Question

Given a class, I would like to limit the number of objects created from this class to a given number, say 4.

Is there a method to achieve this?

Answer 1

The basic idea is to count the number of created instances in some static variable. I would implement it like this. Simpler approaches exist, but this one has some advantages.

template<class T, int maxInstances>
class Counter {
protected:
    Counter() {
        if( ++noInstances() > maxInstances ) {
            throw logic_error( "Cannot create another instance" );
        }
    }

    int& noInstances() {
        static int noInstances = 0;
        return noInstances;
    }

    /* this can be uncommented to restrict the number of instances at given moment rather than creations
    ~Counter() {
        --noInstances();
    }
    */
};

class YourClass : Counter<YourClass, 4> {
}