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Overview of Objective-C

Overview of Objective-C. Objective-C is an object oriented language. follows ANSI C style coding with methods from Smalltalk There is no formal written standard Relies mostly on libraries written by others Flexible almost everything is done at runtime. Dynamic Binding Dynamic Typing

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Overview of Objective-C

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  1. Overview of Objective-C • Objective-C is an object oriented language. • follows ANSI C style coding with methods from Smalltalk • There is no formal written standard • Relies mostly on libraries written by others • Flexible almost everything is done at runtime. • Dynamic Binding • Dynamic Typing • Dynamic Linking

  2. Inventors • Objective-C was invented by two men, Brad Cox and Tom Love. • Both were introduced to Smalltalk at ITT in 1981 • Cox thought something like Smalltalk would be very useful to application developers • Cox modified a C compiler and by 1983 he had a working Object-oriented extension to C called OOPC.

  3. Development • Tom Love acquired a commercial copy of Smalltalk-80 while working for Schlumberger Research • With direct access Smalltalk, Love added more to OOPC making the final product, Objective-C. • In 1986 they release Objective-C through their company “Stepstone”

  4. NeXT and NeXTSTEP • In 1988 Steve Jobs acquires Objective-C license for NeXT • Used Objective-C to build the NeXTSTEP Operating System • Objective-C made interface design for NeXTSTEP much easier • NeXTSTEP was derived from BSD Unix • In 1995 NeXT gets full rights to Objective-C from Stepstone

  5. OPENSTEP API • Developed in 1993 by NeXT and Sun • An effort to make NeXTSTEP-like Objective-C implementation available to other platforms. • In order to be OS independent • Removed dependency on Mach Kernel • Made low-level data into classes • Paved the way for Mac OS X, GNUstep

  6. Apple and Mac OS X • NeXT is taken over by Apple in 1996 and put Steve Jobs and his Objective-C libraries to work • Redesigned Mac OS to use objective-C similar to that of NeXTSTEP • Developed a collection of libraries named “Cocoa” to aid GUI development • Release Mac OS X (ten), which was radically different than OS 9, in March 2001

  7. The Cocoa API • Primarily the most frequently used frameworks nowadays. • Developed by Apple from NeXTSTEP and OPENSTEP • Has a set of predefined classes and types such as NSnumber, NSstring, Nsdate, etc. • NS stands for NeXT-sun • Includes a root class NSObject where words like alloc, retain, and release come from

  8. Dynamic Language • Almost everything is done at runtime • Uses dynamic typing, linking, and binding • This allows for greater flexibility • Minimizes RAM and CPU usage

  9. To Import or Include? • C/C++’s #include will insert head.h into the code even if its been added before. • Obj-C’s #import checks if head.h has been imported beforehand. #import head.h

  10. Messages • Almost every object manipulation is done by sending objects a message • Two words within a set of brackets, the object identifier and the message to send. • Because of dynamic binding, the message and receiver are joined at runtime [Identifier message ]

  11. Basic syntax structure C++ syntax void function(int x, int y, char z); Object.function(x, y, z); Objective-C syntax -(void) function:(int)x, (int)y, (char)z; [Object function:x, y, z];

  12. The word ‘id’ indicates an identifier for an object much like a pointer in c++ This uses dynamic typing For example, if Pen is a class… Keyword: id extern id Pen; id myPen; myPen = [Pen new ];

  13. Memory Allocation • Objects are created dynamically through the keyword, “alloc” • Objects are dynamically deallocated using the words “release” and “autorelease” • autorelease dealocates the object once it goes out of scope. • NOTE: None of these words are built-in

  14. Ownership • Objects are initially owned by the id that created them. • Like C pointers, multiple IDs can use the same object. • However, like in C if one ID releases the object, then any remaining pointers will be referencing invalid memory. • A method like “retain” can allow the object to stay if one ID releases it.

