inheritance

class Element {
public:
    virtual void foo() = 0;
    virtual void bar() {}
};

class Point : Element {
private:
    vector<float> coords;

public:
    virtual void foo();
};

class Quad : Element {
private:
    vector<Point> points;

public:
    virtual void foo();
};

/*
How does one call a function in a derived class using a pointer to the
base class ... where it doesn't make sense to add the function (set_coords())
to the base (i.e., I don't make it virtual because only the Point class
needs set_coords()).
*/

    auto elem = ElementFactory::instance()->create("Point");
    elem->set_coords(v);

/*
The above would complain that class Element has no member named set_coords(v);
*/