Category: Computer Programming

Objective-C Preprocessors ”; Previous Next The Objective-C Preprocessor is not part of the compiler, but is a separate step in the compilation process. In simplistic terms, an Objective-C Preprocessor is just a text substitution tool and it instructs compiler to do required pre-processing before actual compilation. We”ll refer to the Objective-C Preprocessor as the OCPP. All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in first column. Following section lists down all important preprocessor directives − Sr.No. Directive & Description 1 #define Substitutes a preprocessor macro 2 #include Inserts a particular header from another file 3 #undef Undefines a preprocessor macro 4 #ifdef Returns true if this macro is defined 5 #ifndef Returns true if this macro is not defined 6 #if Tests if a compile time condition is true 7 #else The alternative for #if 8 #elif #else an #if in one statement 9 #endif Ends preprocessor conditional 10 #error Prints error message on stderr 11 #pragma Issues special commands to the compiler using a standardized method Preprocessors Examples Analyze the following examples to understand various directives. #define MAX_ARRAY_LENGTH 20 This directive tells the OCPP to replace instances of MAX_ARRAY_LENGTH with 20. Use #define for constants to increase readability. #import <Foundation/Foundation.h> #include “myheader.h” These directives tell the OCPP to get foundation.h from Foundation Framework and add the text to the current source file. The next line tells OCPP to get myheader.h from the local directory and add the content to the current source file. #undef FILE_SIZE #define FILE_SIZE 42 This tells the OCPP to undefine existing FILE_SIZE and define it as 42. #ifndef MESSAGE #define MESSAGE “You wish!” #endif This tells the OCPP to define MESSAGE only if MESSAGE isn”t already defined. #ifdef DEBUG /* Your debugging statements here */ #endif This tells the OCPP to do the process the statements enclosed if DEBUG is defined. This is useful if you pass the -DDEBUG flag to gcc compiler at the time of compilation. This will define DEBUG, so you can turn debugging on and off on the fly during compilation. Predefined Macros ANSI C defines a number of macros. Although each one is available for your use in programming, the predefined macros should not be directly modified. Sr.No. Macro & Description 1 __DATE__ The current date as a character literal in “MMM DD YYYY” format 2 __TIME__ The current time as a character literal in “HH:MM:SS” format 3 __FILE__ This contains the current filename as a string literal. 4 __LINE__ This contains the current line number as a decimal constant. 5 __STDC__ Defined as 1 when the compiler complies with the ANSI standard. Let”s try the following example − Live Demo #import <Foundation/Foundation.h> int main() { NSLog(@”File :%sn”, __FILE__ ); NSLog(@”Date :%sn”, __DATE__ ); NSLog(@”Time :%sn”, __TIME__ ); NSLog(@”Line :%dn”, __LINE__ ); NSLog(@”ANSI :%dn”, __STDC__ ); return 0; } When the above code in a file main.m is compiled and executed, it produces the following result − 2013-09-14 04:46:14.859 demo[20683] File :main.m 2013-09-14 04:46:14.859 demo[20683] Date :Sep 14 2013 2013-09-14 04:46:14.859 demo[20683] Time :04:46:14 2013-09-14 04:46:14.859 demo[20683] Line :8 2013-09-14 04:46:14.859 demo[20683] ANSI :1 Preprocessor Operators The Objective-C preprocessor offers following operators to help you in creating macros − Macro Continuation () A macro usually must be contained on a single line. The macro continuation operator is used to continue a macro that is too long for a single line. For example − #define message_for(a, b) NSLog(@#a ” and ” #b “: We love you!n”) Stringize (#) The stringize or number-sign operator (”#”), when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro that has a specified argument or parameter list. For example − Live Demo #import <Foundation/Foundation.h> #define message_for(a, b) NSLog(@#a ” and ” #b “: We love you!n”) int main(void) { message_for(Carole, Debra); return 0; } When the above code is compiled and executed, it produces the following result − 2013-09-14 05:46:14.859 demo[20683] Carole and Debra: We love you! Token Pasting (##) The token-pasting operator (##) within a macro definition combines two arguments. It permits two separate tokens in the macro definition to be joined into a