[5hoom]

Coming from C++, protocols vs categories still mystify me a bit.

They appear to be methods for doing multiple inheritance type stuff without the multiple inheritance problems. From what I understand protocols set a list of methods that a class implementing that protocol must provide, whereas categories let you 'tack-on' methods to existing classes.

They seem like powerful features & I'm sure there is more to them than I've described, but I sometimes have problems re-wiring my brain to 'think in objective-c'.

Seeing more tutorials that show how to use these interesting language concepts in real projects would be very educational.

[hboon]

Protocols are "interfaces" (in the general sense of the word in programming). So they are useful when you want to write a delegate (or callbacks in the general sense of the word) class. Many classes have a delegate property that is of type id<SomeProtocol>. Equivalent to Java's interfaces, but Cocoa use protocols heavily for delegates only.

Categories are useful not for sharing code, but for expanding a class's functionality. This is especially useful when you write a class X that works with an existing class Y, but some of X's code should be an instance method of Y, or X introduces additional capability for Y to support X. UITableView adding functionality to NSIndexPath to support referencing table sections and rows is a good example. All those Java classes that add static methods to support existing Java classes, or if you used Smalltalk and added methods to existing classes, this is the equivalent.

[DrJokepu]

(I agree with you, this is just nitpicking)

>Equivalent to Java's interfaces, but Cocoa use protocols heavily for delegates only.

Uh, NSCoding, NSCopying and the million other non-delegate protocols? Cocoa uses protocols heavily for a lot of things, not just delegates. The average Cocoa/Cocoa Touch developer, however, uses protocols mostly for delegates.

[masklinn]

A protocol is an interface (a type contract), a category is a mixin (class extension)

[Zev]

Close.

A protocol says that an object can (and in some (but not all) cases, is required to) implement a given list of methods, leaving the implementation/the "how" up to the programmer. Think of protocols as being similar to interfaces in Java.

OTOH, A category says "in addition to the existing methods, this class will also, for sure, implement these additional methods — and here's how."