Copy Constructor

Question

I am new to C++ programming, I got a doubt when I was reading a C++ regarding copy constructor. Why copy constructor will call when we pass object of class to a outside function as pass by value. Please go through my code as below.

#include "stdafx.h"
#include <iostream>
#include <conio.h>
using namespace std;

class Line
{
    public:
      int getLength( void );
      Line( int len );             // simple constructor
      Line( const Line &obj);      // copy constructor
      ~Line();                     // destructor

    private:
      int *ptr;
};

// Member functions definitions including constructor
Line::Line(int len)
{
    cout << "Normal constructor allocating ptr" << endl;
    ptr = new int;
    *ptr = len;
}

Line::Line(const Line &obj)
{
   cout << "Copy constructor allocating ptr." << endl;
   ptr = new int;
  *ptr = *obj.ptr; // copy the value
}

Line::~Line(void)
{
   cout << "Freeing memory!" << endl;
   delete ptr;
}

int Line::getLength( void )
{
   return *ptr;
}

void display(Line obj)//here function receiving object as pass by value 
{
  cout << "Length of line : " << obj.getLength() <<endl;
}

// Main function for the program
int main( )
{
    Line line(10);
    display(line);//here i am calling outside function
   _getch();
   return 0;
}

In the above the I am passing object of class as parameter and display function receiving it as pass by value. My doubt is when I pass object to a function which is not a member of class why copy constructor is calling. if I receive object as reference in display() function[i.e display(Line &Obj)] it is not calling the copy constructor. Please help me what is the difference.

Answer 1

When you pass something by value, the copy constructor is used to initialize the parameter that's passed -- i.e., what's passed is a copy of whatever you give, so of course the copy constructor is used to create that copy.

If you don't want the value copied, pass by (probably const) reference instead.