How do you implement this Python idiom in C++

[benben]

You are heading the wrong way...

There are a number of ways to implement this but the easiest I can think 
of is to use RTTI.

To get around with counting sub-objects you can rely on virtual 
inheritance, which only happens at the top of the inheritance tree.

Here is a simple demo:

#include <iostream>
#include <typeinfo>
#include <map>

class count
{
   public:
     typedef std::map<const std::type_info*, unsigned int> counts_t;
     static counts_t counts;

     const std::type_info* ti;

     static unsigned int get_count(const std::type_info& c)
         {return counts[&c];}

     count(const std::type_info& c):ti(&c){++counts[ti];}
     ~count(){--counts[ti];}
};

count::counts_t count::counts;

class c1: virtual private count
{
     public: c1(): count(typeid(c1)){}
};

class c2: public c1, virtual private count
{
     public: c2(): count(typeid(c2)){}
};

int main()
{
     c1 t1[3];
     c2 t2[5];

     std::cout << count::get_count(typeid(c1)) << "\n";
               << count::get_count(typeid(c2)) << "\n";

     return 0;
}