single token. For example − Live Demo #import <Foundation/Foundation.h> #define tokenpaster(n) NSLog (@”token” #n ” = %d”, token##n) int main(void) { int token34 = 40; tokenpaster(34); return 0; } When the above code is compiled and executed, it produces the following result − 2013-09-14 05:48:14.859 demo[20683] token34 = 40 How it happened, because this example results in the following actual output from the preprocessor − NSLog (@”token34 = %d”, token34); This example shows the concatenation of token##n into token34 and here we have used both stringize and token-pasting. The defined() Operator The preprocessor defined operator is used in constant expressions to determine if an identifier is defined using #define. If the specified identifier is defined, the value is true (non-zero). If the symbol is not defined, the value is false (zero). The defined operator is specified as follows − Live Demo #import <Foundation/Foundation.h> #if !defined (MESSAGE) #define MESSAGE “You wish!” #endif int main(void) { NSLog(@”Here is the message: %sn”, MESSAGE); return 0; } When the above code is compiled and executed, it produces the following result − 2013-09-14 05:48:19.859 demo[20683] Here is the message: You wish! Parameterized Macros One of the powerful functions of the OCPP is the ability to simulate functions using parameterized macros. For example, we might have some code to square a number as follows − int square(int x) { return x * x; } We can rewrite above code using a macro as follows − #define square(x) ((x) * (x)) Macros with arguments must be defined using the #define directive before they can be used. The argument list is enclosed in parentheses and must immediately follow the macro name. Spaces are not allowed between macro name and open parenthesis. For example − Live Demo

Objective-C Numbers ”; Previous Next In Objective-C programming language, in order to save the basic data types like int, float, bool in object form, Objective-C provides a range of methods to work with NSNumber and important ones are listed in following table. Sr.No. Method & Description 1 + (NSNumber *)numberWithBool:(BOOL)value Creates and returns an NSNumber object containing a given value, treating it as a BOOL. 2 + (NSNumber *)numberWithChar:(char)value Creates and returns an NSNumber object containing a given value, treating it as a signed char. 3 + (NSNumber *)numberWithDouble:(double)value Creates and returns an NSNumber object containing a given value, treating it as a double. 4 + (NSNumber *)numberWithFloat:(float)value Creates and returns an NSNumber object containing a given value, treating it as a float. 5 + (NSNumber *)numberWithInt:(int)value Creates and returns an NSNumber object containing a given value, treating it as a signed int. 6 + (NSNumber *)numberWithInteger:(NSInteger)value Creates and returns an NSNumber object containing a given value, treating it as an NSInteger. 7 – (BOOL)boolValue Returns the receiver”s value as a BOOL. 8 – (char)charValue Returns the receiver”s value as a char. 9 – (double)doubleValue Returns the receiver”s value as a double. 10 – (float)floatValue Returns the receiver”s value as a float. 11 – (NSInteger)integerValue Returns the receiver”s value as an NSInteger. 12 – (int)intValue Returns the receiver”s value as an int. 13 – (NSString *)stringValue Returns the receiver”s value as a human-readable string. Here is a simple example for using NSNumber which multiplies two numbers and returns the product. Live Demo #import <Foundation/Foundation.h> @interface SampleClass:NSObject – (NSNumber *)multiplyA:(NSNumber *)a withB:(NSNumber *)b; @end @implementation SampleClass – (NSNumber *)multiplyA:(NSNumber *)a withB:(NSNumber *)b { float number1 = [a floatValue]; float number2 = [b floatValue]; float product = number1 * number2; NSNumber *result = [NSNumber numberWithFloat:product]; return result; } @end int main() { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; SampleClass *sampleClass = [[SampleClass alloc]init]; NSNumber *a = [NSNumber numberWithFloat:10.5]; NSNumber *b = [NSNumber numberWithFloat:10.0]; NSNumber *result = [sampleClass multiplyA:a withB:b]; NSString *resultString = [result stringValue]; NSLog(@”The product is %@”,resultString); [pool drain]; return 0; } Now when we compile and run the program, we will get the following result. 2013-09-14 18:53:40.575 demo[16787] The product is 105 Print Page Previous Next Advertisements ”;

