swap fails in case of int and works in case of string

Question

#include <iostream>
#include <string>
#include <algorithm>    
int main()
{
 std::string str1 = "good", str2 = "luck";
 swap(str1,str2); /*Line A*/
 int x = 5, y= 3;
 swap(x,y); /*Line B*/
}

If I comment Line B the code compiles(http://www.ideone.com/hbHwf) whereas commenting Line A the code fails to compile(http://www.ideone.com/odHka) and I get the following error:

error: ‘swap’ was not declared in this scope

Why don't I get any error in the first case?

Answer 1

swap(str1, str2) works because of Argument dependent lookup

P.S: Pitfalls of ADL

Answer 2

You're not qualifying swap; it works when passing in std::string objects because of ADL, but as int does not reside in namespace std you must fully qualify the call:

std::swap(x, y);

or use a using declaration:

using std::swap;
swap(x, y);