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

Way in C# that sprung in my mind imediately:

class A : IDisposable
{
    static Dictionary _typeCounts = new Dictionary();
    private bool _disposed = false;

    public static int GetCount() where T:A
    {
        if (!_typeCounts.ContainsKey(typeof(T))) return 0;

        return _typeCounts[typeof(T)];
    }

    public A()
    {
        Increment();
    }

    private void Increment()
    {
        var type = this.GetType();
        if (!_typeCounts.ContainsKey(type)) _typeCounts[type] = 0;
        _typeCounts[type]++;
    }

    private void Decrement()
    {
        var type = this.GetType();
        _typeCounts[type]--;            
    }

    ~A()
    {
        if (!_disposed) Decrement();
    }

    public void Dispose()
    {
        _disposed = true;
        Decrement();
    }
}

class B : A
{

}

And how to use it:

        A a1 = new A();
        Console.WriteLine(A.GetCount());
        A a2 = new A();
        Console.WriteLine(A.GetCount());            

        using(B b1 = new B())
        {
            Console.WriteLine(B.GetCount());
        }
        Console.WriteLine(B.GetCount());

The output could be done differently maybe. And its not pure OOP, but nor are C++ or Java examples in this thread. But it doesnt require a bit of code in inheriting class.

And dont forget about correcly disposing your objects!!