Implementation of the C++11 feature nullptr_t and an example of why it works better than the C #define NULL

nullptr_t.cpp

struct nullptr_t
{
  template <typename T>
  operator T*() { return static_cast<T*>(0); }
};

nullptr_t nullptr;


#include <iostream>

const char *foo(int *) { return "ptr"; }
const char *foo(int) { return "int"; }

#define SIMPLE_NULL 0

#define PRINT(expr) std::cout << #expr ": " << expr << std::endl

int main()
{
  PRINT(foo(0));           // "int"
  PRINT(foo((int*)0));     // "ptr"
  PRINT(foo(NULL));        // compile error in GCC 4.6.3, ambiguous call
  PRINT(foo(SIMPLE_NULL)); // "int"
  PRINT(foo(nullptr));     // "ptr"
}