Constants in Objective-C

Question

I m developing a Cocoa application  and I m using constant NSStrings 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

User · Answer

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

User · Answer

The accepted  and correct  answer says that  you can include this  Constants h  file    in the pre-compiled header for the project    As a novice  I had difficulty doing this without further explanation -- here s how  In your YourAppNameHere-Prefix pch file  this is the default name for the precompiled header in Xcode   import your Constants h inside the  ifdef   OBJC   block    ifdef   OBJC      import  lt UIKit UIKit h gt     import  lt Foundation Foundation h gt     import  Constants h   endif   Also note that the Constants h and Constants m files should contain absolutely nothing else in them except what is described in the accepted answer   No interface or implementation

User · Answer

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

User · Answer

If you want something like global constants  a quick an dirty way is to put the constant declarations into the pch file

User · Answer

Prefs h extern NSString   const RAHUL      Prefs m NSString   const RAHUL     rahul

User · Answer

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

User · Answer

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  lt StringConsts h gt   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

User · Answer

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  lt Foundation Foundation h gt    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

User · Answer

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    I had to redefine the macro in the  m file after  undefing it for the macro to be used  I also had to use  include instead of  import for this to work properly and avoid the compiler seeing the previously precompiled values  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

User · Answer

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  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

User · Answer

If you want to call something like this NSString newLine  from objective c  and you want it to be static constant  you can create something like this in swift   public extension NSString        objc public static let newLine     n      And you have nice readable constant definition  and available from within a type of your choice while stile bounded to context of type

User · Answer

If you like namespace constant  you can leverage struct  Friday Q amp A 2011-08-19  Namespaced Constants and Functions     in the header extern const struct MANotifyingArrayNotificationsStruct       NSString  didAddObject      NSString  didChangeObject      NSString  didRemoveObject    MANotifyingArrayNotifications      in the implementation const struct MANotifyingArrayNotificationsStruct MANotifyingArrayNotifications          didAddObject     didAddObject        didChangeObject     didChangeObject        didRemoveObject     didRemoveObject

User · Answer

Try using a class method     NSString  theMainTitle       return   Hello World       I use it sometimes

User · Answer

You should create a header file like     Constants h FOUNDATION EXPORT NSString  const MyFirstConstant  FOUNDATION EXPORT NSString  const MySecondConstant    etc     you can use extern instead of FOUNDATION EXPORT if your code will not be used in mixed C C   environments or on other platforms   You can include this file in each file that uses the constants or in the pre-compiled header for the project    You define these constants in a  m file like     Constants m NSString  const MyFirstConstant     FirstConstant   NSString  const MySecondConstant     SecondConstant     Constants m should be added to your application framework s target so that it is linked in to the final product   The advantage of using string constants instead of  define d constants is that you can test for equality using pointer comparison  stringInstance    MyFirstConstant  which is much faster than string comparison   stringInstance isEqualToString MyFirstConstant    and easier to read  IMO

[ios] Constants in Objective-C

Examples related to ios

Examples related to objective-c

Examples related to cocoa

Examples related to nsstring

Examples related to constants