Where do you declare a constant in Objective-C?

Question

I declared a constant in a header file const double EARTH_RADIUS=6353; that is imported to various other headers and I got a linker error.

Ld /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Products/Debug-iphonesimulator/BadgerNew.app/BadgerNew normal i386
    cd /Users/Teguh/Dropbox/badgers/BadgerNew
    setenv MACOSX_DEPLOYMENT_TARGET 10.6
    setenv PATH "/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
    /Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/llvm-g++-4.2 -arch i386 -isysroot /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk -L/Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Products/Debug-iphonesimulator -F/Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Products/Debug-iphonesimulator -filelist /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Intermediates/BadgerNew.build/Debug-iphonesimulator/BadgerNew.build/Objects-normal/i386/BadgerNew.LinkFileList -mmacosx-version-min=10.6 -Xlinker -objc_abi_version -Xlinker 2 -framework CoreLocation -framework UIKit -framework Foundation -framework CoreGraphics -framework CoreData -o /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Products/Debug-iphonesimulator/BadgerNew.app/BadgerNew

ld: duplicate symbol _EARTH_RADIUS in /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Intermediates/BadgerNew.build/Debug-iphonesimulator/BadgerNew.build/Objects-normal/i386/NearbyIsiKota.o and /Users/Teguh/Library/Developer/Xcode/DerivedData/BadgerNew-bjopcgcgsjkcvcevflfbvsjwfgnu/Build/Intermediates/BadgerNew.build/Debug-iphonesimulator/BadgerNew.build/Objects-normal/i386/FrontPageofBadger.o for architecture i386
collect2: ld returned 1 exit status

Basically I want the constant to be available for all classes on my project. Where should I declare it?

score 96 · Accepted Answer · answered May 31 '11 at 13:58

96

You can declare in the header, define it in a code file. Simply declare it as

extern const double EARTH_RADIUS;

then in a .m file somewhere (usually the .m for the .h you declared it in)

const double EARTH_RADIUS = 6353;

answered May 31 '11 at 13:58

Dan F

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Do you actually need to do the extern in the header ? Without it, it seems to work. – Paweł Brewczynski Jan 06 '15 at 22:13
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`extern` in the header allows other files to know about your constant. If you only need the constant for the scope of a single file, then the `extern` is not necessary. – Dan F Jan 07 '15 at 12:20
in .m, does it matter if before or after `@implementation`? – kraftydevil May 27 '15 at 16:18
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@kraftydevil technically it doesn't matter, but the convention is to do it after your `#import`s and before the `@implementation`. But really anywhere in the global scope of the file is fine (meaning as long as it isn't inside any curly braces `{}` – Dan F May 28 '15 at 10:50

score 65 · Answer 2 · answered Mar 02 '12 at 01:28

65

There are two ways to accomplish this:

1st option- As previous replies pointed, in the .h file:

myfile.h
extern const int MY_CONSTANT_VARIABLE;

and in myfile.m define them

myfile.m    
const int MY_CONSTANT_VARIABLE = 5;

2nd option- My favourite:

myfile.h
static const int MY_CONSTANT_VARIABLE = 5 ;

answered Mar 02 '12 at 01:28

spacebiker

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why declare with static const int MY_CONSTANT_VARIABLE = 5 works? – user4951 Mar 02 '12 at 06:15
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It works, but only if you need a non global constant. Static constant is not visible outside the file. Otherwise use the 1st option. – spacebiker Mar 05 '12 at 20:10
Where in the .h or .m files? Inside curly braces at top? Elsewhere? – Basil Bourque May 22 '14 at 19:34
This seems to be wrong. I get the error from Xcode5: "Interface type cannot be statically allocated" when I do `static const int MY_CONSTANT_VARIABLE = 5 ;` – learner Jul 14 '14 at 17:59
Hi @learner, are you declaring the constant below the import in your header file? – spacebiker Jul 15 '14 at 17:13

score 4 · Answer 3 · answered May 31 '11 at 13:58

4

Declare it in a source file and have external linkage to it( using extern keyword ) so as to use in all other source files.

answered May 31 '11 at 13:58

Mahesh

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score 1 · Answer 4 · edited May 23 '17 at 12:25

1

Best practice would be to declare it in your .h and .m files. See Constants in Objective-C for a very detailed set of answers regarding this same question.

edited May 23 '17 at 12:25

Community

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answered Aug 30 '12 at 15:56

Kyle Clegg

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Where do you declare a constant in Objective-C?

4 Answers4

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