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 Java you can implement the counting function to the common super class of your hirachy.

This base class contains a Map - associating the classes to the number of instances. If a instance of base or one of its sub class is created, then the constructor ins invoked. The constructor increase the number of instances of the concreate class.

import java.util.Map.Entry;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicInteger;

public class Base {

    /** Threadsave counter */
    private static final ConcurrentMap, AtomicInteger>
       instancesByClass 
       = new ConcurrentHashMap, AtomicInteger>(
            10);

    /** The only one constructor of base */
    public Base() {
        Class concreateClass = this.getClass();
        AtomicInteger oldValue = instancesByClass.putIfAbsent(concreateClass,
                new AtomicInteger(1));
        if (oldValue != null) {
            oldValue.incrementAndGet();
        }
    }

    /* DEMO starts here */
    public static class SubA extends Base{
    }

    public static class SubB extends Base{
    }

    public static class SubSubA extends SubA{
    }


    public static void main(String[] args) {
        printNumbers();
        new SubA();
        new SubA();

        new SubB();

        new SubSubA();

        printNumbers();
    }

    private static void printNumbers() {
        // not thread save!
        for (Entry, AtomicInteger> item : instancesByClass
                .entrySet()) {
            System.out.println(item.getKey().getName() + "  :  "
                    + item.getValue());
        }
    }
}