count class instances for every derived class

Question

is there any way to make all derived classes count their instances?How (write code in one of C++, C#, Java)?

Imagine I\'ve got access to the root class (e.g. object), an

Answer 1

In C++, you can do it with a template base class. Basically it is a mixin, so it does still require each class to co-operate by inheriting from the mixin:

// warning: not thread-safe
template 
class instance_counter {
  public:
    static size_t InstancesCount() { return count(); }
    instance_counter() { count() += 1; }
    instance_counter(const instance_counter&) { count() += 1; }
    // rare case where we don't need to implement the copy assignment operator.
  protected:
    ~instance_counter() { count() -= 1; }
  private:
    static size_t &count {
        static size_t counter = 0;
        return counter;
    }
};

class my_class: public instance_counter {};

Since each class using the template has a different base class, it has a different count function and hence a different copy of the static variable counter.

The trick of inheriting from a template class that's instantiated using the derived class as a template parameter is called CRTP.