Pass parent class to constructor of another class

Question

I have several classes in my application, and I'd like to keep the interaction amongst classes restricted to the base classes. To make this concrete, right now I want to do something like this:

    class A
    {
    data 

    public:
       void method();
    };

    class A_child : public A
    {
    //stuff
    };

    class B 
    {
    public:
        B(class A *a);
    };
   class B_child : public B
   {
   public:
        B_child(class A *a);
        B_method(); // does some stuff with data in A and B.
   };

Although in actuality each of the classes have separate header files, so there's a lot of includes back and forth...

In the cpp file for A_child I want to be able to use B_child (or B, for that matter). But I don't know how to avoid having to call something like

void A_child::method()
{
   // stuff

    B_child *example_B;
    example_B = new B_child(this);
    example_B->B_method();

  // more stuff
}

It doesn't have to be

example_B = new B_child(this);

necessarily, it just has to have some way of passing the parent class of A_child into the constructor of B or B_child.

What I'd like to do is avoid having to define B_child as something like:

   class B_child : public B
   {
   public:
        B_child(class A_child *a_child);
        B_method();
   };

instead of what I have above, because this seems to imply that B_child has to work with A_child rather than with A. Although this is true, there is nothing in the material that B or B_child work with that require A_child - the data in A, not A_child, suffice.

I am guessing there is a way to do this in C++? Could someone give me an example for how something like this works or a link to a page that might explain how to do this?

Thanks.

What's the question here exactly? "How can I make B_child take an A* in the constructor?" If so I don't see what's preventing you. — Taylor Brandstetter, Jun 06 '13 at 20:20
`B_child(class A *a);` What's `class` doing in there? This is not legal code. Do you mean `const`? `B_child(A *a);` is also legal. — Matt Phillips, Jun 06 '13 at 20:20
@MattPhillips, It's just like `struct A *a` (so basically C). http://coliru.stacked-crooked.com/view?id=c6161d161a16b1ba1868dceb5edf95ab-5bfb4a3ecda91a191e93557cbeaa6247 — chris, Jun 06 '13 at 20:41

score 0 · Answer 1 · answered Jun 06 '13 at 20:20

0

If the B_child constructor takes an A *, you can pass an A_child * to it, since A_child "is-an" A (via public inheritance). Did you try doing this and encounter an error?

answered Jun 06 '13 at 20:20

Adam H. Peterson

4,511
20
28

score 0 · Answer 2 · answered Jun 06 '13 at 20:29

You can add parent field in A class:

class A {
  B_child* parent;
public:
  void setParent(B_child* p) {
    parent = p;
  }
}

next in B_child constructor:

B_child::B_child(A* a) {
  // stuff
  a->setParent(this);
}

then you access the parent object inside A_child method:

void A_child::method() {
  if(parent)
    parent->B_method();
}

Pass parent class to constructor of another class

2 Answers2