Counting number of objects of different classes

Question

I would like to count the number of objects that my program creates over its lifetime. Based on the solution provided here:

how to count the number of objects created in c++

I have the following code:

#include <iostream>
using namespace::std;
using std::cout;
using std::endl;

template <typename T>
struct counter
{
    counter()
    {
        objects_created++;
        objects_alive++;
    }

    virtual ~counter()
    {
        --objects_alive;
    }
    static int objects_created;
    static int objects_alive;
};
template <typename T> int counter<T>::objects_created(0);
template <typename T> int counter<T>::objects_alive(0);

class X : counter<X>
{
    int a;
};

class Y : counter<Y>
{
    int b;
};

void fooX(class X x) {
    cout << "passing object" << endl;
}

void fooY(class Y& y) {
    cout << "passing reference" << endl;
}

int main()
{
    cout << "created: " << " X:" << counter<X>::objects_created << " Y:" << counter<Y>::objects_created << endl;
    cout << "alive: " << " X:" << counter<X>::objects_alive << " Y:" << counter<Y>::objects_alive << endl;
    X x;
    Y y;
    cout << "created: " << " X:" << counter<X>::objects_created << " Y:" << counter<Y>::objects_created << endl;
    cout << "alive: " << " X:" << counter<X>::objects_alive << " Y:" << counter<Y>::objects_alive << endl;
    fooX(x);
    fooY(y);
    cout << "created: " << " X:" << counter<X>::objects_created << " Y:" << counter<Y>::objects_created << endl;
    cout << "alive: " << " X:" << counter<X>::objects_alive << " Y:" << counter<Y>::objects_alive << endl;
    int ui;
    cin >> ui;
}

I expected that since x is passed by value, a copy of it is made inside fooX making the total number of objects of class X to be 2 while since y is passed by reference, the total number of objects of class Y to be 1.

Yet, the output of the code is as follows:

created:  X:0 Y:0
alive:  X:0 Y:0
created:  X:1 Y:1
alive:  X:1 Y:1
passing object
passing reference
created:  X:1 Y:1
alive:  X:0 Y:1

Why does not the number of Xs created be 2?

Answer 1

A copy constructor is automatically added to your counter class, and that automatically created copy constructor doesn't increment your static variables.

Write a copy constructor which does that:

counter(counter const&)
{
    objects_created++;
    objects_alive++;
}

---

Note that your destructor should probably not be virtual, unless you intend to delete dynamically created instances of derived classes via pointers or references to counter. As it stands, it's just premature pessimisation because it needlessly increases the size of your objects.