You should use:
NSString * const kPolygonNumberOfSides = @"..."; // const pointer
instead of:
NSString const * kPolygonNumberOfSides = @"..."; // pointer to const
The first is a constant pointer to an NSString object, while the second is a pointer to a constant NSString object.
It is a subtle difference. The compiler warning happens because setObject:forKey: is declared as follows:
- (void)setObject:(id)value forKey:(NSString *)defaultName;
It is expecting the defaultName argument to be of type NSString *. When you instead pass in a pointer to a constant, you’ve given it something different.
Update: I want to point out that these constants should be defined as static if they are only going to be used from within a single file. I say this because I have run across this problem myself: if you do not declare them as static, then they will exist in the global namespace, and you will not be able to use a variable with the same the same name in another file. see Constants in Objective-C for more information. To explain by example, this is what I currently use for keys that I only need to use in one .m file:
static NSString * const kSomeLabel = @"...";