Preventing Virtual Method Implementation in C++

Question

I have the following class hierarchy in C++:

class Base {
    virtual void apply() = 0;
};

class Derived : public Base {
    virtual void apply() {
        // implementation here that uses derived_specialty
    }

    virtual void derived_specialty() = 0;
};


class Implementation : public Derived {   
    virtual void derived_specialty() {
        // implementation
    }
};

I'd like to guarantee that classes at the level of Implementation don't supply their own implementation of apply, and that they only implement derived_specialty. Is there any way to guarantee that classes inheriting from Derived will not implement apply, so that the Derived::apply implementation is used? My understanding is that in C++, a method made virtual in the Base class is virtual all the way down the inheritance hierarchy, but if there are any tricks in C++ to accomplish, I'd be interested in hearing about them.

I'm always surprised by the things that are allowed by C++, so I thought it was worth asking. :)

Answer 1

You could make Implementation a delegate class rather than a specialization of Derived

class Derived : public Base
{
    Derived()

    void apply() 
    {
        //whatever, delegate to impl class instance
        impl->apply_specialization();
    }


    Impl* impl;
};

class Impl : public WhateverImplInterface
{
      void apply_specialization(){}
};

Then the implementation doesn't have access to the apply function and is separated from the hierarchy. The Derived class is then parameterized by an instance of Impl class.

Answer 2

You can kind of do it by composition:

class Base {
    virtual void apply();
};

class Derived : public Base {

    class IImplementation {
        virtual void derived_specialty() = 0;
    };

    IImplementation& m_implementation;

    Derived(IImplementation& implementation)
        : m_implementation(implementation)
    {}

    virtual void apply() {
        // implementation here that uses m_implementation.derived_specialty
    }

};


class Implementation : Derived::IImplementation {   
    virtual void derived_specialty() {
        // implementation
    }
};

Other classes can still subclass Derived and override the apply method, but your Implementation class is no longer one of these classes.