How do I define two C++ classes with pointers to each other?

**lowerlogic** · 06-21-2007

I'm trying to make two classes that hold a pointer or variable of the other, like this (pseudo-code):
---unit.h---

Code:

#ifndef UNIT_H
#define UNIT_H
#include "action.h"
#include <vector>
using namespace std;

class Unit {
  vector<Action> actions;
  int hp;
};
#endif

---action.h---

Code:

#ifndef ACTION_H
#define ACTION_H
#include "unit.h"

class Action {
  Unit *target_unit;
  int action_type;
};
#endif

But since one is defined inside the other, it can't compile the program. It reads in action.h and sees unit.h is included, when it read unit.h, it says Action is not defined. Is there a way around error?

**laserlight** · 06-21-2007

You can use a forward declaration:

Code:

#ifndef ACTION_H
#define ACTION_H

class Unit;

class Action {
  Unit *target_unit;
  int action_type;
};
#endif

**kroiz** · 06-21-2007

you do a forward reference.

Code:

#ifndef UNIT_H
#define UNIT_H
class Action;
#include <vector>
using namespace std;

class Unit {
  vector<Action*> actions;
  int hp;
};
#endif

Code:

#ifndef ACTION_H
#define ACTION_H
class Unit;

class Action {
  Unit *target_unit;
  int action_type;
};
#endif

notice that the vector now has a pointer to Action.
the include will go into the cpp file.

**laserlight** · 06-21-2007

kroiz, why do you recommend that the vector should store Action pointers?

**lowerlogic** · 06-21-2007

Thanks for the fast replies.

**kroiz** · 06-21-2007

Originally Posted by laserlight

kroiz, why do you recommend that the vector should store Action pointers?

I think that if it is not a pointer then he can not use forward reference it must use include.
I am pretty sure. Am I wrong?

**laserlight** · 06-21-2007

I think that if it is not a pointer then he can not use forward reference it must use include.
I am pretty sure. Am I wrong?

You are right, but the Action class has a Unit* member variable, so the forward declaration can be used there and action.h can continue to be included in unit.h, requiring no change to the vector.

**kroiz** · 06-21-2007

yes ofcourse you are right.
I was in such a hurry to post first because it is my first reply to someone on the forum that I missed the details.
and you beat me to it after all.
not fair )-:

**laserlight** · 06-21-2007

Haha, no worries, I lost a "fastest finger first" reply race recently to two people

**AverageSoftware** · 06-21-2007

Originally Posted by kroiz

I think that if it is not a pointer then he can not use forward reference it must use include.
I am pretty sure. Am I wrong?

You're sort of wrong. Forward declarations can be used with references too.

**kroiz** · 06-21-2007

I did not know that,That is good to know.
thanks.

Thread: How do I define two C++ classes with pointers to each other?

Thread Tools

Search Thread

Display

How do I define two C++ classes with pointers to each other?

Thanks

Similar Threads

size of an integer pointer

Accessing syscalls from C

LISP (DrScheme) any one?

keyboard control

queued signals problem, C programming related