  15. Prototyping functions • When declaring or implementing functions for a class, they must begin with a + or - • + indicates a “class method” that can only be used by the class itself. In other words, they’re for private functions. • - indicates “instance methods” to be used by the client program (public functions).

  16. Class Declaration (Interface) node.h @interface Node : NSObject { Node *link; int contents; } +(id)new; -(void)setContent:(int)number; -(void)setLink:(Node*)next; -(int)getContent; -(Node*)getLink; @end

  17. Class Definition (Implementation) node.m #import "node.h” @implementation Node +(id)new { return [Node alloc];} -(void)setContent:(int)number {contents = number;} -(void)setLink:(Node*)next { [link autorelease]; link = [next retain]; } -(int)getContent {return contents;} -(Node*)getLink {return link;} @end

  18. Adds OOP, meta programming and generic programming to C Comes with a std library Has numerous uses Large and complex code for OOP Only adds OOP to C Has no standard library; is dependant on other libraries Mostly used for application building Simpler way of handling classes and objects C++ VS. Objective-C

  19. Objective-C 2.0 • In October 2007, Apple Inc. releases Objective-C 2.0 for Mac OS 10.5 (Leopard) • Adds automatic garbage collection • Instance Methods (public functions) are defined differently using @property

  20. #import "linkList.h" @implementation linkList +(id)new {return [linkList alloc];} -(void)insert:(int)value { id temp = [Node new]; [temp setContent:value]; [temp setLink:head]; head = [temp retain]; [temp release]; } -(void)append:(int)value { id last = [head getLink]; while ([last getLink] != nil) {last = [last getLink];} id temp = [Node new]; [temp setContent:value]; [last setLink:temp]; [temp release]; } -(void)remove { id temp = head; head = [head getLink]; [temp release]; } -(int)getValue { return [head getContent];} @end linkList class linkList.m

  21. #import "linkList.h” @interface Stack : linkList {} +(id)new; -(void)push:(int)value; -(int)pop; @end #import "stack.h” @implementation Stack +(id)new {return [Stack alloc];} -(void)push:(int)value {[self insert:value];} -(int)pop { int ret = [self getValue]; [self remove]; return ret; } @end Stack class stack.h stack.m

  22. #import "stack.h” int main(){ Stack *s = [Stack new]; [s push:1]; [s push:2]; printf("%d\t", [s pop]); [s push:3]; printf("%d\t", [s pop]); printf("%d\t", [s pop]); [s release]; return 0; } Example: main.c main.c $ gcc -x objective-c node.m linkList.m stack.m main.c -framework Cocoa -o stackTest $./stackTest 2 3 1

  23. C and Objective-C Programming

  24. Why learn C? • Objective-C is based on C • Better control of low-level mechanisms • Performance better than Java • Java hides many details needed for writing OS code But,…. • Memory management responsibility • Explicit initialization and error detection • More room for mistakes

  25. Goals of this tutorial • To introduce some basic C concepts to you • so that you can read further details on your own • To warn you about common mistakes made by beginners • so that you get your homework done quickly • You can write more complicated code

  26. Simple Example #include <stdio.h> void main(void) { printf(“Hello World. \n \t and you ! \n ”); /* print out a message */ return; } $Hello World. and you ! $

  27. Summarizing the Example • #include <stdio.h> = include header file stdio.h • No semicolon at end • Small letters only – C is case-sensitive • void main(void){ … } is the only code executed • printf(“ /* message you want printed */ ”); • \n = newline \t = tab • Dessert: \ in front of other special characters within printf. • printf(“Have you heard of \”The Rock\” ? \n”);

  28. Simple Data Types • data-type # bytes(typical) values short-hand • int               4       -2,147,483,648 to 2,147,483,647 %d • char              1       -128 to 127 %c • float             4       3.4E+/-38 (7 digits) %f • double            8       1.7E+/-308 (15 digits long) %lf • long              4       -2,147,483,648 to 2,147,483,647 %l • short             2       -32,768 to 32,767 • Lookup: • signed / unsigned - int, char, long, short • long double • ex: • int num=20000; • printf(“Cornell has about %d students.\n”, num);