Objective-C Data Encapsulation ”; Previous Next All Objective-C programs are composed of the following two fundamental elements − Program statements (code) − This is the part of a program that performs actions and they are called methods. Program data − The data is the information of the program which is affected by the program functions. Encapsulation is an Object-Oriented Programming concept that binds together the data and functions that manipulate the data and that keeps both safe from outside interference and misuse. Data encapsulation led to the important OOP concept of data hiding. Data encapsulation is a mechanism of bundling the data and the functions that use them, and data abstraction is a mechanism of exposing only the interfaces and hiding the implementation details from the user. Objective-C supports the properties of encapsulation and data hiding through the creation of user-defined types, called classes. For example − @interface Adder : NSObject { NSInteger total; } – (id)initWithInitialNumber:(NSInteger)initialNumber; – (void)addNumber:(NSInteger)newNumber; – (NSInteger)getTotal; @end The variable total is private and we cannot access from outside the class. This means that they can be accessed only by other members of the Adder class and not by any other part of your program. This is one way encapsulation is achieved. Methods inside the interface file are accessible and are public in scope. There are private methods, which are written with the help of extensions, which we will learn in upcoming chapters. Data Encapsulation Example Any Objective-C program where you implement a class with public and private members variables is an example of data encapsulation and data abstraction. Consider the following example − Live Demo #import <Foundation/Foundation.h> @interface Adder : NSObject { NSInteger total; } – (id)initWithInitialNumber:(NSInteger)initialNumber; – (void)addNumber:(NSInteger)newNumber; – (NSInteger)getTotal; @end @implementation Adder -(id)initWithInitialNumber:(NSInteger)initialNumber { total = initialNumber; return self; } – (void)addNumber:(NSInteger)newNumber { total = total + newNumber; } – (NSInteger)getTotal { return total; } @end int main(int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; Adder *adder = [[Adder alloc]initWithInitialNumber:10]; [adder addNumber:5]; [adder addNumber:4]; NSLog(@”The total is %ld”,[adder getTotal]); [pool drain]; return 0; } When the above code is compiled and executed, it produces the following result − 2013-09-22 21:17:30.485 DataEncapsulation[317:303] The total is 19 Above class adds numbers together and returns the sum. The public members addNum and getTotal are the interfaces to the outside world and a user needs to know them to use the class. The private member total is something that is hidden from the outside world, but is needed for the class to operate properly. Designing Strategy Most of us have learned through bitter experience to make class members private by default unless we really need to expose them. That”s just good encapsulation. It”s important to understand data encapsulation since it”s one of the core features of all Object-Oriented Programming (OOP) languages including Objective-C. Print Page Previous Next Advertisements ”;

Objective-C Error Handling ”; Previous Next In Objective-C programming, error handling is provided with NSError class available in Foundation framework. An NSError object encapsulates richer and more extensible error information than is possible using only an error code or error string. The core attributes of an NSError object are an error domain (represented by a string), a domain-specific error code and a user info dictionary containing application specific information. NSError Objective-C programs use NSError objects to convey information about runtime errors that users need to be informed about. In most cases, a program displays this error information in a dialog or sheet. But it may also interpret the information and either ask the user to attempt to recover from the error or attempt to correct the error on its own NSError Object consists of − Domain − The error domain can be one of the predefined NSError domains or an arbitrary string describing a custom domain and domain must not be nil. Code − The error code for the error. User Info − The userInfo dictionary for the error and userInfo may be nil. The following example shows how to create a custom error NSString *domain = @”com.MyCompany.MyApplication.ErrorDomain”; NSString *desc = NSLocalizedString(@”Unable to complete the process”, @””); NSDictionary *userInfo = @{ NSLocalizedDescriptionKey : desc }; NSError *error = [NSError errorWithDomain:domain code:-101 userInfo:userInfo]; Here is complete code of the above error sample passed as reference to an pointer − Live Demo #import <Foundation/Foundation.h> @interface SampleClass:NSObject -(NSString *) getEmployeeNameForID:(int) id withError:(NSError **)errorPtr; @end @implementation SampleClass -(NSString *) getEmployeeNameForID:(int) id withError:(NSError **)errorPtr { if(id == 1) { return @”Employee Test Name”; } else { NSString *domain = @”com.MyCompany.MyApplication.ErrorDomain”; NSString *desc =@”Unable to complete the process”; NSDictionary *userInfo = [[NSDictionary alloc] initWithObjectsAndKeys:desc, @”NSLocalizedDescriptionKey”,NULL]; *errorPtr = [NSError errorWithDomain:domain code:-101 userInfo:userInfo]; return @””; } } @end int main() { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; SampleClass *sampleClass = [[SampleClass alloc]init]; NSError *error = nil; NSString *name1 = [sampleClass getEmployeeNameForID:1 withError:&error]; if(error) { NSLog(@”Error finding Name1: %@”,error); } else { NSLog(@”Name1: %@”,name1); } error = nil; NSString *name2 = [sampleClass getEmployeeNameForID:2 withError:&error]; if(error) { NSLog(@”Error finding Name2: %@”,error); } else { NSLog(@”Name2: %@”,name2); } [pool drain]; return 0; } In the above example, we return a name if the id is 1, otherwise we set the user-defined error object. When the above code is compiled and executed, it produces the following result − 2013-09-14 18:01:00.809 demo[27632] Name1: Employee Test Name 2013-09-14 18:01:00.809 demo[27632] Error finding Name2: Unable to complete the process Print Page Previous Next Advertisements ”;

Objective-C Polymorphism ”; Previous Next The word polymorphism means having many forms. Typically, polymorphism occurs when there is a hierarchy of classes and they are related by inheritance. Objective-C polymorphism means that a call to a member function will cause a different function to be executed depending on the type of object that invokes the function. Consider the example, we have a class Shape that provides the basic interface for all the shapes. Square and Rectangle are derived from the base class Shape. We have the method printArea that is going to show about the OOP feature polymorphism. Live Demo #import <Foundation/Foundation.h> @interface Shape : NSObject { CGFloat area; } – (void)printArea; – (void)calculateArea; @end @implementation Shape – (void)printArea { NSLog(@”The area is %f”, area); } – (void)calculateArea { } @end @interface Square : Shape { CGFloat length; } – (id)initWithSide:(CGFloat)side; – (void)calculateArea; @end @implementation Square – (id)initWithSide:(CGFloat)side { length = side; return self; } – (void)calculateArea { area = length * length; } – (void)printArea { NSLog(@”The area of square is %f”, area); } @end @interface Rectangle : Shape { CGFloat length; CGFloat breadth; } – (id)initWithLength:(CGFloat)rLength andBreadth:(CGFloat)rBreadth; @end @implementation Rectangle – (id)initWithLength:(CGFloat)rLength andBreadth:(CGFloat)rBreadth { length = rLength; breadth = rBreadth; return self; } – (void)calculateArea { area = length * breadth; } @end int main(int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; Shape *square = [[Square alloc]initWithSide:10.0]; [square calculateArea]; [square printArea]; Shape *rect = [[Rectangle alloc] initWithLength:10.0 andBreadth:5.0]; [rect calculateArea]; [rect printArea]; [pool drain]; return 0; } When the above code is compiled and executed, it produces the following result − 2013-09-22 21:21:50.785 Polymorphism[358:303] The area of square is 100.000000 2013-09-22 21:21:50.786 Polymorphism[358:303] The area is 50.000000 In the above example based on the availability of the method calculateArea and printArea, either the method in the base class or the derived class executed. Polymorphism handles the switching of methods between the base class and derived class based on the method implementation of the two classes. Print Page Previous Next Advertisements ”;