You cannot set a callback to class functions, instead, you need to make polymorphic listeners or use a message system.

//your problem is:
set_close_button_callback(game.quit_game);

The basic idea is game.quit_game() actually points to a function that takes in an invisible pointer to your class instance (this) and so C++ won't know to use your 'game' instance because of how the language works. To do what you want, Google 'signal slot pattern' . std::function from c++0x can bind the this pointer so that could do it too.

A quick fix is to make quit_game() a static function. The disadvantage is that the function will be like a C one so it won't have access to your class's instance variables.

You would also access the static function like a scope:

set_close_button_callback(game::quit_game);

If you are just starting to learn Allegro, move to Allegro 5. It is better, and it supports proper events which will allow you to make your game object oriented more easily than Allegro 4.

There's no reason not to use a global variable here. Don't be paranoid about globals. Sometimes they're the right tool for the job.

bool quit = false;

void quit_game() {quit = true;}

// in main
set_close_button_callback(quit_game);

And yes, you could use a static quit variable and a static quit_game method. It would be effectively the same, except it would be affected by it's class access level.

Yes, download the examples for A5 from the allegro files page.

I'll use a global; in what situations is a global okay to use?

Also, just tried out some Allegro 5 examples and the demos. It's really cool (works) but before some games hadn't worked with me...

Non of the audio examples work with me nor the "acodec" examples.
OpenGL of course doesn't work with me since I think I need a good video card for that.

In some of the examples I can choose from a memory or video bitmap. Memory is really slow while video is fast. Is this to demonstrate a point? Allegro 4 used memory blending right?

Some other examples run a bit in their log windows but close quickly (I can't tell what some of them do just by trying the executable out )

The code for A5 is a lot different though
Where do I start? the manual? Or the wiki? Is there a guide for people that have been using A4 and want to transfer smoothly?

Removing globals does not guarantee a good design. If you put all of your globals into a struct and expose that struct, then you've accomplished nothing. The most important thing is to try to keep implementation details as private as possible.

If your computer is old, then I'd not bother with A5 until you at least upgrade the video card (if it's a desktop machine...).

'extern' declarations go in headers, never in source files. All they do is say that 'this variable is available to you'. They still have to be defined somewhere else though.

Add 4 int members to your snake class that hold the value of the keys you want to check and then check if (key[MOVE_LEFT]) {...}. Look up what an initialization list is, as you don't use one. Give them default values in the initialization list.

Noted, but is it unnecessary or destructive? Or both? I'll not do it, but I don't know why other than you said not to right now.

Oh okay. I thought it'd function the same in the source file since it worked for the external Boolean variable quit. I'll try it in a header. I guess I'll need to make a globals declaration header. I'm hoping the creation of player in the main.cpp counts as a definition.

Okay, I'm trying to look up "initialization lists", though I'm not sure what my goal is. So far it looks like just a constructor, but I'll keep reading into it I guess.

I'm not sure if this is supposed to be plainer to me, but I assume I'll know what you mean when I read more on these lists. Right now it looks ambiguous.

I've never succeeded in storing the keys flags in variables. I've asked for help in the past but I guess my wording was inaccurate cause I never got a satisfactory answer. I mean I don't know know how to store what keys is needed as a variable (I'm assuming you mean so I can initialize it with a constructor taking those keys as parameters, but I could be inferring wrongly)

int my_special_key = KEY_ENTER;

// ...
if (key[my_special_key]) {/*...*/}

Thanks, I haven't tested that, but I assume that works. I guess I didn't know they were integers. Seriously, thanks. I didn't know how to let my user configure keys. Does this work for shift/ctrl/alt flag, too?

Hmm. I didn't know you could do that in constructors. I guess you'd have to use constants, right? I mean to take parameters I'd need a constructor with parameters, yes (like parameters for keys to use, for example) I could always assign them later.
Oh I see you just have an overloaded constructor.

