Global array question [modified]

[_Superman_]

In the header file declare it as extern int someMap[MAPSIZE_X][MAPSIZE_Y]; Give the declaration int someMap[MAPSIZE_X][MAPSIZE_Y]; in any one of the .cpp files.

«_Superman_»  _I love work. It gives me something to do between weekends.

_Microsoft MVP (Visual C++)

Polymorphism in C

[Cristoff]

Thank you for your reply. Well, my code is supposed to be a C code.

[_Superman_]

Just replace CPP in my reply with C.

«_Superman_»  _I love work. It gives me something to do between weekends.

_Microsoft MVP (Visual C++)

Polymorphism in C

[Cristoff]

Thank you! It was a lame problem. BTW Very interesting article.

[Luc Pattyn]

int someMap[MAPSIZE_X][MAPSIZE_Y]; allocates memory for an array; if you include that statement in a header file, then include that header file in N different C files, you end up allocating N arrays, all with the same name, and the linker will complain as it can't have data blocks with global scope and conflicting names. extern int someMap[MAPSIZE_X][MAPSIZE_Y]; says: "there is an array somewhere, but this by itself does not allocate it"; so inserting that in the header file tells your C files they get access to the array, without allocating memory at all, and without creating linker problems. Of course you still need a single int someMap[MAPSIZE_X][MAPSIZE_Y]; in one of the C files to actually allocate the memory; and that C file better also include the header file with the extern statement. [ADDED] One elegant way of dealing with all this is described in Stefan63's message below. [/ADDED] :)

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modified on Thursday, April 7, 2011 9:02 AM

[Stefan_Lang]

If you have plenty of such variables or declarations you can use a macro to automatically use the declaration in your header file as a definition exactly once:

// globals.h
#ifndef GLOBALS_H
#define GLOBALS_H

#ifdef GLOBALS_C

define MYGLOBAL_API

#else

define MYGLOBAL_API extern

#endif

MYGLOBAL_API int someMap[MAPSIZE_X][MAPSIZE_Y];

#undef MYGLOBAL_API // clean up - we won't need that symbol outside this header
#endif

// globals.c
#define GLOBALS_C // MYGLOBAL_API will be resolved to empty string
#include "globals.h" // defines the globals

// every_other_soucre_file.c
#include "globals.h" // declares the globals, but doesn't define them
//...

Using this technique, adding another global variable just requires adding it to globals.h, you never have to touch globals.c (or any other file) for that purpose! That said, it is always a good idea to keep the number of globals to a minimum, or, if possible, not use any at all. It's always very difficult to track an error that leaves a global in a bad state, because the problem may be caused anywhere in your program!

[Albert Holguin]

macros are wonderful when you know how to use them :)

[Cristoff]

Wow! Amazing explanation! Thank you so much! :)

[Cristoff]

That's very interesting thank you!

[Luc Pattyn]

you're welcome. In software development, getting things to work may be the goal, understanding how each part works is a necessary step to get things to work reliably. :)

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Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.