NSNumber constants in Obj-C

Question

I want to make some NSNumber constants via the same style used for NSStrings in this topic. That is, I'm creating separate constants.h/.m files and importing them into classes that need to access them.

Answer 1

The trouble with doing this is that there isn't such a thing as a compile-time constant NSNumber. Only NSString gets that distinction. NSNumbers are always created dynamically. You can fake it by using a function that runs at your program's startup to initialize the variables. Your options:

1. Create a class with a +load method that performs the initialization.

2. In the file with the constants, include a function with __attribute__((constructor)). So, for example:

   // Constants.m
   
   NSNumber *someGlobalNumber;
   
   __attribute__((constructor))
   static void InitGlobalNumber() {
       someGlobalNumber = [[NSNumber numberWithInteger:1] retain];
   }
   

But of course then you can't reliably use these numbers in any other functions which are run that early in the startup process. This usually isn't a problem, but is worth keeping in mind.

The other option, which I've seen crop up a few times, is to have a class with accessors for the numbers instead of giving raw access to the variables. It's a bit of a heavier design, but it also feels less voodooish, which has its charms.

Answer 2

Unfortunately you cannot currently generate NSNumber constants in the same way you can generate NSString constants. When you try to do you will get a compiler error

NSNumber * const kNumberConstant = @2; // This doesn't work.

However, you can use primitives instead.

NSInteger const kSomeIntValue = 10;