Access child members within parent class, C++

Question

I am facing a situation where I need to access child member variables inside the parent class. I know this is against OO principles but I have to deal with a scenario where

Answer 1

Would making an intermediate abstract class that contains the childMember you need to access be an option?

If so:

class Parent
{
    virtual void DoSomething() //Parent must be a polymorphing type to allow dyn_casting
    {
        if (AbstractChild* child = dynamic_cast<AbstractChild*>(this)) {
            child->childMember = 0;
        } else //Its not an AbstractChild
    }
}

class AbstractChild : Parent 
{ 

     int childMember; 
     virtual void DoSomething() = 0;
}

class Child1 : AbstractChild {
     virtual void DoSomething() { }
}

Answer 2

static_cast<Child*>(this)->childMember = 0; should work.

Answer 3

You can use the curiously recurring template pattern to achieve this.

template<typename T>
class Parent
{
    void DoSomething()
    {
        // This is what I need
        T::childMember = 0;
    }

    virtual ~Parent() {}
};

class Child1 : Parent<Child1>
{
  int childMember;

  friend class Parent<Child1>;
};