Why is the copy constructor called when we return an object from a method by value

why copy constructor is called when we return an object from a method by value. please see my below code in that i am returning an object from a method while returning control is hitting the copy constructor and then returning. i am not understood following things:
1) why it is calling copy constructor.
2)which object is passing implicitly to copy constructor,
3)to which object copy constructor will copy the content,
4)what is the necessity of copying the object content while returning. so plz help.

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

class ClassA
{
   int a, b;
     public:
   ClassA()
   {
     a = 10;
     b = 20;
   }
   ClassA(ClassA &obj)
   {
    cout << "copy constructor called" << endl;
   }
 };

 ClassA function (ClassA &str)
 {
  return str;
 }

 int main ()
 {
   ClassA str;
   function(str);
   //function(str);
   return 0;
 }

The copy constructor is called because you call by value not by reference. Therefore a new object must be instantiated from your current object since all members of the object should have the same value in the returned instance. Because otherwise you would be returning the object it self, which would be returning by reference. In this case modifying the reference object would change the original as well. This is generally not a behavior one wants when returning by value.