C++ string size()

#include <iostream>
#include <string>
using namespace std;
int main()/*  w  ww. jav a  2  s  . c  o m*/
{
   string s1("Quick! from book 2s.com.");
   string s2("Lord test ");
   string s3("Don't test");
   s1.erase(0, 7);
   s1.replace(9, 5, s2);
   s1.replace(0, 1, "s");
   s1.insert(0, s3);
   s1.erase(s1.size()-1, 1);
   s1.append(3, '!');
   int x = s1.find(' ');         //find a space
   while( x < s1.size() )        //loop while spaces remain
   {
      s1.replace(x, 1, "/");     //replace with slash
      x = s1.find(' ');          //find next space
   }
   cout << "s1: " << s1 << endl;
   return 0;
}