Constants in Objective-C

I'm developing a [Cocoa][1] application, and I'm using constant `NSString`s as ways to store key names for my preferences.

I understand this is a good idea because it allows easy changing of keys if necessary.
Plus, it's the whole 'separate your data from your logic' notion.

> Anyway, is there a good way to make these constants defined once for the whole application?

I'm sure that there's an easy and intelligent way, but right now my classes just redefine the ones they use.

[1]:

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As Abizer said, you could put it into the PCH file. Another way that isn't so dirty is to make a include file for all of your keys and then either include that in the file you're using the keys in, or, include it in the PCH. With them in their own include file, that at least gives you one place to look for and define all of these constants.

Easiest way:

// Prefs.h
#define PREFS_MY_CONSTANT @"prefs_my_constant"

Better way:

// Prefs.h
extern NSString * const PREFS_MY_CONSTANT;

// Prefs.m
NSString * const PREFS_MY_CONSTANT = @"prefs_my_constant";

One benefit of the second is that changing the value of a constant does not cause a rebuild of your entire program.

// Prefs.h
extern NSString * const RAHUL;

// Prefs.m
NSString * const RAHUL = @"rahul";

A slight modification of the suggestion of @Krizz, so that it works properly if the constants header file is to be included in the PCH, which is rather normal. Since the original is imported into the PCH, it won't reload it into the `.m` file and thus you get no symbols and the linker is unhappy.

However, the following modification allows it to work. It's a bit convoluted, but it works.

You'll need **3** files, `.h` file which has the constant definitions, the `.h` file and the `.m` file, I'll use `ConstantList.h`, `Constants.h` and `Constants.m`, respectively. the contents of `Constants.h` are simply:

// Constants.h
#define STR_CONST(name, value) extern NSString* const name
#include "ConstantList.h"

and the `Constants.m` file looks like:

// Constants.m
#ifdef STR_CONST
#undef STR_CONST
#endif
#define STR_CONST(name, value) NSString* const name = @ value
#include "ConstantList.h"

Finally, the `ConstantList.h` file has the actual declarations in it and that is all:

// ConstantList.h
STR_CONST(kMyConstant, "Value");
…

**A couple of things to note:**

1. I had to redefine the macro in the `.m` file **after** `#undef`ing it for the macro to be used.

2. I also had to use **`#include`** instead of `#import` for this to work properly and avoid the compiler seeing the previously precompiled values.

3. This will require a recompile of your PCH (and probably the entire project) whenever any values are changed, which is not the case if they are separated (and duplicated) as normal.

Hope that is helpful for someone.

I am generally using the way posted by Barry Wark and Rahul Gupta.

Although, I do not like repeating the same words in both .h and .m file.
Note, that in the following example the line is almost identical in both files:

// file.h
extern NSString* const MyConst;

//file.m
NSString* const MyConst = @"Lorem ipsum";

Therefore, what I like to do is to use some C preprocessor machinery.
Let me explain through the example.

I have a header file which defines the macro `STR_CONST(name, value)`:

// StringConsts.h
#ifdef SYNTHESIZE_CONSTS
# define STR_CONST(name, value) NSString* const name = @ value
#else
# define STR_CONST(name, value) extern NSString* const name
#endif

The in my .h/.m pair where I want to define the constant I do the following:

// myfile.h
#import <StringConsts.h>

STR_CONST(MyConst, "Lorem Ipsum");
STR_CONST(MyOtherConst, "Hello world");

// myfile.m
#define SYNTHESIZE_CONSTS
#import "myfile.h"

et voila, I have all the information about the constants in .h file only.

I use a singleton class, so that I can mock the class and change the constants if necessary for testing. The constants class looks like this:

#import <Foundation/Foundation.h>

@interface iCode_Framework : NSObject

@property (readonly, nonatomic) unsigned int iBufCapacity;
@property (readonly, nonatomic) unsigned int iPort;
@property (readonly, nonatomic) NSString * urlStr;

@end

#import "iCode_Framework.h"

static iCode_Framework * instance;

@implementation iCode_Framework

@dynamic iBufCapacity;
@dynamic iPort;
@dynamic urlStr;

- (unsigned int)iBufCapacity
{
return 1024u;
};

- (unsigned int)iPort
{
return 1978u;
};

- (NSString *)urlStr
{
return @"localhost";
};

+ (void)initialize
{
if (!instance) {
instance = [[super allocWithZone:NULL] init];
}
}

+ (id)allocWithZone:(NSZone * const)notUsed
{
return instance;
}

@end

And it is used like this (note the use of a shorthand for the constants c - it saves typing `[[Constants alloc] init]` every time):

#import "iCode_FrameworkTests.h"
#import "iCode_Framework.h"

static iCode_Framework * c; // Shorthand

@implementation iCode_FrameworkTests

+ (void)initialize
{
c = [[iCode_Framework alloc] init]; // Used like normal class; easy to mock!
}

- (void)testSingleton
{
STAssertNotNil(c, nil);
STAssertEqualObjects(c, [iCode_Framework alloc], nil);
STAssertEquals(c.iBufCapacity, 1024u, nil);
}

@end

I myself have a header dedicated to declaring constant NSStrings used for preferences like so:

extern NSString * const PPRememberMusicList;
extern NSString * const PPLoadMusicAtListLoad;
extern NSString * const PPAfterPlayingMusic;
extern NSString * const PPGotoStartupAfterPlaying;

Then declaring them in the accompanying .m file:


NSString * const PPRememberMusicList = @"Remember Music List";
NSString * const PPLoadMusicAtListLoad = @"Load music when loading list";
NSString * const PPAfterPlayingMusic = @"After playing music";
NSString * const PPGotoStartupAfterPlaying = @"Go to startup pos. after playing";

This approach has served me well.

Edit: Note that this works best if the strings are used in multiple files. If only one file uses it, you can just do `#define kNSStringConstant @"Constant NSString"` in the .m file that uses the string.

There is also one thing to mention. If you need a non global constant, you should use `static` keyword.

Example

// In your *.m file
static NSString * const kNSStringConst = @"const value";

Because of the `static` keyword, this const is not visible outside of the file.

---

**Minor correction by [@QuinnTaylor][1]:** static variables are visible within a *compilation unit*. Usually, this is a single .m file (as in this example), but it can bite you if you declare it in a header which is included elsewhere, since you'll get linker errors after compilation


[1]:

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Try using a class method:

+(NSString*)theMainTitle
{
return @"Hello World";
}

I use it sometimes.