c_str() == NULL not working

Question

I was going through legacy code to fix a problem and found out that checking c_str in the following manner does not work. However if I change the comparison to string.empty() it works. So my problem is solved, but I am not sure why the c_str comparison does not work.Any idea why?
Thanks!
I am using - gcc version 4.1.2 20080704 (Red Hat 4.1.2-46).

#include <iostream>  
using namespace std;  
int main()
{
   string a="";
   if (a.c_str() == NULL)
   {
     cout <<"char says empty"<<endl;
   }
   if (a.empty())
  {
    cout << "string says empty"<<endl;
  } 
 }

The output is-
string says I am empty

Answer 1

a is not null in this case. The string object has an internal pointer, which is not null, and that pointer is what c_str returns.

To verify this, try

printf("%zu\n", a.c_str());

and you will get a real address.

Answer 2

A null terminated string is never NULL, it always points to valid memory with a null terminator. The empty string is just a single \0. Since c_str() is contracted to return a null terminated string it can never return NULL. The correct way to test for an empty string is indeed empty().