class Base {
    virtual Base* GetInterface(string type_I_want)=0;
    virtual void SomeBaseMethod()=0;
};
class Derv1: public Base {
    virtual Base* GetInterface(string  type_I_want) {
        if(type_I_want == "Derv1") {
            return this;
        } else {
            return NULL;
        }
    }
    virtual void SomeBaseMethod() {
        //implementation
    }
    void SomeDerv1SpecificMethod() {
        //implementation
    }
};
int main() {
    Base* ptr; //I got this from somewhere, I want a Derv1 pointer so I can call SomeDerv1SpecificMethod
    Derv1* dptr = (Derv1*)ptr->GetInterface("Derv1");
    if(dptr == NULL) {
        cout << "Umm... that wasn't actually a Derv1";
        return 0;
    }
    dptr->SomeDerv1SpecificMethod();
}