Curiously Recurring Template Pattern

curiously_recurring_template_pattern.cpp

////
//  Curiously Recurring Template Pattern
#include <stdio.h>


template <typename T>
struct Counter {
  //  Static class members variables
  static int objects_created;
  static int objects_alive;

  Counter(){
    ++objects_created;
    ++objects_alive;
  }

  protected:
  ~Counter(){  // Don't let people delete via pointer
    --objects_alive;
  }
};

//  Set the initial value, this will come not come into effect 
//  unless Counter's template is instantiated, and will instantiate
//  a separate template class for each T.
template <typename T> int Counter<T>::objects_created(0);
template <typename T> int Counter<T>::objects_alive(0);


//  X and Y are derived from a base class which takes themselves as
//  a template argument to Counter, this will run the above static
//  initializations, and run Counter<X>'s constructors, which are 
//  separate classes.
//  When an X or Y is destructed, Counter<>'s destructor will run too.
class X : Counter<X> {};

class Y : Counter<Y> {};



int main(int argc, char **argv){
  printf("Interesting Object Counter with Templates\n");

  //  Create some Xs on the stack.
  X x1;
  X x2;
  X x3;

  {
    //  Create some Ys inside an inner scope.
    Y y1;
    Y y2;

    printf("X Objects Created %d\n", Counter<X>::objects_created);
    printf("X Objects Alive %d\n", Counter<X>::objects_alive);
    printf("Y Objects Created %d\n", Counter<Y>::objects_created);
    printf("Y Objects Alive %d\n", Counter<Y>::objects_alive);
    //  The Ys are destroyed here.
  }

  printf("X Objects Created %d\n", Counter<X>::objects_created);
  printf("X Objects Alive %d\n", Counter<X>::objects_alive);
  printf("Y Objects Created %d\n", Counter<Y>::objects_created);
  printf("Y Objects Alive %d\n", Counter<Y>::objects_alive);

  /*  Outputs 
    Interesting Object Counter with Templates
    X Objects Created 3
    X Objects Alive 3
    Y Objects Created 2
    Y Objects Alive 2
    X Objects Created 3
    X Objects Alive 3
    Y Objects Created 2
    Y Objects Alive 0
  */

  return 0;
}