Yeah I typed that before I saw the overloaded constructor

I just saw this Test() : key1(KEY_1) , key2(KEY_2) {}

Hey, do these count as the function declarations and definitions? Since the only code is there?

Edit:
I hit a problem when I made a header for my global variables.
In the global variables header, it says snake does not name a type.
I assume this is what's causing the gamedata.h to say that the instance of snake, player has not been declared.

My gamedata struct works, but the snake class isn't. Is there some different syntax to this or did I do something stupid?

gamedata.h

global_variables.h

#ifndef GLOBAL_VARIABLES
#define GLOBAL_VARIABLES

#include "snake.h"
#include "gamedata.h"

bool quit;
gamedata game;
snake player; 

#endif

snake.h

Ahh, somehow I forgot -- I had even done it before, declaring the struct and class instances inside of the header that defines them. I moved the game and player declarations to gamedata.h and snake.h.

Yeah, I already have a function that is passed the address of player because of the same problem (not knowing player was [supposed to be] declared) I thought it'd be interesting to see if the code could handle another snake (just for testing, to see how easy it'd be with "correct" code)

forgetting to put "extern" was stupid of me. I don't know how I missed that (although I didn't realize that would define them)

gamedata.h still isn't liking this line, though

It doesn't know snake is a type, still (I thought including the snake.h would fix that)

I only planned on one more global at most, but I probably will add the snake instance to gamedata. I had already pondered on it.

I'm not sure if I should just declare the bool quit in main.cpp or not (getting rid of the global_variables.h) since only main is using quit

I can tell circular includes is gonna be a problem for me. I thought the

#ifndef GLOBAL_VARIABLES
#define GLOBAL_VARIABLES
#endif

made it okay. Hmm. I was originally just including the headers in other headers using classes/structs from them (eg including snake.h in gamedata.h but not working as I planned)

I think I did everything you said... Here let me paste what I got:

global_variables.h (will probably get rid of this)

#ifndef GLOBAL_VARIABLES
#define GLOBAL_VARIABLES

#include "gamedata.h"

extern bool quit;

#endif

gamedata.h

snake.h

I appreciate your patience in helping me with this. This is really not coming easy to me, and I struggled with learning it in the past and gave up and coded everything in one file

Sweet. Something clicked for me -- I had been including whole headers (in other headers) when a certain struct/class wasn't recognized. So I do forward declarations instead, and in source files (cpp) there, I include the headers. I might be wrong with this, but I compiled with no warnings (I had to modify pretty much every file, and I eliminated global_variables.h)

main.cpp

gamedata.h

gamedata.cpp

#include <allegro.h>
#include "gamedata.h"
#include "snake.h"

...
}

snake.h

snake.cpp

#include "snake.h"
void snake::position(int x, int y)
{
    head_x = x;
    head_y = y;
}

global_functions.cpp

It compiles fine, but are there some bad include practices in there I should know about? I really appreciate your help so far, I got a lot of this down (finally), but I'm not sure this is 100% "correct".

I probably don't need to include <vector> in snake.h, do I? I should use a forward declaration, but what is a vector? A struct/class? Hmm. Googling. the std:: thing takes care of it I think. I know that is supposed to mean standard namespace or something, but I don't really know why that works.

Oops, missed those. I must've hurried through it.

I'll have to read up on namespaces, I guess, cause I still don't really get them (as far as usefulness of custom namespaces)

edit: I fixed those includes and globals (I brought back global_variables.h for quit for global_functions.cpp

I was removing including custom headers in other headers, but I got a problem when I don't include segment.h in snake.h, even with a forward declaration of segment

snake.h

segment.h

#ifndef SEGMENT
#define SEGMENT

class segment
{
    public:
    int x;
    int y;
};

#endif

It says error: invalid use of undefined type `struct segment'
Not sure why it says struct and not class...
Hmm? Segmentation fault? :B

Actually, currently until I make a segment.cpp, nothing includes segment.h -- is it still declared, then?

Oh, I get it now. Thanks, man!