Information AboutObjective-c |
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Today it is used primarily on Mac OS X and GNUstep , two environments based on the OpenStep standard, and is the primary language used for the NeXTSTEP , OPENSTEP , and Cocoa application frameworks. Generic object-oriented Objective-C programs that do not make use of these libraries can also be compiled for any system supported by Gcc , which includes an Objective-C compiler. HISTORY In the early 1980s , common Software Engineering practice was based on Structured Programming . Structured programming was implemented in order to help "break down" programs into smaller parts, primarily to make them easier to work on as they grew increasingly large. However, as the problems being solved grew in size, structured programming became less useful as more and more methods had to be written, leading to Spaghetti Code and poor code reuse. Many saw to run in. This virtual machine was very large and tended to require huge amounts of Memory for the time and ran very slowly. ObjC was created primarily by Brad Cox in the early 1980s at his company Stepstone . He had become interested in the problems of true reusability in Software Design and programming. In order to demonstrate that real progress could be made, Cox set about to show that making interchangeable Software Component s really needed only a few practical changes to existing tools. Specifically, they needed to support objects in a flexible manner, come supplied with a set of libraries that were usable, and allow for the code (and any resources needed by the code) to be bundled into a single cross-platform format. The main description of Objective-C in its original form was published in his book, ''Object-Oriented Programming, An Evolutionary Approach'' in 1986 . Cox was careful to point out that there is more to the problem of reusability than just the language, but it appears this fell on deaf ears. Instead, the system often found itself compared on a feature-for-feature basis with other languages. In 1988 , NeXT licenced Objective-C from StepStone and released their own Objective-C compiler and libraries on which the NeXTstep user interface and interface builder were based. The success of the tools and quality of the resultant operating system helped NeXT become a fairly popular niche workstation provider. Steve Jobs demonstrated the power of the system in 1990 , showing its ease of interoperation with Macintosh , Sun , and Windows machines, as well as the advanced features of drag and drop. The GNU project started work on their free clone of NeXTStep based on the OpenStep standard, GNUStep . Dennis Glatting wrote the first gnu-objc Runtime in 1992 , and Richard Stallman followed with a second one shortly after. The GNU Objective-C runtime that has been in use since 1993 is the one developed by Kresten Krab Thorup when he was a university student in Denmark. Kresten also worked at NeXT for a while. After acquiring NeXT in 1996 , Apple used NeXTstep as the basis for its main operating system, Mac OS X . This includes Objective-C and NeXT's Objective-C based developer tool, Project Builder (later replaced by Xcode ), as well as its interface design tool, Interface Builder . Most of the Cocoa API is based on NextStep interface objects. SYNTAX Objective-C is a very "thin" layer on top of C. Objective-C is a ''strict'' Superset of C. That is, it is possible to compile any C program with an Objective-C compiler, which cannot be said of C++ . Objective-C borrows its syntax from both C and Smalltalk. Most of the syntax, including the traditional function calls, is inherited from C, while the syntax for certain object-oriented features, including message-passing, was partially borrowed from Smalltalk. Messages The added syntax is for built-in support of Object-oriented Programming . The Objective-C model of Object-oriented Programming is based on sending Messages to objects, similar to the model of Smalltalk . This is unlike the Simula programming model, which is used by C++ among other programming languages. This distinction is semantically important. The basic difference is that in Objective-C, one does not ''call a method''; one ''sends a message''. An object called obj whose class has a method doSomething implemented is said to ''respond'' to the message doSomething. If we wish to send a doSomething message to obj, we write doSomething ; This differs from Statically Typed languages such as C++ and Java , in that one can send messages to objects that do ''not'' respond to them. See the Dynamic Typing section below. Interfaces and implementations Objective-C requires the interface and implementation of a class to be in separate specially declared code blocks. By convention, the interface is put in a header file and the implementation in a code file; these header files, suffixed .h, are similar to C header files. Interface The interface of the class is usually defined in a header file. Convention is usually to create the name of the header file based on the name of the class. So if we have the class Thing, Thing's interface goes in the file Thing.h. The interface declaration is in this form (NOTE: the class methods need not precede the instance methods): @interface ''classname'' : ''superclass name'' { ''instance variables'' } + ''class method'' + ''class method'' ... - ''instance method'' - ''instance method'' ... @end Implementation The interface only declares the prototypes for the methods, and not the methods themselves, which go in the implementation. The implementation is usually stored in a main file, for example, Thing.m. The implementation is written @implementation ''classname'' + ''class method'' { ''implementation'' } - ''instance method'' { ''implementation'' } ... @end Methods are written in a different way from C-style functions. For example, a function in both C and Objective-C follows this general form: int do_something(int i) { return square_root(i); } with int do_something(int) as the prototype. When this is implemented as a method, this becomes: - (int) do_something: (int) i { return square_root: i ; } The hyphen lets us know that this is an Instance Method , and not a Class Method (which uses a +). This syntax may appear to be more troublesome but it allows the Naming Of Parameter s, for example - (int) changeColorWithRed: (int) r green: (int) g blue: (int) b ''...'' which can be invoked thus: changeColorWithRed:5 green:2 blue:6 ; Internal representations of this method vary between different implementations of Objective-C. If myColor is of the class Color, internally, instance method -changeColorWithRed:green:blue: might be labeled _i_Color_changeColorWithRed_green_blue. The i is to refer to an instance method, with the class and then method names