Why we use string library while using string data type?

[ZahoorKhan]

According to book from which I am learning C++, you should add string library before using string data type, but suddenly when I missed adding string library the program was still working fine why?
( https://i.imgsafe.org/2bb7b68.png )

#include <iostream>
#include <string>
using namespace std;
int main()
{
 string day = "Monday";
 
 cout << day << " Go to school\n";
 
 return 0;
}

and also working without string library:-

#include <iostream>
using namespace std;
int main()
{
 string day = "Monday";
 
 cout << day << " Go to school\n";
 
 return 0;
}

So, why we write a additional line if program is working fine without it, or it is doing something else that I am missing to know?

[TarikNeaj]

http://www.cplusplus.com/forum/general/4202/

Edit: Might I add that both my and @cire's link can be found with a simple google search.

[cire]

A little more informative:
http://stackoverflow.com/questions/9539650/why-does-omission-of-include-string-only-sometimes-cause-compilation-failur