

Well you're returning a reference to that object, so the copy constructor is not going to be called. Change that function signature to return a copy, and then you'll see it being called.

Same goes for the func(). Change the parameter to be passed by value instead of by reference.

[yepMe]

Thanks, for object passing as the argument is working but for returning an object from a function gives an error.

c_const func1 (int x , int y)
{
c_const pq(5,6);
cout << x << endl;
cout << y << endl;
return pq;
}
int main()
{
{c_const cc(10,20);
// c_const cc1(cc);
//c_const cc2 = cc1;
// cc2 = cc1;
cc.print();
// cc1.print();
func(cc);
//c_const rr(7,8);
/ c_const rr = func1(2,3);
}
system("pause");
return 0;
}

Wat's wrong?

[ResidentBiscuit]

What's the error? And why didn't you use code tags?

[yepMe]

sorry..

#include<iostream>

using namespace std;

class c_const
{
      int x;
      int y;
      public :
             c_const(int a, int b)
             {
                          cout <<"in constructor!!"<<endl;
                          x = a;
                          y = b;
                          
             }
             void print()
             {
              cout << "x  " << x << endl;
              cout << "y  " << y << endl;
             }
             ~c_const()
             {
                       cout << "detructed " << endl;
                       cout << "x_d " << x <<endl;;
                       cout << "y_d  " << y << endl;;
             }
             c_const( c_const & pp)
             {
                      cout <<"in copy constructor!!!"<< endl;
                      x = pp.x;
                      y = pp.y;
                      
             }
};

void func(c_const  cp)
{
     cout <<"hey ya "<<endl;
     cp.print();
     
 }
 
 c_const  func1  (int x , int y)
{
     c_const pq(5,6);
     cout << x << endl;
     cout << y << endl;
     return pq;    
}
int main()
{
    {c_const cc(10,20);
   // c_const cc1(cc);
    //c_const cc2 = cc1;
   // cc2 = cc1;
    cc.print();
   // cc1.print();
    func(cc);
    c_const rr(7,8);
    rr = func1(2,3);
     }
    system("pause");
    return 0;
}

It's giving error, no matching function for call to 'c_const::c_const(c_const)'
I just removed the '&' from return type of the function definition

[ResidentBiscuit]

Oh you're missing the const qualifier for your copy ctor. It should take a parameter of a const reference. Not just a reference.

[yepMe]

Now it is working but again copy constructor is getting called .

#include<iostream>

using namespace std;

class c_const
{
      int x;
      int y;
      public :
             c_const(int a, int b)
             {
                          cout <<"in constructor!!"<<endl;
                          x = a;
                          y = b;
                          
             }
             void print()
             {
              cout << "x  " << x << endl;
              cout << "y  " << y << endl;
             }
             ~c_const()
             {
                       cout << "detructed " << endl;
                       cout << "x_d " << x <<endl;;
                       cout << "y_d  " << y << endl;;
             }
             c_const(const c_const & pp)
             {
                      cout <<"in copy constructor!!!"<< endl;
                      x = pp.x;
                      y = pp.y;
                      
             }
};

/*void func(c_const  cp)
{
     cout <<"hey ya "<<endl;
     cp.print();
     
 }*/
 
 c_const   func1  (int x , int y)
{
     c_const pq(5,6);
     cout << x << endl;
     cout << y << endl;
     return pq;    
}
int main()
{
    {c_const cc(10,20);
   // c_const cc1(cc);
    //c_const cc2 = cc1;
   // cc2 = cc1;
    cc.print();
   // cc1.print();
    //func(cc);
    c_const rr(7,8);
    rr = func1(2,3);
     }
    system("pause");
    return 0;
}

[yepMe]

And also what is the significance of 'const'? why was it throwing error without it?

[ResidentBiscuit]

The technical reason is a temporary object cannot bind to a non-const reference.

A more intuitive (?) reason is because you don't want to be changing that object, and marking objects as const is a good defensive programming technique.