C++: design advice

Question

I need some advice on the design of my class hierarchy. The 'skeleton' of my current design is

template <class X>
class BASE {
  public:
    virtual void f() {
        x_.f2(m_);
    }

    void g() {
        /* do stuff here*/
        x_.g();
    }

    /* more member functions here*/

  protected:
    X x_;
    int m_;
};

template <class X>
class DERIVED : BASE<X> {
    public:
      virtual f() override {
         x_.f1();
      }

     protected:
     using BASE<X>::x_;
     using BASE<X>::m_;
};

and finally I have two more classes like this

struct X1 {
  void f1();
  void g();
};

struct X2 : X1 {
void f2(int m);
};

I would like to be able to create an instance of DERIVED<X1> . To do that, the compiler will create first an instance of BASE<X1> , in which case it will complain that X1 does not have a member function f2 (even though in practice it would never be called since the calling function f() is virtual).

I understand that this is a bad design since in order to have a templated class, the template arguments must have the same interface. In my case X1 and X2 have a common interface but X2 has more functionality, which creates the above problem. I understand I could create an f2 function in X1 which does nothing, but ideally I would like to avoid that.

I would welcome any suggestions for improving the design. Thanks in advance!

Answer 1

First advice is not to use protected atributes. Is is breaking incapsulation.

The second advice is to make virtual void f()=0 pure virtual without implementation. In such there will be no error in the derived class.

Answer 2

Avoid deriving from a concrete class. Lift interfaces/ABCs to the top, make all concrete classes leaves. It's a general advice for many kinds of design difficulties.

In your particular case, instead of this:

             BASE    // concrete
              | 
           DERIVED

use this:

             BASE   // fully abstract
            /   \
BASIC_DERIVED    ADVANCED_DERIVED

Now f is pure virtual in BASE and all is well.