adding method to derived class and calling it from base pointer

Question

Let's say I have the following class hierarchy:

class A
{
public:
    virtual ~A();
};

class B : public A
{};

class AUser
{
public:
    virtual ~AUser();
protected:
    A* m_pA;
};

class BUser : public AUser
{};

Now I want to add a method to B which is not relevant for A and will be called only from BUser.

class B : public A
{
public:
    void addedFunctionality()
    {
        // do something
    }
};

To call the added method I have two options:

1. Add an empty method to A:

   class A
   {
   public:
       virtual ~A();
       void addedFunctionality()
       {
           // do nothing
       }
   };
   
   class BUser : public AUser
   {
       void useB()
       {
           m_pA->addedFunctionality();
       }
   };
   

2. call the added method using downcast:

   class BUser : public AUser
   {
       void useB()
       {
           static_cast<B*>(m_pA)->addedFunctionality();
       }
   };
   

I know downcasts should be avoided but I don't like the first option as it bloats A's code for no reason.

Is there any other design alternative?

Answer 1

The simplest solution looks like this (also suggested by Ulrich Eckhardt):

class AUser
{
public:
    AUser(A* a)
    {
      m_pA = a;
    }
protected:
    A* m_pA;
};

class BUser : public AUser
{
public:
    BUser(B* b) : AUser(b)
    {
      m_pB = b;
    }

    void useB()
    {
        m_pB->addedFunctionality();
    }
protected:
    B* m_pB;
};

Less clean, but you may also consider this:

class BUser : public AUser
{
public:
    BUser(B* b) : AUser(b)
    {
    }
    void useB()
    {
        getB()->addedFunctionality();
    }
protected:
    B *getB()
    {
      return dynamic_cast<B*>(m_pA);
    }

};