Linker error on same named constants in different scopes

Question

I have a constant named "ID_KEY" that is declared at the top of 3 separate .m files, neither of which include the other files.

The declaration is as follows:

#import "PublicGamesResponse.h"

NSString *const ID_KEY = @"id";
...
@implementation PublicGamesResponse

And similarly for the two other classes. However I am getting a linker error complaining about multiple definitions of the same name(If I was to comment out two of the definitions, this goes away).

My question is why on earth is the linker complaining about this? Each definition of ID_KEY is outside of the scope of all of the others, so I don't see why the linker is complaining.

As a disclaimer, I cleaned the project and restarted xCode, and looked for similar questions on the site, but had no luck.

Answer 1

When you define variables or constants outside a function, they are placed in the global scope. Linker resolves global references, and it complains when it finds the same name more than once.

To give the constants a scope of their compilation unit (i.e. the file where they are defined) add static in front of their definitions:

static NSString *const ID_KEY = @"id";

This way all functions and methods inside the same file will have access to ID_KEY, but the name would remain in the scope of the file. Essentially, static "hides" the the name from the linker.