appended, colons translated to underscores. However, internal names of the function are rarely used directly, and generally even message-sends are converted to a call to a function defined in a run-time library rather than directly accessing the internal name. This is partially because it is rarely known at compile-time which method will actually be called, because the class of the receiver (i.e. the object being sent the message) is rarely known until runtime. Protocols Objective-C was extended at NeXT to introduce the concept of multiple inheritance of specification, but not implementation, through the introduction of protocols. This is a pattern achievable as an abstract multiply inherited base class in C++ or, more popularly adopted in Java as an "interface". Objective-C makes use of both ad-hoc protocols, called informal protocols, and compiler enforced protocols called formal protocols. An informal protocol is a list of methods that a class can implement. It is specified in the documentation, since it has no presence in the language. Informal protocols often include optional methods, where implementing the method can change the behavior of a class. For example, a text field class might have a delegate that should implement an informal protocol with an optional autocomplete method. The text field discovers whether the delegate implements that method (via Reflection ), and if so, calls it to support autocomplete. A formal protocol is similar to an interface in Java. It is a list of methods that any class can declare itself to implement. The compiler will emit an error if the class does not implement every method of its declared protocols. The Objective-C concept of protocols is different from the Java concept of interfaces in that a class may implement a protocol without being declared to implement that protocol. The difference is not detectable from outside code. The syntax @protocol Locking - (void)lock; - (void)unlock; @end denotes that there is the abstract idea of locking that is useful and when stated in a class definition @interface SomeClass : SomeSuperClass ... @end that instances of SomeClass will provide an implementation for the two instance methods using whatever means they want. This abstract specification is particularly useful to describe the desired behaviors of plug-ins for example, without constraining at all what the implementation hierarchy should be. Dynamic typing Objective-C is a Dynamically Typed language, as is Smalltalk. This means that we can send to an object a message that is not specified in its interface. This may seem like a bad idea, but in fact this allows for a great level of flexibility - in Objective-C an object can "capture" this message, and depending on the object, can send the message off again to a different object (who can respond to the message correctly and appropriately, or likewise send the message on again). This behaviour is known as ''message forwarding'' or ''delegation'' (see below). Alternatively, an error handler can be used instead, in case the message cannot be forwarded. However if the object does not forward the message, handle the error, or respond to it, a Runtime error occurs. Like other dynamically typed languages, there is the potential problem of Run-time Error s that come from sending the wrong message to the wrong object. However, Objective-C allows the programmer to optionally specify the class of an object, and in such cases the Compiler will apply static-typing. In an Objective-C function declaration, a special type, id, casts a parameter as a Pointer to an object, without requiring that object to be of any class: -(void)setMyValue:(id)aStringOrANumber; If the programmer wants to require that the parameter is an object of a specific class, he can specify the type in the same way:
Perhaps most usefully, the programmer can allow the parameter to be dynamically-typed, but require that the parameter conform to a given protocol: -(void)setMyValue:(id When one does need dynamic typing, it becomes tremendously powerful. Consider a simple example, placing an object in a container. In statically-typed languages without generics like pre-1.5 Java, the programmer is forced to write a Container Class for a generic type of object, and then cast back and forth between the abstract generic type and the real type. Sadly, Casting breaks the discipline of Static Typing —if you put in an Integer and read out a String , you get an error. This is annoying when you consider that it was a string when you put it in there three lines earlier, and now you have to tell the compiler that it is a string coming back out just to call toUpperCase(). Dynamic typing avoids this problem by delaying type-checking until runtime.Forwarding As mentioned, since Objective-C permits the sending of a message to an object that might not respond to it, the object has a number of things it can do with the message. One of these things could be to forward the message on to an object that can respond to it. Forwarding can be used to implement certain Design Patterns , such as the Observer Design Pattern or the Proxy Design Pattern very simply. See Examples Of Message Forwarding In Objective-C for such an implementation. The Objective-C runtime specifies a pair of methods in Object
- (retval_t) forward: (SEL) sel : (arglist_t) args; // with GCC - (id) forward: (SEL) sel : (marg_list) args; // with NeXT/Apple systems
- (retval_t) performv: (SEL) sel : (arglist_t) args; // with GCC - (id) performv: (SEL) sel : (marg_list) args; // with NeXT/Apple systems and as such an object wishing to implement forwarding needs only to override the forwarding method to define the forwarding behaviour. The action methods performv:: need not be overriden as this method merely performs the method based on the selector and arguments. Example Here is an example of a program that demonstrates the basics of forwarding. ; ''Forwarder.h'' #import @interface Forwarder : Object { id recipient; //The object we want to forward the message to. } //Accessor methods - (id) recipient: (id) _recipient; - (id) recipient; @end ; ''Forwarder.m'' #import "Forwarder.h" @implementation Forwarder - (retval_t) forward: (SEL) sel : (arglist_t) args {
if( respondsTo:sel ) return performv: sel : args ; else return error:"Recipient does not respond" ; } - (id) recipient: (id) _recipient { recipient = _recipient; return self; } - (id) recipient { return recipient; } @end ; ''Recipient.h'' #import // A simple Recipient object. @interface Recipient : Object - (id) hello; @end ; ''Recipient.m'' #import "Recipient.h" @implementation Recipient - (id) hello { printf("Recipient says hello! "); return self; } @end ; main.c #import "Forwarder.h" #import "Recipient.h" int main(void) { recipient:recipient ; //Set the recipient.
