1. private

myfile.m
NSString *const SelectionSort = @”SelectionSort”;

2. public

myfile.h
extern NSString *const SelectionSort;
and in myfile.m define them

myfile.m
NSString *const SelectionSort = @”SelectionSort”;

It works, but only if you need a non global constant. Static constant is not visible outside the file. Otherwise use the 1st option.

http://stackoverflow.com/questions/6188672/where-do-you-declare-constant-in-objective-c

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The #define is a pre-processor macro. That means that it basically goes through your code and replace your macro with what you’ve defined.

If you use a const, it’s going to be a pointer to the string in memory. It’s way more efficient than having the same string being allocated wherever/whenever it is used.

To do that, you’ll need both .h and .m files. Your .h file will look something like:

extern NSString * const YOUR_STRING;
And your .m file:

NSString * const YOUR_STRING = @”your string”;

no need Class.THIS_IS_A_CONST , just THIS_IS_A_CONST then ok

===============================================

把NSString宣告為常數
錯誤的宣告方式
會出現warning:(Sending ‘const NSString *__strong’ to parameter of type ‘NSString *’ discards qualifiers)
.h
extern NSString * SelectionSort;
.m
const NSString *InsertionSort = @”SelectionSort”;

正確的宣告方式
.h
extern NSString *const SelectionSort;
.m
NSString *const SelectionSort = @”SelectionSort”;

http://mywayonobjectivec.blogspot.hk/2014/05/nsstring.html