How can I create two classes that both know about each other?, C++ FAQ

C++ FAQ / Section 39 / FAQ 39.11

Section 39:

39.1	How do I convert a value (a number, for example) to a `std::string`?
39.2	How do I convert a `std::string` to a number?
39.3	Can I templatize the above functions so they work with other types?
39.4	What should be done with macros that contain `if`?
39.5	What should be done with macros that have multiple lines?
39.6	What should be done with macros that need to paste two tokens together? Updated!
39.7	Why can't the compiler find my header file in `#include "c:\test.h"` ?
39.8	What are the C++ scoping rules for `for` loops?
39.9	Why can't I overload a function by its return type?
39.10	What is "persistence"? What is a "persistent object"?
39.11	How can I create two classes that both know about each other?
39.12	What special considerations are needed when forward declarations are used with member objects?
39.13	What special considerations are needed when forward declarations are used with inline functions?
39.14	Why can't I put a forward-declared class in a `std::vector<>`?
39.15	Why do some people think `x = ++y + y++` is bad?
39.16	What's the deal with "sequence points"?

[39.11] How can I create two classes that both know about each other?

Use a forward declaration.

Sometimes you must create two classes that use each other. This is called a circular dependency. For example:

class Fred {
public:
  Barney* foo();  // Error: Unknown symbol 'Barney'
};

class Barney {
public:
  Fred* bar();
};

The Fred class has a member function that returns a Barney*, and the Barney class has a member function that returns a Fred*. You may inform the compiler about the existence of a class or structure by using a "forward declaration":

class Barney;

This line must appear before the declaration of class Fred. It simply informs the compiler that the name Barney is a class, and further it is a promise to the compiler that you will eventually supply a complete definition of that class.