Problem with #includes, friend classes, and forward declarations.

[boxfish]

Hi everyone, I'm working on a 3d maze game, and I just messed up the classes so it won't compile, and I need help sorting it out. There's a class Maze, whose members are, among other things, a list of the walls, the user who is an object of class MazePerson, and a yellow square who is an object of class MazeMonster. MazePerson and MazeMonster are both inherited from class MazeThing, which takes care of stuff like collisions. MazePerson, MazeMonster, and MazeThing used to be structs inside of class Maze like this:

Maze.h:

#ifndef MAZE_H
#define MAZE_H
...
using namespace std;
 
class Maze {
public:
    ...
private:
    ...
    struct MazeThing {
        ...
    };
    struct MazePerson : public MazeThing {
        ...
    };
    struct MazeMonster : public MazeThing {
        ...
    };
    ...
    MazePerson person;
    MazeMonster monster;
    ...
};
 
#endif
 

Now I want to take MazeThing, MazePerson, and MazeMonster out of class Maze, give each of them their own header file, let them be real classes, make Maze be a friend of all of them, and make all of them friends of class Maze. Here's what I did:

Maze.h:

#ifndef MAZE_H
#define MAZE_H
...
#include "MazePerson.h"
#include "MazeMonster.h"
 
using namespace std;
 
class Maze {
public:
    friend class MazeThing;
    ...
private:
    ...
    MazePerson person;
    MazeMonster monster;
    ...
};
 
#endif
 

MazeThing.h:

#ifndef MAZETHING_H
#define MAZETHING_H
 
class Maze;
 
class MazeThing {
public:
    friend class Maze;
protected:
    ...
    Maze *myMaze;
    ...
};
 
#endif
 

MazePerson.h

#ifndef MAZEPERSON_H
#define MAZEPERSON_H
 
#include "MazeThing.h"
 
class Maze;
 
class MazePerson : public MazeThing {
public:
    friend class Maze;
    ...
};
 
#endif
 

MazeMonster.h

#ifndef MAZEMONSTER_H
#define MAZEMONSTER_H
 
#include "MazeThing.h"
 
class Maze;
 
class MazeMonster : public MazeThing {
public:
    friend class Maze;
    ...
};
 
#endif
 

But wherever I use MazeThing's member myMaze in MazeThing.cpp I get these errors:

invalid use of undefined type `struct Maze'
forward declaration of `struct Maze'

I've tried changing around the #includes and forward declarations, but I always get errors. Any help would be greatly appreciated.

[weaknessforcats]

This compiles:

class MazeThing {
        //...
    };
struct MazePerson : public MazeThing {
        //...
    };
struct MazeMonster : public MazeThing {
        //...
    };
 
 
#ifndef MAZE_H
#define MAZE_H
//...
//#include "MazePerson.h"
//#include "MazeMonster.h"
 
using namespace std;
 
class Maze {
public:
    friend class MazeThing;
    //...
private:
    //...
    MazePerson person;
    MazeMonster monster;
    //...
};
#endif
 

Basically, all I did was change your struct MazeThing to class MazeThing. I did that because you declared friend class rather than friend struct.

[boxfish]

Thanks so much for your reply! But MazeThing already was a class. But when I put everything in one file like you did, it works! So it seems like I have to figure out how to get the #includes right so the classes end up in that order after the preprocessor does its thing. Oh wait, maybe it's because I didn't use namespace std on all of them...
Okay I didn't, let me try that...
Thanks again!

[boxfish]

Okay, that didn't work. There's another error I can get instead if, instead of a forward declaration of class Maze in MazeThing.h, I #include "Maze.h".
It goes like this:

In file included from Maze.h
from MazeThing.h
from MazeThing.cpp
parse error before `{' token
parse error before `}' token
In file included from Maze.h
from MazeThing.h
from MazeThing.cpp
parse error before `{' token
parse error before `}' token
In file included from MazeThing.h
from MazeThing.cpp
field `person' has incomplete type
field `monster' has incomplete type

When it says `{' and `}' tokens, It points to the opening and closing braces of a class:

#ifndef MAZEMONSTER_H
#define MAZEMONSTER_H
 
#include "MazeThing.h"
 
using namespace std;
 
class Maze;
 
class MazeMonster : public MazeThing { // << This one
public:
    friend class Maze;
    ...
}; // << and this one..
 
#endif
 

It does this in MazePerson.h and MazeMonster.h.

When It says field monster and field person, it points to the MazePerson person and MazeMonster monster members of class Maze.

Does anyone have any ideas?

I'm using Dev-C++ 4.9.8.0 on Windows XP, if that helps.

Thanks for any help you can give me.

[mkborregaard]

How does your MazeThing constructor look?

[weaknessforcats]

class MazeMonster : public MazeThing { // << This one

When you compiler says parse error before {, it is trying tell you that when the { was encountered it was unexpected. Therefore, the parse knows there is trouble either with the { or the code preceding it. Like maybe MazeThing.

Keep in mind that you are using templates. The compiler requires the templates to precede any use of the templates. You either a) put all the templates together in the correct order in one header file and include it or b) use multiple header files that are included in the correct order.

You cannot have some of your templates in other files because templates are not code. They are patterns the compiler copies, substitutes a real type for the template type and this becomes the real code.

[boxfish]

Hi, Thanks for your replies. My problem magically went away just now. I put everything into one file, and that made it work, and I just put it back into separate files and it still works. I thought that maybe not everything had been recompiled, so I clicked rebuild all, and it still worked. Ok, I just found out what I changed that made it work: MazePerson.cpp and MazeMonster.cpp now both include Maze.h instead of their own header files. It seems weird to me that that made it work, but I guess If they include their own header files, stuff ends up in the wrong order, and by including Maze.h, they indirectly include their own header files, because Maze.h includes them.
@mkborregaard: MazeThing currently does not have a constructor. Maze initializes its MazeThing objects' variables in its own constructor. That is not good and I need to fix it.
@weaknessforcats: I don't know what you mean by templates. I thought templates were functions and classes that could be instantiated to work with new variable types as needed. I didn't think I was using them. Does it have to do with inheritance? Eventually I want to go through all the files, figure out (by trial and error) which includes are unnessecary (I have such a hard time spelling that), and get rid of them. They seem like a mess right now.
Thanks again.

[mkborregaard]

@mkborregaard: MazeThing currently does not have a constructor. Maze initializes its MazeThing objects' variables in its own constructor. That is not good and I need to fix it.

OK. I was just wondering whether your problem was related to the uninitialized Maze* pointer (which i guess you intend to initialize in the MazeThing constructor to point to the Maze object owning MazeThing). Cool you got it solved, though.

[boxfish]

OK. I was just wondering whether your problem was related to the uninitialized Maze* pointer (which i guess you intend to initialize in the MazeThing constructor to point to the Maze object owning MazeThing). Cool you got it solved, though.

Yeah. But if it was a problem with an uninitialized pointer, I probably would have gotten an access violation, not a compiler error. I'm going to go try to make it a multi-story maze with ramps leading between the levels now.