Circular Dependencies / Incomplete Types

Question

In C++, I have a problem with circular dependencies / incomplete types. The situation is as follows:

Stuffcollection.h

#include "Spritesheet.h";
class Stuffcollection {
    public:
    void myfunc (Spritesheet *spritesheet);
    void myfuncTwo ();
};

Stuffcollection.cpp

void Stuffcollection::myfunc(Spritesheet *spritesheet) {
    unsigned int myvar = 5 * spritesheet->spritevar;
}
void myfunc2() {
    //
}

Spritesheet.h

#include "Stuffcollection.h"
class Spritesheet {
    public:
    void init();
};

Spritesheet.cpp

void Spritesheet::init() {
    Stuffcollection stuffme;
    myvar = stuffme.myfuncTwo();
}

• If I keep the includes as shown above, I get the compiler error spritesheet has not been declared in Stuffcollection.h (line 4 in the above). I understand this to be due to a circular dependency.
• Now if I change #include "Spritesheet.h" to the Forward Declaration class Spritesheet; in Stuffcollection.h, I get the compiler error invalid use of incomplete type 'struct Spritesheet' in Stuffcollection.cpp (line 2 in the above).
• Similarly, if I change #include "Stuffcollection.h" to class Stuffcollection; in Spritesheet.h, I get the compiler error aggregate 'Stuffcollection stuffme' has incomplete type and cannot be defined in Spritesheet.cpp (line 2 in the above).

What can I do to solve this problem?

Answer 1

You should include Spritesheet.h in Stuffcollection.cpp
Just use forward declaration in the header file not the cpp file, that solves the circular dependency of the header file. The source file has no circular dependency actually.

Stuffcollection.cpp needs to know the complete layout of class Spritesheet(because you dereference it), So you need to include the header which defines the class Spritesheet in that file.

From your previous Q here, I believe that class Stuffcollection is used in the class declaration of Spritesheet header file and hence the above proposed solution.

Answer 2

Use this form for your nested includes:

Stuffcollection.h

#ifndef STUFFCOLLECTION_H_GUARD
#define STUFFCOLLECTION_H_GUARD
class Spritesheet;
class Stuffcollection {
  public:
  void myfunc (Spritesheet *spritesheet);
  void myfuncTwo ();
};
#endif

Stuffcollection.cpp

#include "Stuffcollection.h"
#include "Spritesheet.h"

void Stuffcollection::myfunc(Spritesheet *spritesheet) {
  unsigned int myvar = 5 * spritesheet->spritevar;
}

void Stuffcollection::myfuncTwo() {
  //
}

Spritesheet.h

#ifndef SPRITESHEET_H_GUARD
#define SPRITESHEET_H_GUARD
class Spritesheet {
  public:
  void init();
};
#endif

Spritesheet.cpp

#include "Stuffcollection.h"
#include "Spritesheet.h"

void Spritesheet::init() {
  Stuffcollection stuffme;
  myvar = stuffme.myfuncTwo();
}

General rules I follow:

• Don't include an include from an include, dude. Prefer forward declarations if possible.
  • Exception: include system includes anywhere you want
• Have CPP include everything it needs, not relying upon H recursively including it files.
• Always use include guards.
• Never use pragma