Re: conversion operator problem

i encounter a problem as following:

#include <iostream>
#include <string>
using namespace std;

class X
{
public:
   X(const string& s, int i):s_(s),i_(i)
   {}

   operator string ()
   {
       return s_;
   }

   operator int ()
   {
       return i_;
   }

private:
   string s_;
   int i_;
};

int main()
{
   string s("abc");
   int i(10);

   X x("abc", 10);

   if (x == i)
   {
       cout<<"int ok"<<endl;
   }

   return 0;
}

================================
x can be compared with the integer i and the output is "int ok",
but when i wanna compare with string s, it is a compile error:
if (x == s)
{
   ...
}

If convert x to string explicitly, it is ok:
if ((string)x == s)
{
   ...
}

Why couldn't it do a implicit type conversion to string like build in
type?