Question

In the header of the class, outside of interface declaration, I’ve declared global constants:

NSString * const gotFilePathNotification = @"gotFilePath";
NSString * const gotResultNotification = @"gotResultOfType";

gotResultNotification is used only in this class (yet), but I reference gotFilePathNotificaion in another class implementation. To do it, I import this header.

When I try to compile, I get a duplicate symbol linker error about gotFilePathNotification in this header. Why does it happen?

Answer 1

You have two identifier(s) with same name across two different compilation unit(s) at file scope. This violates One Definition Rule. Instead you need to –

1. Declare the global variables marking to have external linkage in a header file.

   extern NSString * const gotFilePathNotification;
   

2. Now provide the definition in only one source file.

   NSString * const gotFilePathNotification = @"gotFilePath";
   

Now where ever you need to use these variables, include the header in the source file.