hello ; return 0; } Notes If we were to compile the program, the compiler would report that $ gcc -x objective-c -Wno-import Forwarder.m Recipient.m main.c -lobjc main.c: In function `main': main.c:12: warning: `Forwarder' does not respond to `hello' $ The compiler is reporting the point that was made earlier, Forwarder does not respond to hello messages. In certain circumstances, such a warning can help us find errors, but in this circumstance however, we can safely ignore this warning, since we have implemented forwarding. If we were to run the program $ ./a.out Recipient says hello! For more examples, see Examples Of Message Forwarding In Objective-C Categories Cox's main concern was the maintainability of large code bases. Experience from the structured programming world had shown that one of the main ways to improve code was to break it down into smaller pieces. Objective-C added the concept of ''Categories'' to help with this process. A category collects method implementations into separate files. The programmer can place groups of related methods into a category to make them more readable. For instance, one could create a "SpellChecking" category "on" the String object, collecting all of the methods related to spell checking into a single place. Furthermore, the methods within a category are added to a class at Runtime . Thus, categories permit the programmer to add methods to an existing class without the need to recompile that class or even have access to its source code. For example, if the system you are supplied with does not contain a Spell Checker in its String implementation, you can add it without modifying the String source code. Methods within categories become indistinguishable from the methods in a class when the program is run. A category has full access to all of the instance variables within the class, including private variables. Categories provide an elegant solution to the Fragile Base Class Problem for methods. If you declare a method in a category with the same Method Signature as an existing method in a class, the category's method is adopted. Thus categories can not only add methods to a class, but also replace existing methods. This feature can be used to fix bugs in other classes by rewriting their methods, or to cause a global change to a class's behavior within a program. If two categories have methods with the same method signature, it is undefined which category's method is adopted. Other languages have attempted to add this feature in a variety of ways. TOM took the Objective-C system to its logical conclusion and allowed for the addition of variables as well. Other languages have instead used Prototype Oriented solutions, the most notable being Self . Example usage of categories This example builds up an Integer class, by defining first a basic class with only Accessor Method s implemented, and adding two categories, Arithmetic and Display that extend the basic class. Whilst categories can access the base class's private data members, it is often good practice to access these private data members through the accessor methods, which helps keep categories more independent from the base class. This is one typical usage of categories—the other is to use categories to add or replace certain methods in the base class (however it is not regarded as good practice to use categories for subclass overriding). ; ''Integer.h'' #include @interface Integer : Object { int integer; } - (int) integer; - (id) integer: (int) _integer; @end ; ''Integer.m'' #import "Integer.h" @implementation Integer - (int) integer { return integer; } - (id) integer: (int) _integer; { integer = _integer; } @end ; ''Arithmetic.h'' #import "Integer.h" @interface Integer (Arithmetic)
@end ; ''Arithmetic.m'' #import "Arithmetic.h" @implementation Integer (Arithmetic)
{ return integer: [self integer + integer ]; }
{ return integer: [self integer - integer ]; } @end ; ''Display.h'' #import "Integer.h" @interface Integer (Display) - (id) showstars; - (id) showint; @end ; ''Display.m'' #import "Display.h" @implementation Integer (Display) - (id) showstars { int i, x = integer ; for(i=0; i < x; i++)
printf(" "); return self; } - (id) showint { printf("%d ", integer ); return self; } @end ; ''main.c'' #import "Integer.h" #import "Arithmetic.h" #import "Display.h" int main(void) { printf("Enter an integer: "); scanf("%d", &x); integer:x ; showstars ; printf("Enter an integer: "); scanf("%d", &x); integer:x ; showstars ; add:num2 ; showint ; } = Notes Compilation is performed, for example, by gcc -x objective-c main.c Integer.m Arithmetic.m Display.m -lobjc You can experiment by omitting the #import "Arithmetic.h" and add:num2 lines and omit Arithmetic.m in compilation. The program will still run. This means that it is possible to "mix-and-match" added categories if necessary - if one does not need to have some capability provided in a category, one can simply not compile it in. Posing Objective-C permits a class to wholly replace another class within a program. The replacing class is said to "pose as" the target class. All messages sent to the target class are then instead received by the posing class. There are several restrictions on which classes can pose:
Posing, similarly to categories, allows globally augmenting existing classes. Posing permits two features absent from categories:
For example, @interface CustomNSApplication : NSApplication @end @implementation CustomNSApplication
{ // do something with menu } @end class_poseAs ( class , class ); This intercepts every invocation of setMainMenu to NSApplication. OTHER FEATURES Objective-C in fact included a laundry-list of features that are still being added to other languages, or simply don't exist at all. These led from Cox's (and later, NeXT 's) realization that there is considerably more to programming than the language. The system has to be usable and flexible as a whole in order to work in a real-world setting.
