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 use a global Multiset:

import com.google.common.collect.ConcurrentHashMultiset;

public abstract class InstanceCounted {

    protected InstanceCounted() {
        COUNT_MAP.add(this.getClass());
    }

    protected static final ConcurrentHashMultiset> COUNT_MAP =
        ConcurrentHashMultiset.create();

}

Alternatively you can use a Map if you don't want the dependency on guava.

Note: this only tracks instance creation, not garbage collection, so the count will never decrease. You can also track collection using PhantomReferences if you are willing to take a performance hit:

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;

import com.google.common.collect.HashMultimap;
import com.google.common.collect.Multimap;
import com.google.common.collect.Multimaps;

public abstract class InstanceCounted {

    public static int getInstanceCount(Class clazz) {
        reap();
        return INSTANCES.get(clazz).size();
    }

    protected InstanceCounted() {
        reap();
        INSTANCES.put(getClass(), new CountingReference(this));
    }

    static final Multimap, CountingReference> INSTANCES =
        Multimaps.synchronizedSetMultimap(HashMultimap., CountingReference>create());

    static final ReferenceQueue QUEUE =
        new ReferenceQueue();

    private static void reap() {
        Reference ref;
        while ((ref = QUEUE.poll()) != null) {
            ((CountingReference) ref).clear();
        }
    }

    private static class CountingReference extends PhantomReference {

        public void clear() {
            super.clear();
            INSTANCES.remove(clazz, this);
        }

        CountingReference(InstanceCounted instance) {
            super(instance, QUEUE);
            this.clazz = instance.getClass();
        }

        private final Class clazz;

    }

}