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GNUstep: Fun with Objective-C(Note: This article originally appeared in cscene)Objective-C is Funby Adam FedorObjective-C is a language based upon C, with a few additionsthat make it a complete, object-oriented language. Why do I thinkObjective-C is fun? Precisely because of this emphasis onsimplicity. Absolutely nothing was added to the language unless itreally made a big improvement on the useability of Objective-C.Fun, of course, can also lead to danger. And there is plenty tobeware of with Objective-C. Since Objective-C doesn't try toimprove the C language, you have to deal with all the faults andcaveats of C. In addition, the philosophy of Objective-C is toallow for a great deal of flexibility, leaving the programmer towatch out for potential problems rather than forcing variousrestrictions. For instance, Objective-C provides an objectdefinition that is completely untyped. This is the idvariable type, which stands for any object. With this type, you cansend a message to any object, without knowing at all what theobject is or what it can do.Like C, this flexibility allows you to perform great tricks,while forcing the programmer to introduce his own conventions toprevent errors. But, ahh, there is fun in danger, isn't there?Note: In the following article, I describe a particular variantof Objective-C known as OpenStep (or GNUstep, the freeimplementation of OpenStep). This is by far the most popularimplementation of Objective-C (there is no standard Objective-Clanguage yet...).An IntroductionIn truth, there is actually only one syntax addition to thelanguage -- the syntax for sending a message to an object: [anObject doSomethingWith: anotherObject];This statement simply sends the message doSomethingWith: toanObject, with an argument of anotherObject. AnObjective-C message call can be used anywhere a C statement can beused, such as in a conditional statement or within anotherObjective-C message. On compilers that support it, Objective-Cmessages can be mixed in C++ code, allowing the programmer to pickand choose the best aspects of either language.In addition, there are some keywords that have been added toallow for the definition and implementation of classes, such as thedefinition for the class of which anObject is amember:@interface MyClass : NSObject{ int aVariable; id subObject;}+ alloc;+ defaultObject;- init;- (int) doSomethingWith: (id)anotherObject;@endHere, @interface tells the compiler we are defining aninterface for the class named MyClass, which is a subclassof the class NSObject. NSObject is the root classused by GNUstep (The traditional Objective-C root classObject, is not used at all in GNUstep, although it isavailable). The root class is not a subclass of any other class.Although it is not required that a class descend from a root classlike NSObject, this hierarchy is so firmly ingrained inthe language, that it is rare to see classes that are a root classby themselves. The root class contains some of the most basic andoften used functionality, which gives you the advantage of knowingthat all your classes will responds to these basic messages. Out ofthe 300 or so classes in the GNUstep core library, only two areroot classes.The curly brackets after the class definition enclose the datathat is encapsulated in the class. These variables are calledinstance variables, because they are variables that belongto an instance of a class.The rest of the interface defines the methods implemented by theclass. A method is a description of a message a class responds to.A '+' indicates a class method. Class methods are oftencalled factory methods, because typically a factory methodis used to create or manufacture instances of the class.The most popular class method is alloc, which allocatesspace for an instance of the class (but does not initialize theclass -- that is usually reserved for the instance methodinit). A '-' indicates an instance method. Forexample, the doSomethingWith: method defined above takes oneargument, the object anotherObject, and returns aninteger. Class messages can only be sent to a class. Instancemessages can only be sent to an instance of a class.Dynamic BindingObjective-C is a language that implements true dynamicbinding (which is required for a language to be trulyobject-oriented). This means that messages sent to an object aren'tbound to a specific function implementation in a specificclass until the program is actually run. Stating this another way,the programmer does not know how an object will react to aspecific message until the program is actually run.Dynamic binding comes about because in every Objective-Cprogram, there is a runtime that works behind the scenesto connect a specific message with a specific object. Every time amessage like doSomethingWith: is sent to an object, theObjective-C runtime looks up the definition of the class of theobject, finds the function (method) that corresponds to the messagedoSomethingWith:, and then calls that function.It may seem like this type of lookup could really slow a programdown. In practice, however, most runtimes are optimized for thissort of lookup, and in general, it can be shown that message lookupis often not the dominant time factor of a program (particularly inGUI programs). Anywhere that time is critical, it's possible inObjective-C to pre-bind a message to its implementation,thus avoiding the expensive message lookup.An interesting aspect of a method definition, is that once amethod is defined, that method name and definition are, in a senseglobal, and can be used anywhere. For instance, say I hadan object newObject that did not recognize