在包含头文件的内部包含其他头文件导致错误

Question

I have few different classes. Few pairs need to know about each other. To prevent the headache I've created one file to keep forward declarations of all troublesome classes along with proper include order.

#ifndef GLPROJECT_H
#define GLPROJECT_H

class MainWindow;
class Scene;
class ShaderProgram;
class Shape;

#include "ShaderProgram.h"
#include "MainWindow.h"
#include "Shape.h"
#include "Scene.h"

#endif

Every file who needs another one from the given set include this header. I came up with an idea to put all includes inside include guard, so for example Shape.h class looks like:

#ifndef SHAPE_H
#define SHAPE_H

#include "GLProject.h" //file above

//...class definition code

#endif

However this example produces error: field 'x' has incomplete type 'GLProject::Shape' in file Scene.h , which is stated after the Shape.h in the main header (other files don't include Scene.h explicitly).

(Note that following flow is correct only for files which directly include GLProject.h ) If I'd trace the inclusion of files starting with GLProject.h , then: 1)

1. it includes the first header which doesn't include any of these headers,
2. MainWindow which include GLProject.h but it's completely omited due to include guards,
3. Shape.h which defines Shape.h (after trying to include guarded GLProject.h )
4. Scene.h which declares viariable of type Shape that should be already defined.

So no idea why does it complains that Shape is incomplete type.

What's more interesting moving inclusion of file GLProject.h in file Shape.h above include guards fixes the problem. (the most important fact)

Answer 1

Ok. The actual problem was connected with not mentioned file Shape.cpp which of course included Shape.h and the fact that moving #include "GLProject.h" before the guard were fixing the problem. The flow with the include directive inside the guard:

Shape.h (def guard)
     GProject.h (def guard) 
          ...(MainWindow.h and ShaderProgram.h)...//not interesting

          Shape.h (omit, because of guard)

          Scene.h
                GProject.h (omit, because of guard)
          back to Scene.h (def guard) ERROR

While with the directive outside:

Shape.h
     GProject.h (def guard)
          ...(MainWindow.h and ShaderProgram.h)...//not interesting

          Shape.h
                GProject.h (omit, cause of guard)
          back to Shape.h (now def guard and class)

          Scene.h
                GProject.h (omit, because of guard)
          back to Scene.h (def guard) No error - Shape defined