CODEWITHSUNDEEP
c++classpointersparent
Francesco Noferi
Francesco Noferi

Reputation: 482

C++: parent pointer to class that includes the child

I have a class design problem that could simplified with this example:

// foo.h
#include "foo2.h" 
class foo
{
public:
    foo2 *child;
// foo2 needs to be able to access the instance
// of foo it belongs to from anywhere inside the class
// possibly through a pointer
};

// foo2.h
// cannot include foo.h, that would cause an include loop

class foo2
{
public:
    foo *parent;
// How can I have a foo pointer if foo hasn't been pre-processed yet?
// I know I could use a generic LPVOID pointer and typecast later
// but isn't there a better way?
};

Is there any other way other than using a generic pointer or passing the parent pointer to every call of foo2 members?

Upvotes: 1

Views: 4860

Answers (4)

zyndor
zyndor

Reputation: 1468

Forward declaration is a friend:

// foo.h
class foo2;

class foo
{
  foo2 *pFoo2;
};

// foo2.h
#include "foo.h"
class foo2
{
  foo *pFoo;
};

As Pubby says, though, classes that need to know about each other should probably just be one class, or maybe a class, with two members, both of which know about the parent class, but not as a two-way relationship.

As far as parenthood and being generic goes:

template  <class Parent>
class  ChildOf
{
public:
  // types
  typedef Parent  ParentType;

  // structors
  explicit ChildOf(Parent& p);
  ~ChildOf();

  // general use
  Parent&  GetParent();
  const Parent&  GetParent() const;

  void  SetParent(Parent& p);

private:
  // data
  Parent  *m_pParent;
};

/*
  implementation
*/
template  <class ParentType>
ChildOf<ParentType>::ChildOf(ParentType& p)
: m_pParent(&p)
{}

template  <class Parent>
ChildOf<Parent>::~ChildOf()
{}

template  <class ParentType>
inline
ParentType&  ChildOf<ParentType>::GetParent()
{
  return *m_pParent;
}

template  <class ParentType>
inline
const ParentType&  ChildOf<ParentType>::GetParent() const
{
  return *m_pParent;
}

template  <class ParentType>
void  ChildOf<ParentType>::SetParent(ParentType& p)
{
  m_pParent = &p;
}

Upvotes: 3

Caleb
Caleb

Reputation: 124997

Use a forward declaration to tell the compiler that foo2 is a class that will be defined subsequently.

class foo2;

class foo {
    foo2 *child;
};

class foo2 {
    foo *parent;
};

Upvotes: 0

pnezis
pnezis

Reputation: 12321

You should use forward declarations and include the header files in your cpp

// foo.h
#ifndef FOO_H_
#define FOO_H_
class foo2;

class foo
{
public:
    foo2 *child;
};
#endif

// foo2.h
#ifndef FOO_2_H_
#define FOO_2_H_
class foo;

class foo2
{
public:
    foo *parent;
};
#endif

Upvotes: 2

Pubby
Pubby

Reputation: 53037

You don't need to include the file if you're only using a pointer, and you won't have looping trouble if you include them in .cpp files:

// foo.h
class foo2; // forward declaration
class foo
{
public:
    foo2 *child;
};

// foo2.h
class foo;
class foo2
{
public:
    foo *parent;
};

//foo.cpp
#include "foo.h"
#include "foo2.h"

//foo2.cpp
#include "foo2.h"
#include "foo.h"

Although you may be better off by rethinking your design.

Upvotes: 9

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