Equivalent of public static final variables

Question

I understand that placing the word extern before a variable declaration in a header file declares the existence of a global static variable without initialising it. I also understand that if I import the file containing the extern variables, I can reference them without a class/file name. But where does one define them and their values?

What I am trying to do is create a class of constants with global constants that I want to use throughout an iOS application's code.

Does one put them inside the interface like this?

Example.h

#import <Foundation/Foundation.h>
@interface Constraints : NSObject
{

extern   NSString * const PREFS_NAME;

}

Or does one put then outside of the interface like this

Example.h

#import <Foundation/Foundation.h>

extern   NSString * const PREFS_NAME;    

@interface Constraints : NSObject
{

}

Then in the implementation .m file how would one initialise the extern values?

Inside the implementation area like this?

Example.m

#import "Constraints.h"

@implementation Constraints

/**PRefecences name for the application**/
const  NSString * PREFS_NAME = @"MyApp_Prefs";

@end

Or do initialise them outside of the implementation area like this:

Example.m

#import "Constraints.h"

/**PRefecences name for the application**/
const  NSString * PREFS_NAME = @"MyApp_Prefs";

@implementation Constraints

@end

Or do I provide them their initial values in a constructor? or some arbitrary a static style method with + in front of it i.e. +(void) setAppConstraints;

I have tried several combinations, but always run into errors, such as "Redefinition of 'xVariable' with a different type". Or a something about "extern does not have an initialise interface" (or something like that, I forget). So I want to know how to declaire and initialise them properly to form the same role as public static final variables in Java.

Also what are the limits of the extern command? I know I can extern an NSInteger or NSString, but what about NSArray?

I am asking this question because there seems to be to much misleading, or incomplete, information regarding the use of extern in Objective-C. Many of the answers seem speculatory. My hope is for this question to be a good resource not only for me, but to limit further similar questions about the basics of extern.

Answer 1

You define it's value in the file inside which it's declared, which in your case is Example.m; You can still re-assign this variable, so the declaration in Example.h would look like this:

extern  NSString * PREFS_NAME;

This way every file that imports Example.h has access to this variable. The equivalent of public static final in Objective-C is const. If you also want it to be public you should make it be a class instance variable, but in this case you don't need it because it's already accessible everywhere. So in this case it would be:

// .m file
NSString* const PREFS_NAME = @"MyApp_Prefs";
// .h file
extern NSString* const PREFS_NAME;

Also notice that const NSString* is different from NSString* const. The latter is a const pointer to NSString. The former hasn't sense even if it's a correct syntax. In Objective-C the const qualifier doesn't affect objects, instead there are mutable and immutable classes. It would have sense in C++ meaning that you can use just const methods on the instance.

Answer 2

extern is used to signal the compiler that you will be using a variable or a function that is defined in another compilation unit.

When you say extern const NSString *PREFS_NAME, you're saying "Replace all references in this compilation unit to PREFS_NAME to the variable PREFS_NAME as it is defined in another file." So when you try to assign PREFS_NAME in your .m, all you're doing is trying to assign a variable that, though it has a name, it doesn't exist. Declaring a variable extern is only a declaration of a variable or function, not a definition of that variable or function. It lets the compiler know that the name is in use, and that the linker will take care of what to do with it, but even if you provide a type here, it doesn't actually set aside space for the variable, it's expecting the space to be set aside in the compilation unit that's actually defining the variable.

You compile three or four different source code files together, three of them may declare:

extern int buffer[];

And one may declare

int buffer[BUFSIZE];

In its global scope, and the linker's job is to resolve the three declared references to extern buffer to the fourth's actual definition of the buffer.

extern is to C variables and functions much as @class is to Objective-C classes, it's a forward declaration, a promise to the compiler that you don't have to freak out when you see a name that's undefined here, because the linker will answer whatever lingering questions you may have.