Use CRTP to record the object counter

Counter_CRTP.cpp

template <typename T>
struct counter
{
    static int objects_created;
    static int objects_alive;
 
    counter()
    {
        ++objects_created;
        ++objects_alive;
    }
 
    counter(const counter&)
    {
        ++objects_created;
        ++objects_alive;
    }
protected:
    ~counter() // objects should never be removed through pointers of this type
    {
        --objects_alive;
    }
};
template <typename T> int counter<T>::objects_created( 0 );
template <typename T> int counter<T>::objects_alive( 0 );
 
class X : counter<X>
{
    // ...
};
 
class Y : counter<Y>
{
    // ...
};