Counting instances of individual derived classes

Question

I'd like to be able to count instances of classes that belong in the same class hierarchy.

For example, let's say I have this:

class A;
class B: public A;
class C: public B;

and then I have this code

A* tempA = new A;
B* tempB = new B;
C* tempC = new C;
C* tempC2 = new C;

printf(tempA->GetInstancesCount());
printf(tempB->GetInstancesCount());
printf(tempC->GetInstancesCount());

the result of that should print
1
1
2

The counting, ideally, should be done internally. Each class, and not some kind of manager, should know how many instances it has.

Any ideas?

Thanks!

Answer 1

There is a problem with proposed solutions: when you create B you A constructor will be called automatically and thus increment count of A.

class A
{
public:
    A(bool doCount = true)
    {
        if (doCount)
            ++instanceCount_;
    }

    static std::size_t GetInstanceCount()
    {
        return instanceCount_;
    }

    virtual ~A(){}
private:
    static std::size_t instanceCount_;
};

class B: public A
{
public:
    B(bool doCount = true):A(false)
    {
        if (doCount)
            ++instanceCount_;
    }

    static std::size_t GetInstanceCount()
    {
        return instanceCount_;
    }
private:
    static std::size_t instanceCount_;
};

std::size_t A::instanceCount_ = 0;
std::size_t B::instanceCount_ = 0;

Answer 2

Directly off my head:

• Create an integer static field in each class. Watch out for integer overflow.
• Initialize it to 0 in an emulated static constructor.
• Increment it at each (nonstatic) constructor body. Decrement it in the destructor.
• GetInstancesCount() is a static function that returns the value of your integer static field.

Note: See Mykola's comments. This would print 4 for A, 3 for B and 2 for C i.e. it would count one instance of B as "one A and one B", and one C as "one A, one B and one C". Which is in a way true, but is not what the question asks for. In other words, my answer is wrong :)