class Base {
private:
    // pure virtual member function used for initialization of derived classes.
    virtual void construct() = 0;
public:
    Base() {
        // wrong: the virtual table of `Derived` has not been initialized yet. So this
        // call will resolve to `Base::construct`, which cannot be called as it is a pure
        // virtual function.
        construct();
    }
};

class Derived : public Base {
    int field;

    void construct() override {
        field = 1;
    }
};