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APPENDIX: Coming to Objective-C from Other Languages
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can avoid some of the runtime errors you might get. You can also use categories to add methods to other classes. Because of this metadata, it s easier to reverse engineer the classes used in a program. You can determine the instance variables, their layout in the object structure, and the methods defined by the class. Even stripping the executable of its debugging information doesn t remove the Objective-C metadata. If you have highly confidential algorithms, you may want to implement them in C++ or at least obfuscate their names don t use class or method names like SerialNumberVerifier, for example. In Objective-C, you can send messages to the nil (zero) object. There is no need to check your message sends against NULL. Messages to nil are no-ops. The return values from messages sent to nil depend on the return type of the method. If the method returns a pointer type (such as an object pointer), the return value will be nil, meaning you can safely chain messages to a nil object the nil will just propagate. If the method returns an int the same size of a pointer or smaller, it will return zero. If it returns a float or a structure, you will have an undefined result. Because of this, you can use a nil object pattern to keep you from having to test object pointers against NULL. On the other hand, this technique can mask errors and cause bugs that are difficult to track down. All objects in Objective-C are dynamically allocated. There are no stack-based objects and no automatic creation and destruction of temporary objects or automatic type conversion between class types, so Objective-C objects are more heavyweight than C++ stack-based objects. That s one of the reasons why small lightweight entities (like NSPoint and NSRange) are structures instead of first-class objects. Finally, Objective-C is a very loose language. Where C++ has public, protected, and private member variables and member functions, Objective-C has some basic support for protected instance variables, which are easy to circumvent, but no protection at all for member functions. Anyone who knows the name of a method can send that message to the object. Using the Objective-C reflection features, you can see all the methods supported by a given object. Methods are callable even if they never appear in a header file, and you have no reliable way to figure out which object is calling the method, because message sends can come from C functions (as discussed earlier in this appendix). As you ve seen, you don t have to redeclare methods you override in subclasses. There are two schools of thought on whether this is a good idea. One camp says that redeclaring provides information to the reader about which changes the class makes to its superclasses, while the other faction says that these are just implementation details that class users don t have to be bothered with and are not worth causing recompilations of all dependent classes when a new method is overridden.
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APPENDIX: Coming to Objective-C from Other Languages
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Objective-C has no class variables. You can simulate them by using file-scoped global variables and providing accessors for them. An example class declaration might look like this (other stuff, like declarations for instance variables and method declarations, is included):
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@interface Blarg : NSObject { } + (int) classVar; + (void) setClassVar: (int) cv; @end // Blarg
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And then the implementation would look like this:
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#import "Blarg.h" static int g_cvar; @implementation Blarg + (int) classVar { return (g_cvar); } // classVar + (void) setClassVar: (int) cv { g_cvar = cv; } // setClassVar @end // Blarg
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The Cocoa object hierarchy has a common ancestor class: NSObject. When you create a new class, you ll almost always subclass NSObject or an existing Cocoa class. C++ object hierarchies tend to be several trees with distinct roots.
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