Question

I recently picked up the BigNerdRanch book on Cocoa for Mac OSX and am looking for a good explanation around the use of asterisks * and underscores _ that seem to be inconsistently used throughout the language. Some examples:

These appear to be functionally equivalent:

NSString* myString;
NSString *myString;

Sometimes new variables get an underscore, while others dont:

NSString _myString;
NSString myString;

And sometimes it gets all crazy:

NSString *myString;
NSString *_myString;

I’ve also seen variations with asterisks in methods:

- (void)speechSynthesizer:(NSSpeechSynthesizer *)sender
            willSpeakWord:(NSRange)characterRange
                 ofString:(NSString *)string;

So what is the functional use of the asterisk or the underscore, when should you use (or not use) either, and when should they be used in combination?

Answer 1

The * indicate a pointer, which all Objective-C objects are. (You pass around pointers to these objects in memory). At a basic level these are normal C pointers. If I remember correctly You could access some data in an Objective-C object by going object->data, just like you do with pointers to C structs.

The _ is a Cocoa (and some other languages) convention, meaning “a piece of data that should be considered private to this object”.

Objective-C has a @private declaration, but it’s also a relatively new addition to the language – if your code is more than 2 or 3 years old (or targeting much older versions of OS X) it might not use @private

Because of this initial lacking of language infrastructure, the _ is (often) used by the Cocoa community to mark “Hey, you probably shouldn’t set or read this directly, please”.

So:

• When dealing with Objective-C classes you always need the * to follow the class name (like NSString), because they are always pointers. I’m confused about your NSString somestring line in your code – either that’ll generate a complier warning or will crash when you try to use it
• The _ indicates private data. You would do something like NSString* _name in a @interface section of your Objective-C class. You would use _name by itself if you were calling or operating on that data in a method in your class.

So say you created a Student class:

// Student.h
@interface Student : NSObject {
  NSString* _name;

}

- (NSString*) name_as_caps;
@end


// Student.m

@implementation Student
- (NSString*) name_as_caps {
   return [_name uppercase];
}

Here we declare a class with a “private” data member: _name. Our Student class needs to return this capitalized for some reason, so we created a method to do that, where we use _name and call the uppercase method on it.

We needed to refer to the type (or class name) or _name a few times here: once, to declare the variable. For name_as_caps we needed to say: this method returns a pointer to an NSString object, thus we used NSString *.