Proper declaration of global C-constants in Objective-C

Question

I'm trying to declare and initialize global C-variables.

const int numberOfTickMarks = 6;
const double tickValues[numberOfTickMarks] = {500, 2000, 3000, 4000, 6000, 8000};

When I do this in my header file (before the @interface), I get a linker error. But when I do this in the .m file (before @implementation), things seem to work as desired.

Is the latter the accepted way to declare global constants for C/Objective-C?

http://stackoverflow.com/questions/4481554/global-variables-in-objective-c-difference-in-extern-and-top-of-m-file-declar?rq=1 — matt, May 08 '13 at 18:16
The question is whether you want them for just one file or shared over multiple files. You are not going to #import a _.m_ file, so "the latter" (`const double` in the _.m_ file) would not suffice in the second case. — matt, May 08 '13 at 18:18

score 1 · Accepted Answer · edited May 08 '13 at 18:27

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Your global variables should be declared like this in the header file:

extern const int numberOfTickMarks;
extern const double tickValues[numberOfTickMarks];

Without extern, linker errors are inevitable.

In the implementation file, you would have to define them again like this:

const int numberOfTickMarks = 6;
const double tickValues[numberOfTickMarks] = {500, 2000, 3000, 4000, 6000, 8000};

edited May 08 '13 at 18:27

answered May 08 '13 at 18:14

user123

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So basically you're saying I just have to add `extern` to the declarations I put in the header file. Of course `numberOfTickMarks` is still undefined there, so can't be used for the length of the array. I'd like to make it so I have to change as little code as possible. Is there any way to allow the `numberOfTickMarks` in the definition of the array? – JohnK May 08 '13 at 18:27
(Sorry, got declaration and definition mixed up.) Is there any way for the compiler to get the size of an array? If so, I could use that to *define*/set `numberOfTickMarks`. – JohnK May 08 '13 at 18:38
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`sizeof(tickValues)/sizeof(*tickValues)` = size of the array – user123 May 08 '13 at 18:53

score 1 · Answer 2 · answered May 08 '13 at 18:22

What I would do in a case where I have a "magic number" is a #define:

#define NUM_TICK_MARKS 6

It can go in the .m file, but if it is to be shared widely, I might put it in the .pch file.

And by the way:

The numberOfTickMarks is not necessary:

const double tickValues[numberOfTickMarks] = {500, 2000, 3000, 4000, 6000, 8000};

The initializer says quite clearly how big the array is!

score 0 · Answer 3 · edited May 23 '17 at 11:53

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Why not define them on the main.m file ?

type myVar;

And declare on the header, use:

extern type myVar;

Edit

By what you wrote on the comments, I think you need to use class variables. Since this resource is not available in objective-c, I see 2 alternatives:

1) Use c++ as in this SO answer

2) Use a shared instance, as described in this answer

Personally, I'd go with number 2

edited May 23 '17 at 11:53

Community

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answered May 08 '13 at 18:14

luksfarris

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Doesn't seem appropriate since the variables are proper to my viewController and not to the project as a whole. – JohnK May 08 '13 at 18:17
than why use global variables ? Class and instance variables are what you should be looking for – luksfarris May 08 '13 at 18:18
Because I want them accessible to some static functions defined in the viewController.m and proper to that viewController. – JohnK May 08 '13 at 18:20
@JohnK Then why do you need them to be extern *at all?* – May 08 '13 at 18:28
@H2CO3, where did I say I needed them to be extern? – JohnK May 08 '13 at 20:32

Proper declaration of global C-constants in Objective-C

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