Variably modified array at file scope

89
votes

I want to create a constant static array to be used throughout my Objective-C implementation file similar to something like this at the top level of my ".m" file:

static const int NUM_TYPES = 4;
static int types[NUM_TYPES] = { 
  1,
  2, 
  3, 
  4 };

I plan on using NUM_TYPES later on in the file so I wanted to put it in a variable.

However, when I do this, I get the error

"Variably modified 'types' at file scope"

I gather that this may have something to do with the array size being a variable (I don't get this message when I put an integer literal there, like static int types[4]).

I want to fix this, but maybe I am going about it all wrong...I have 2 goals here:

  1. To have an array which is accessible throughout the file
  2. To encapsulate NUM_TYPES into a variable so I don't have the same literal scattered about different places in my file

Any suggestions?

[EDIT] Found this in the C Faq: http://c-faq.com/ansi/constasconst.html

6
cobjective-carraysstaticconstants
What happens if you do it as a define instead? #define kNUM_TYPES 4 ? - Jorge Israel Peña
That works...for some reason I was trying to stay away from using the preprocessor because I thought I remembered reading that somewhere, but I just did some more research and couldn't find a good reason not to use it in this case. I think it may be less desirable if I'm creating objects in the preprocessor (like @"An NSString literal") The only thing wrong with your piece of code is that there's no need for the semicolon. - Sam
Ah yes, thanks for the heads up, and glad I could help. - Jorge Israel Peña

6 Answers

64
votes

The reason for this warning is that const in c doesn't mean constant. It means "read only". So the value is stored at a memory address and could potentially be changed by machine code.

33
votes

If you're going to use the preprocessor anyway, as per the other answers, then you can make the compiler determine the value of NUM_TYPES automagically:

#define NUM_TYPES (sizeof types / sizeof types[0])
static int types[] = { 
  1,
  2, 
  3, 
  4 };
22
votes
#define NUM_TYPES 4
11
votes

It is also possible to use enumeration.

typedef enum {
    typeNo1 = 1,
    typeNo2,
    typeNo3,
    typeNo4,
    NumOfTypes = typeNo4
}  TypeOfSomething;
5
votes

As it is already explained in other answers, const in C merely means that a variable is read-only. It is still a run-time value. However, you can use an enum as a real constant in C:

enum { NUM_TYPES = 4 };
static int types[NUM_TYPES] = { 
  1, 2, 3, 4
};
3
votes

Imho this is a flaw in many c compilers. I know for a fact that the compilers i worked with do not store a "static const"variable at an adress but replace the use in the code by the very constant. This can be verified as you will get the same checksum for the produced code when you use a preprocessors #define directive and when you use a static const variable.

Either way you should use static const variables instead of #defines whenever possible as the static const is type safe.