Objective-C keywords

Here in this section we will know about the keywords used in objective-C language. Objective-C is a superset of C language, so program written in c and C++ should compile as objective-c. It provides some additional keywords, to avoid conflict with keywords in other language it uses ‘@’ at the beginning of keyword. These keyword are called Compiler Directives.

Directives used to declare and define classes, categories and protocols:

Directive	Definition
@interface	used to declare of class or interface.
@implementation	used to define a class or category.
@protocol	used to declare a formal protocol.
@end	ends the declaration, definition, category or protocol.

Directive used to specify the visibility of the instance. Default is @protected.

Directive Definition

@private Limits the scope of an instance variable to the class that declares it.

@protected Limits instance variable scope to declaring and inheriting classes.

@public Removes restrictions on the scope of instance variables.

Exception handling directives.

Directive	Definition
@try	Defines a block within which exceptions can be thrown.
@throw	Throws an exception object.
@catch	Catches an exception thrown within the preceding @try block.
@finally	A block of code that is executed whether exceptions were thrown or not in a @try block.

Directive used for particular purpose.

Directive	Definition
@class	Declares the names of classes defined elsewhere.
@selector(method_name)	It returns the compiled selector that identifies method_name.
@protocol(protocol_name)	Returns the protocol_name protocol (an instance of the Protocol class). (@protocol is also valid without (protocol_name) for forward declarations.)
@encode(type_spec)	Yields a character string that encodes the type structure of type_spec.
@"string"	Defines a constant NSString object in the current module and initializes the object with the specified 7-bit ASCII-encoded string.
@"string1" @"string2" ... @"stringN"	Defines a constant NSString object in the currentmodule. The string created is the result of concatenating the strings specified in the two directives.
@synchronized()	Defines a block of code that must be executed only by one thread at a time.

Some keywords of Objective-C are not reserved outside. These are…..

in	out	inout	bycopy
byref	oneway

Keyword for memory management in Objective-C
These are looking as keywords but infact these are methods of root class NSObject.

alloc

retain

release

autorelease

Some other keywords:

1. bool is a keyword used in objective-C but its value is here YES or NO. In C and C++ it has value either TRUE or FALSE.
2. 'super' and 'self' can be treated as keywords but self is a hidden parameter to each method and super gives the instructions to the compiler that how to use self differently.

Preprocessor Directives
The preprocessor directives are special notations:

Directive	Definition
//	This is used to comment a single line.
#import	Like C and C++ it is used to include a file but it doesn't include more than once.