为什么前向声明不能解决循环依赖关系？

Question

I have three classes: a TopClass which contains an instance of a BottomClass pointer. The BottomClass contains a pointer to a HelperClass . The HelperClass keeps a pointer to the TopClass . Circular dependency pops up and a forward declaration is needed in the HelperClass .

All of this is illustrated with the following code snippets:

#include "BottomLevel.h"

namespace foo
{
  class TopLevel
  {
  private:
    BottomLevel* item;
  };
}

-

#include "HelperClass.h"

namespace foo
{
  class BottomLevel
  {
  private:
    HelperClass* item;
  };
}

-

class TopLevel; // forward declaration here

namespace foo
{
  class HelperClass
  {
  public
    HelperClass(TopLevel* item);
  };
}

The issue comes when trying to do things in the implementation file. If I #include "TopClass.h" in the cpp file, I get compilation errors stating "overloaded member function not found -- use of undefined type ' TopLevel '" (ERRORS C2511 and C2027).

Then, if I don't do the #include I'm still left with C2027 errors because I try to use the forward declared type.

I just know there is a method to fix this, I'm sure I've done it before, but I can't for the life of me remember what I'm supposed to do. Any help please?

Answer 1

The problem is that you're forward declaring TopLevel outside the foo namespace so the compiler is never finding the class foo::TopLevel .

Try moving the forward declaration of TopLevel inside the foo namespace.

Answer 2

You need a forward declaration of the form

namespace foo {
class TopLevel;
class BottomLevel;
class HelperClass;
}

Then the order wont matter [because you use pointers]