Introduction to Objective-C in Objective-C

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Introduction to Objective-C
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string open back, 5 string resonator, fretless, etc.) you use inheritance to customize behavior. In the inheritance relationship, the general class is called the base, or parent class, and the class that specifies custom behavior is the derived, or child class. Inheritance enables you to create class hierarchies that derive specific behavior from a common parent or set of parents (multiple inheritance). An alternative to inheritance is composition. Whereas inheritance derives general behavior from a parent class, composition enables specification by assembling various classes within another class and calling the composed objects through their class interface. One of the best discussions of object-oriented concepts is the first chapter of Design Patterns: Elements of Reusable Object-Oriented Software, listed in the reference section at the end of the book.
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5.2.2 Classes
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Like all object-oriented languages, Objective-C supports classes. A class is a definition, or blueprint, of a user-defined type. Classes are a fundamental piece of any object-oriented language; they encapsulate data members and the methods that operate on the data members. Classes in Objective-C are implemented in two files: the interface definition resides in the .h file, and the implementation resides in the .m file. The following example shows a skeleton of an interface definition (.h file):
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@interface Class Name : <Super Class> <Protocol List> { // Instance variables } // Methods @end
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An Objective-C class definition begins with the @interface directive. In Objective-C, the @ token is a compiler directive. Following the interface keyword are the class name and its super class. The class name is the name you give to your class; the super class is optional. If it s included, it specifies the parent class from which your class derives its behavior. If it s omitted, your class is a root or base class. Next is the protocol list. Protocols are discussed in section 5.2.5. This is followed by the class s data members, enclosed in a right and left brace, and any class methods. You terminate the class definition with the @end directive. Class implementations reside in a .m file. Here s a skeleton of a class s implementation:
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@implementation <Class Name> // Implement class methods here @end
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Objective-C and the Cocoa development frameworks
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You control access to the class through the private, protected, and public keywords. The private keyword means that class members are only accessible from within the class that declared them. Protected restricts access to inheriting classes, and public permits anyone to access the class. Data members Data members (sometimes called fields) store the data state of a class and provide runtime data persistence for the class. Objective-C uses the same built-in types as C, including int, long, float, double, char, and pointers. In addition, it defines further types exclusive to Objective-C (see table 5.1).
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Table 5.1 Objective-C uses C s data types, but also defines further types. Type id Class SEL IMP BOOL nil Nil Description Holds an object (pointer); capable of holding any object type Class definition Selector; an internal identifier for a method name Pointer to a method returning an id Boolean data type: YES or NO Null object pointer Null class pointer
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Methods Method is an object-oriented term for what we call a function in procedural programming languages. However, in OO languages, we tend to think of methods as receiving messages. In Objective-C, like other OO languages, you refer to a method through a class instance variable or the class itself. In the latter case, the methods are called static methods. The following listing shows some examples of Objective-C methods:
// A method with no arguments, returning an Object - foo; // A method with no arguments, returning an integer - (int)foo // A method with one argument, returning an integer - (int)foo : (int) n; // A method with two arguments, returning void - (void)foo: (int) x and: (int) y; // A method with three arguments, returning void - (void)foo3: (int) x and: (int) y and: (int) z;
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