Comments on: Java Developer’s Guide to String Constants in Objective-C

By: dnd

dnd — Fri, 12 Feb 2010 11:10:20 +0000

@Rodney

>> My experience is with J2ME/Java and CF/C#; in both of those platforms, String literals “add up” and should always be avoided.

I know this is an old post, but I came across it while googling for ‘constants in objective c’. I can’t speak for the other environments, but this is not true of Java.

String x = “xxx”;
String y = “xxx;

will result in only one String object being created. Both x and y will reference the same String object.

By: Hasnat

Hasnat — Mon, 23 Nov 2009 15:51:42 +0000

@david
yes you are correct
you can define in separate header file

By: stevebert

stevebert — Thu, 05 Feb 2009 19:38:34 +0000

You’re not incurring any more expense importing 999 unused #defines just to get that one string constant. The compiler does not incur any storage allocation just because a #define exists, it has to be used in the code being compiled and then it’s only a pointer reference to a global DATA item where the string constant is stored.

Of course if you’re using the same .h file across multiple executables, of course each binary will contain a copy of the string constant, but you knew that already. ;-)

By: stevebert

stevebert — Thu, 05 Feb 2009 19:34:37 +0000

There’s nothing magic about #defines so including them all in a massive .h is fine. The pre-processor will substitute the string constants and the compiler will correlate multiple instances of the same string constant.

By: david

david — Thu, 05 Feb 2009 12:54:19 +0000

so whats the conclusion..whats the better way to declare NSString constants across the application ?

———————————————————————–
I created a .h file and it contents look like

//myconstants.h
#define stringglobal1 @”ssss”
#define string2 @”zzzz”

#define kMywidth 123

//more than 50 constats are there
and in other files I am using #import myconstants.h
I am also importing this header file only for using a single constant
———————————————————————–

is it okay ?

By: david

david — Thu, 05 Feb 2009 12:48:25 +0000

so whats the conclusion..whats the better way to declare NSString constants across the application ?

———————————————————————–
I created a .h file and it contents look like

//myconstants.h
#define stringglobal1 @”ssss”
#define string2 @”zzzz”

#define kMywidth 123

and in other files I am using #import myconstants.h
———————————————————————–

is it okay ?

By: stevebert

stevebert — Wed, 29 Oct 2008 18:16:54 +0000

I have no special insight into the workings of the compiler, but I think the compiler detects the use of @”" string constants and makes instances of an “NSString-like” object in the DATA section of the executable (note the low-address reference for the string). I say NSString-like because this type of object can never be sent retain or release messages, so is probably not a 1st class NSString instance. Repeated uses of the string are probably matched by hashing the string literal and matching to an existing instance of the constant.

By: Rodney

Rodney — Wed, 29 Oct 2008 17:25:02 +0000

@Stevebert,

Thanks for the response. I stand corrected. It is interesting to see how different ObjC is compared to other languages. My experience is with J2ME/Java and CF/C#; in both of those platforms, String literals “add up” and should always be avoided. This is, yet again, an example of how Objective-C excels as a language for mobile application development.

I’ll update the article to include your feedback. Thanks again!
- Rodney

By: Steve Wetherill

Steve Wetherill — Wed, 29 Oct 2008 16:43:54 +0000

OK, answer is here:

http://www.gnustep.org/resources/documentation/Developer/Base/Reference/NSString.html#003000000000

By: Steve Wetherill

Steve Wetherill — Wed, 29 Oct 2008 16:42:13 +0000

That looks correct.

It might help to understand this if we could see what @”some literal” actually translates to in the code. Presumably this maps to some instance of a specialization of NSString. I have not looked this up, but is it as simple as the @ being a special macro? Do you know off-hand how that mapping occurs?