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: pu         


        

Answer 1

One way to get around the double-counting when counting objects in the constructor is to count the objects at the point of need, rather than in the constructor, using RTTI. This is minimally intrusive:

#include 
#include 
#include 
#include 


class A
{

public:

    A();
    virtual ~A() { }

};

class B: public A
{

public:

    virtual ~B() { }
};

class C: public B
{

public:

    virtual ~C() { }

};

template
struct TypeIdsEqual: public std::binary_function
{
    bool operator() (const T& obj1, const T& obj2) const
    {
        return typeid(*obj1) == typeid(*obj2);
    }
};

struct Counter
{
    static std::vector objects;

    static void add(A* obj)
    {
        objects.push_back(obj);
    }

    static int count(A* obj)
    {
        return std::count_if(objects.begin(), objects.end(),
                             std::bind1st(TypeIdsEqual(), obj));
    }

};

std::vector Counter::objects;

// One intrusive line in the base class constructor.  None in derived classes...
A::A()
{
    Counter::add(this);
}

int main(int *argc, char* argv[])
{
    A* a  = new A;
    B* b  = new B;
    C* c  = new C;
    C* c2 = new C;
    std::cout << Counter::count(*a) << std::endl;  // Output: 1
    std::cout << Counter::count(*b) << std::endl;  // Output: 1
    std::cout << Counter::count(*c) << std::endl;  // Output: 2
}