Programming iOS 6, 3rd Edition

Pointer Parameters and the Address Operator

Objective-C is chock-a-block with pointers (and asterisks). Objective-C methods typically expect pointer parameters and return a pointer value. But this doesn’t make things more complicated, because, as I’ve already mentioned, your variables referring to Objective-C objects are pointers. Pointers are what Objective-C expects, but pointers are also what Objective-C gives you. Pointers are exactly what you’ve got, so there’s no problem.

For example, one way to concatenate two NSStrings is to call the NSString method stringByAppendingString:, which the documentation tells you is declared as follows:

- (NSString *)stringByAppendingString:(NSString *)aString

This declaration is telling you (after you allow for the Objective-C syntax) that this method expects one NSString* parameter and returns an NSString*. That sounds messy, but it isn’t, because every NSString is really an NSString*. So nothing could be simpler than to obtain a new NSString consisting of two concatenated NSStrings:

NSString* s1 = @"Hello, ";
NSString* s2 = @"World!"
NSString* s3 = [s1 stringByAppendingString: s2];

Sometimes, however, a function or method expects as a parameter a pointer to a thing, but what you’ve got is not that pointer but the thing itself. Thus, you need a way to create a pointer to that thing. The solution is the address operator (K&R 5.1), which is an ampersand before the name of the thing.

For example, there’s an NSString method for reading from a file into ...