C++ String类

#include <iostream>
#include <string>

using namespace std;

int main () {
   string str1 = Hello;
   string str2 = World;
   string str3;
   int  len ;

   // copy str1 into str3
   str3 = str1;
   cout << str3 :  << str3 << endl;

   // concatenates str1 and str2
   str3 = str1 + str2;
   cout << str1 + str2 :  << str3 << endl;

   // total length of str3 after concatenation
   len = str3.size();
   cout << str3.size() :   << len << endl;

   return 0;
}