C++ one base class multiple child classes

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C++11

Hi,
i like to do some basic stuff in c++. I want to have one solely base class and many child class whome all are derived from the base class. So in the base class i like to have a member that just counts up each time there is a new child class. So the base class should be for all child classes the same and not initialize several times again as a new one. Can anyone give me a hint.

Thanks

What I have tried:

class App {
private:

	int	count = 0;

public:
	App(void);
	App(int);
	virtual ~App(void);

	int getCount(void);
};

class Derived : virtual public App {
public:
    Derived();

}

int main()
{
    Derived d1;//count 1
    Derived d2;//count should be 2 now
}