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#1 Guest_stankefa*


Reputation:

Small code which i don't understand

Posted 04 February 2011 - 03:32 AM

#include <iostream>
using namespace std;
class X {
public:
   X () : myID(id) {}
   X (const X& right) : myID(++id) {}
   friend ostream& operator<< (ostream&, const X&);
private:
   static int id;
   int myID;
};

int X::id = 0;

ostream& operator<< (ostream& os, const X& x) {
   return os<<x.myID;
}

X f (X x1, X& x2) {
   x1 = x2;
   return x2;
}
void main () {
   X x1, x2 = x1;
   X x3 = f(x1,x2);
   cout<<x1<<','<<x2<<','<<x3;
}



What I don't understand is how the 25. line of the program works (3rd line of main() function). I know that it should use a copy constructor, and it does it at first,but after finishing copy constructor function it calls the f function and after that initializes x3 with the returned value of function f. Why does it call function f at all after copy constructor? Isn't it supposed to call only copy constructor?
What i'm trying to say is that I don't understand how copy constructor works when I have a function on the right side... Please help.

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Replies To: Small code which i don't understand

#2 oscode  Icon User is offline

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Re: Small code which i don't understand

Posted 04 February 2011 - 05:02 AM

Here is what happens step by step, assuming no compiler opimizations:
x1 is constructed
x2 is copy constructed
f1 is called
x1 is copy constructed into a local x1 (pass by value)
x1 is assigned x2
f2 returns - x2 is copy constructed to x3

This post has been edited by oscode: 04 February 2011 - 05:03 AM

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