thedoSomethingWith: message. Well I could send the message tothe object anyway, and the compiler would not complain aboutit: id newObject; // could be any object; newObject = [[NewObject alloc] init]; // create and initialize the object [newObject doSomethingWith: anotherObject]; // send it a message.Only when the program was run would the Objective-C runtimediscover that newObject did not recognize thedoSomethingWith: message. However, instead of immediatelyissuing an error, the runtime instead sends another message tonewObject, called the forwardInvocation: message. Thisgives newObject a chance to handle messages sent to it that itdoesn't understand. In this case, if newObject knew about anObject,it could forward the message on to anObject and let it handle themessage:- (void) forwardInvocation: (NSInvocation*)anInvocation{ if ([anObject respondsToSelector: [anInvocation selector]]) return [anInvocation invokeWithTarget: anObject]; else return [self doesNotRecognizeSelector: [anInvocation selector]];}Here, the variable self refers to the object whichreceived the message (similar to the this variable inC++). anInvocation is another object which contains allthe information about the message that was sent, including the nameof the message (the selector) and any arguments.This type of usage is called delegation, and it is a powerfulmethod of implementing various different constructs such asmultiple inheritance, journaling, and dispatching messages todynamically loaded code.Incidentally, there are proper ways to handle unrecognizedmessage, using Protocols, that allow for compile time checking andmore robust code. Most programmers avoid defining un-typed objectsunless it's really useful. A better way to write the first codesegment would be: NewObject *newObject; // newObject will be an instance of the NewObject class newObject = [[NewObject alloc] init]; // create and initialize the object [newObject doSomethingWith: anotherObject];In this case, the compiler would issue a warning stating thatnewObject does not respond to the messagedoSomethingWith:.ImplementationI've already given an example of how to implement a method withthe forwardInvocation: method. That example wasn'tentirely correct. In Objective-C, a method implementation must beassociated with a certain class by enclosing the implementationbetween the keywords @implementation and@end.@implementation MyClass- (int) doSomethingWith: anotherObject{ return [anotherObject multiply: 3 by: 4];}@endThe @implementation keyword, is a counterpart to the@interface keyword (Note that it is not necessary toindicate that MyClass is a subclass of NSObject. The compileralready knows this from the interface definition). You canimplement as many methods as you like within an@implementation block (even methods that were notdescribed in the class interface).Methods that are implemented in the implementation section butare not described in the interface become, in essence, privatemethods that can only be used by that class and not by anotherclass.Power through CustomizationOne of the most frustrating aspects of using classes written byother people is that these classes often don't contain some keyfunctionality that you need. One way to add this functionality(assuming you don't have the source to the original class, or don'twant to change it for some other reason), is to simply make asubclass and implement the methods you need in that class. But thiscreates another problem. Now you have to tell everyone else to useyour particular subclass and not the original class. Or what ifsomeone else has already subclassed this class and added differentfunctionality. How do you combine the two subclasses?Objective-C provides a very simple and elegant way to addfunctionality to an existing class using Categories. Acategory is defined simply with an @interface line and thename of the category (e.g. MyAdditionalMethods):@interface MyClass (MyAdditionalMethods)- (int) doSomethingWith: thisObject andThenWith: thatObject;@endThe additional methods are then implemented in a complementary@implementation section. There is one restriction,however. You can't define additional instance variables, whichwould change the amount of data held by the class. Other than thisrestriction, the things you can do with categories is incredible(and bizarre). For instance, if a method is defined both in themain class interface and a category, the category implementationwins. A message sent to the class will use the categoryimplementation and ignore the original class implementation. If amethod is defined in two separate categories, the implementationthat is used is indeterminate, since you never know which categorywill be loaded first. In a situation where dynamic loading is used,this sort of behavior can create all sorts of headaches for asecurity conscious programmer...Most often, however, categories are used to implement additionalfunctionality that may be useful to only one part of the program.For instance, some additional methods may be defined to allow aclass to better interact with the GUI section of the program, orthe distributed objects section of the program.And More...I've only touched on a few of the features of Objective-C. Ialso haven't answered a lot of typical first-time Objective-C userquestions, like why is the allocation of space for a class(alloc) separated from the initialization of the class(init)? How do you get rid of an object once your done withit (garbage collecting)? Future articles might tackle thesequestions.To learn more, I'd encourage you to visit the GNUstep home page(www.gnustep.org) or the AppleDevelopers page ( |
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