C ++使用标头的正确方法

Question

I'm by no means new to using headers in programs, but I've recently run into a strange problem.

#ifndef PLAYER_HEADER_GUARDS
#define PLAYER_HEADER_GUARDS

#include "CollidableObject.h"
#include "Includes.h"

extern const int ANIMATION_EXTENDER;
extern class MapSystem *mapSystem;

class Player : public CollidableObject
{
     //declaration of rest of the class
};

#endif

I have a lot of classes split across several files (and their respective headers), but here is my current problem : even though I have already included CollidableObject.h, the compiler (CodeBlocks) throws an error

Player.h|12|error: expected class-name before '{' token|

The problem arises from the " : public CollidableObject" part of the code, since the program worked fine before I added that. Shouldn't including the CollidableObject header (I'm not posting it, but it basically looks just the same, and defines the class CollidableObject) remove this kind of a problem? Further,

extern class MapSystem *mapSystem;

This statement should, to the best of my knowledge, compile fine without the extra "class" after extern, but if I remove the class keyword, it throws an error "expected type qualifier before * token". The class MapSystem has also been defined previously, and it should work fine - but without the class keyword, it doesn't.

So that brings me to my question - what mistake am I making in my header files, that is leading to such problems? No multi-filed project that I worked on before had such problems.

Answer 1

It sounds like either "CollidableObject.h" or "Includes.h" tries to reinclude "Player.h" , resulting in a circular dependency. The result is that Player ends up being defined before CollidableObject , so the latter name is not declared at that point.

Make sure neither of these headers, nor any of their dependencies, include "Player.h" . You might need to add forward declarations of class Player; if anything needs to use that name.

Also check that "CollidableObject.h" does actually define class CollidableObject (perhaps there's a spelling mistake?), and has a uniquely named include guard.

As for the second question:

extern class MapSystem *mapSystem;

This doubles up as a declaration of class MapSystem , so doesn't require a previous declaration.

extern MapSystem *mapSystem;

This does not declare MapSystem (it only indicates that it's a type name, not a class), so it does require a previous declaration.