class Base
{
    int m_var;

    public:
    
    virtual void mf_0() = 0;  //The contract. The compulsory Customization

    virtual void mf_1();    //The customizable "default behaviour" 

    int m_f2(); // the invariant. Redefining this in derived class would mean
                // the derived class is-not-really-a base class .. or that this 
                // non virtual function should really be virtual, i.e. customizable

    virtual ~Base() {};
};

class Derived : public Base
{
    public:

    virtual void mf_0()
    {

    }

};

int main()
{
    Derived D1;
}

This produces undefined reference to `vtable for Base’