views:

96

answers:

4

How do I implement mutually recursive classes in C++? Something like:

/*
 * Recursion.h
 *
 */

#ifndef RECURSION_H_
#define RECURSION_H_

class Class1
{
  Class2* Class2_ptr;
public:
  void Class1_method()
  {
      //...
      (*Class2_ptr).Class2_method();
      //...
  }
};

class Class2
{
    Class1* Class1_ptr;
public:
    void Class2_method()
    {
        //...
        (*Class1_ptr).Class1_method();
        //...
    };
};


#endif /* RECURSION_H_ */
+4  A: 

Use forward declaration.

class Class2;

class Class1
{
  Class2* Class2_ptr;
};

class Class2 
{
  Class1* Class1_ptr;
}

Because the methods in Class1 will depend on the actual definition of Class2, method definitions must occur after the Class2 declaration, since you can't use methods from only a forward declaration.

On the other hand, this kind of tight coupling is usually indicative of bad design.

Jherico
+1  A: 

Predeclare one of the classes, for example Class2

#ifndef RECURSION_H_
#define RECURSION_H_
class Class2;
class Class1
{
   Class2* Class2_ptr;
   public:
   void Class1_method()
   {
      //...
      (*Class2_ptr).Class2_method();
      //...
   }
};

class Class2
{
     // ...
}  
deinst
+1  A: 

Forward declare one of the classes (or both) on the top, eg.:

class Class2;
class Class1 { ... };

and define the methods after both of the classes are defined (that is, out-of-line):

class Class1
{
 ...
 void Class1_method(); // just declare
 ...
};

class Class2
{
 ...
};

// once definition of Class2 is known, we can define the method of Class1
void Class1::Class1_method()
{
      //...
      (*Class2_ptr).Class2_method();
      //...
}
jpalecek
What do you mean? How can you define the classes without defining the methods?
See edit. More characters.
jpalecek
+4  A: 
Beta
You forget to put the forward declaration, but this answer still has the code that is clearest to me.
@drenami: Ack! Sorry, I had it in my test code, but goofed in the cut-and-paste. I'll fix it...
Beta