Multiple file problem/forward declarations

[DoctorSalt]

Background: I have a year-ish of experience programing from a class and through a book/lots of internet searching for c++.

From scratch, I've been trying to make a program that uses a character array to simulate pixels on a screen. Theres two classes so far, Point and Screen.

[url=http://rapidshare.com/files/253273296/Graphics_Program.rar.html]I compressed with 7zip, rapidshared here[/url].

So anyways, Point functions use a Screen object called screen, and Screen functions use points. I only need one Screen object, and I want it to be globally used. I've been experimenting with the use of extern variables, and forward declaration things, but with my current build, I am getting a ton of errors that say:

forward declaration of 'struct Point'
invalid use of undefined type 'struct Point'

As well as an occasional

'a' has incomplete type

Where a is of class Point.

If you don't mind could you go through my code to find generally what I need to do to get every working smoothly? If you want, you can delete/comment out code that isn't written well, as long as the inclusions crap/variables/whatever is working all right. Also, a lot of things are commented out and slightly messy because I've been moving things around to find an answer.

[jA_cOp]

-snip-

Nevermind that, I didn't read what you wanted properly.

You should include both point.h and screen.h from screen.cpp and point.cpp, providing full definitions. You can't create code actually [I]using[/I] a declared object unless it's defined.

You should also move "extern Screen screen;" to screen.h, providing "screen" for both main.cpp and point.cpp.

[DoctorSalt]

[QUOTE=jA_cOp;15914882]-snip-

Nevermind that, I didn't read what you wanted properly.

You should include both point.h and screen.h from screen.cpp and point.cpp, providing full definitions. You can't create code actually [I]using[/I] a declared object unless it's defined.

You should also move "extern Screen screen;" to screen.h, providing "screen" for both main.cpp and point.cpp.[/QUOTE]

Thank you, trying that right now..... *Hopes*

EDIT: So would putting extern Screen screen in Screen.h declare it at the same time or something? I always thought extern is used when it is externally declared somewhere else.

[jA_cOp]

[QUOTE=DoctorSalt;15921635]Thank you, trying that right now..... *Hopes*

EDIT: So would putting extern Screen screen in Screen.h declare it at the same time or something? I always thought extern is used when it is externally declared somewhere else.[/QUOTE]

extern Screen screen; [I]declares[/I] screen, but it doesn't define it. It tells the linker to look for the definition possibly elsewhere, looking like this:
[cpp]Screen screen;[/cpp]

screen.cpp, main.cpp and point.cpp all get their own object files. If you want to share data between them, you need [I]declarations[/I] for that data, with the same name ("screen").

[cpp]
extern int Hello; //declares Hello
int Hello; //gives Hello linkage
[/cpp]