nullptr

The nullptr is a new literal of type std::nullptr_t introduced in C++11. It provides a specific expression for null pointers. Before C++11, it was common to use 0, 0x0 or the preprocessor macro NULL to indicate a pointer to nothing. This is not type save as the following example illustrates:

void printMe(int i) { cout << "I am an integer: " << i << endl; }
void printMe(int *i) { cout << "I am a pointer to an integer: " << i << endl; }

int main() {
  printMe(0);
  printMe(NULL);      // error: call to 'printMe' is ambiguous
  printMe(nullptr);   // works fine
}

To be backward compatible, you can: #define nullptr NULL (ROOT already does this for you).