Objective-C++ Objective-C++ is a front-end to the Mac OS X version of the GNU Compiler Collection that can compile source files that use both C++ and Objective-C, with certain restrictions:
TODAY Objective-C today is often used in tandem with a fixed library of standard objects (often known as a "kit" or "framework"), such as libraries come with the OPENSTEP operating system and Cocoa comes with Mac OS X . One can however bypass the framework and inherit directly from the root object, Object, and create one's own functionality. The aforementioned libraries however implement NSObject, which adds some additional features to Object, such as Reference Counting . History note: Earlier versions of NeXT 's NeXTSTEP operating system had objects inheriting from Object, but migrated to the newer NSObject root class, which was named to distinguish it from the original Object root (see note on namespaces below). All newer versions of the NeXTSTEP libraries which had objects inheriting from NSObject were prefixed with "NS", while those which did not inherit from NSObject did not. Later, the entire library codebase as OpenStep moved to use the NSObject class outright, and to maintain compatibility with the NeXTSTEP libraries, the prefix was maintained, and is still maintained in Cocoa today. On the other hand, Cocoa has a class which does not inherit from NSObject: NSProxy. ANALYSIS OF THE LANGUAGE Objective-C is a very pragmatic language. Objective-C implementations use a thin Runtime written in C that adds little to the size of the application. In contrast, most OO systems at the time that it was created used large VM runtimes that took over the entire system. Programs written in ObjC tend to be not much larger than the size of their code and that of the libraries (which generally don't need to be included in the Software Distribution ), in contrast to Smalltalk systems where a large amount of memory was used just to open a window. Likewise, the language can be implemented on top of existing C compilers (in the GCC , first as a preprocessor, then as a module) rather than as a new compiler. This allowed ObjC to leverage the huge existing collection of C code, libraries, tools, and mindshare. Existing C libraries—even in object code libraries—can be wrapped in ObjC Wrappers to provide an OO-style interface. All of these practical changes lowered the Barrier To Entry , likely the biggest problem for the widespread acceptance of Smalltalk in the 1980s. Objective-C can thus be described as offering much of the flexibility of the later Smalltalk systems in a language that is deployed as easily as C. The first versions of Objective-C did not support Garbage Collection . At the time this decision was a matter of some debate, and many people considered long "dead times" (when Smalltalk did collection) to render the entire system unusable. Although some 3rd party implementations have added this feature (most notably GNUstep), Apple has not implemented it as of Mac OS X V10.4 , but is planning to do so in the near future. Another common criticism is that Objective-C does not have language support for Namespaces . Instead programmers are forced to add prefixes to their class names, which can cause collisions. As Of 2004 , all Mac OS X classes and functions in the Cocoa programming environment are prefixed with "NS" (as in NSObject or NSButton) to clearly identify them as belonging to the Mac OS X core; the "NS" derives from the names of the classes as defined during the development of NEXTSTEP . Since Objective-C is a strict superset of C, it does not treat C primitive types as First-class Object s either. Unlike C++ , Objective-C does not support Operator Overloading , though it does support Overloading . Also unlike C++, Objective-C allows an object only to directly inherit from one class (forbidding Multiple Inheritance ). As Java was influenced by the design of Objective-C, the decision to use single inheritance was carried into Java. Categories and protocols may be used to provide many of the benefits of multiple inheritance, without many of the disadvantages, such as extra runtime overhead and binary incompatibilities. EXTERNAL LINKS
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