Why is the copy constructor called when we pass an object as an argument by value to a method?

Question

Why is the copy constructor called when I pass an object as an argument by value to a function? Please see my below code: I am passing an object of a class as an argument by value to a function display(), but it is calling the copy constructor before control is hitting the display() function. I don't understand why.

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

class ClassA
{
    private:
       int a, b;
    public:
       ClassA()
       {
           a = 10, b = 20;
       }
       ClassA(ClassA &obj)
       {
           cout << "copy constructor called" << endl;
           a = obj.a;
           b = obj.b;
       }
};
void display(ClassA obj)
{
   cout << "Hello World" << endl;
}
int main()
{
   ClassA obj;
   display(obj);
   return 0;
}

Answer 1

To elaborate the two answers already given a bit:

When you define variables to be "the same as" some other variable, you have basically two possibilities:

ClassA aCopy = someOtherA; //copy
ClassA& aRef = someOtherA; //reference

Instead of non-const lvalue references, there are of course const references and rvalue references. The main thing I want to point out here is, that aCopy is independent of someOtherA, while aRef is practically the same variable as someOtherA, it's just another name (alias) for it.

With function parameters, it's basically the same. When the parameter is a reference, it gets bound to the argument when the function is called, and it's just an alias for that argument. That means, what you do with the parameter, you do with the argument:

void f(int& iRef) {
  ++iRef;
}

int main() {
  int i = 5;
  f(i); //i becomes 6, because iRef IS i
}

When the parameter is a value, it is only a copy of the argument, so regardless of what you do to the parameter, the argument remains unchanged.

void f(int iCopy) {
  ++iCopy;
}

int main() {
  int i = 5;
  f(i); //i remains 5, because iCopy IS NOT i
}

When you pass by value, the parameter is a new object. It has to be, since it's not the same as the argument, it's independent. Creating a new object that is a copy of the argument, means calling the copy constructor or move constructor, depending on whether the argument is an lvalue or rvalue. In your case, making the function pass by value is unnecessary, because you only read the argument.

There is a Guideline from GotW #4:

Prefer passing a read-only parameter by const& if you are only going to read from it (not make a copy of it).