  29. Example ! #include <stdio.h> void main(void) { int nstudents = 0; /* Initialization, required */ printf(“How many students does ITU have ?:”); scanf (“%d”, &nstudents); /* Read input */ printf(“ITU has %d students.\n”, nstudents); return ; } $How many students does ITU have ?: 20000 (enter) ITU has 20000 students. $

  30. Type conversion #include <stdio.h> void main(void) { int i,j = 12; /* i not initialized, only j */ float f1,f2 = 1.2; i = (int) f2; /* explicit: i <- 1, 0.2 lost */ f1 = i; /* implicit: f1 <- 1.0 */ f1 = f2 + (int) j; /* explicit: f1 <- 1.2 + 12.0 */ f1 = f2 + j; /* implicit: f1 <- 1.2 + 12.0 */ } • Explicit conversion rules for arithmetic operation x=y+z; • convert y or z as • double <- float <- int <- char, short • then type cast it to x ’s type • Moral: stick with explicit conversions - no confusion !

  31. Like Java, like C • Operators same as Java: • Arithmetic • int i = i+1; i++; i--; i *= 2; • +, -, *, /, %, • Relational and Logical • <, >, <=, >=, ==, != • &&, ||, &, |, ! • Syntax same as in Java: • if ( ) { } else { } • while ( ) { } • do { } while ( ); • for(i=1; i <= 100; i++) { } • switch ( ) {case 1: … } • continue; break;

  32. Example • #include <stdio.h> • #define DANGERLEVEL 5 /* C Preprocessor - • - substitution on appearance */ • /* like Java ‘final’ */ • void main(void) • { • float level=1; • /* if-then-else as in Java */ • if (level <= DANGERLEVEL){ /*replaced by 5*/ • printf(“Low on gas!\n”); • } • else printf(“Good driver !\n”); • return; • }

  33. One-Dimensional Arrays #include <stdio.h> void main(void) { int number[12]; /* 12 cells, one cell per student */ int index, sum = 0; /* Always initialize array before use */ for (index = 0; index < 12; index++) { number[index] = index; } /* now, number[index]=index; will cause error:why ?*/ for (index = 0; index < 12; index = index + 1) { sum += number[index]; /* sum array elements */ } return; }

  34. More arrays • Strings char name[6]; name = {‘C’,’S’,’4’,’1’,’4’,’\0’}; /* ’\0’= end of string */ printf(“%s”, name); /* print until ‘\0’ */ • Functions to operate on strings • strcpy, strncpy, strcmp, strncmp, strcat, strncat, strstr,strchr • #include <strings.h> at program start • Multi-dimensional arrays int points[3][4]; points [1][3] = 12; /* NOT points[3,4] */ printf(“%d”, points[1][3]);

  35. Like Java, somewhat like C • Type conversions • but you can typecast from any type to any type • c = (char) some_int; • So be careful ! • Arrays • Always initialize before use • int number[12]; printf(“%d”, number[20]); • produces undefined output, may terminate, may not even be detected. • Strings are terminated by ’\0’ character char name[6] = {‘C’,’S’,’4’,’1’,’4’,’\0’}; /* ’\0’= end of string */ printf(“%s”, name); /* print until ‘\0’ */

  36. Memory layout and addresses int x = 5, y = 10; float f = 12.5, g = 9.8; char c = ‘c’, d = ‘d’; 5 10 12.5 9. 8 c d 4300 4304 4308 4312 4316 4317

  37. Pointers made easy - 1 any float f f_addr ? ? ? any address 4300 4304 f f_addr ? 4300 4300 4304 • Pointer = variable containing address of another variable • float f; /* data variable */ • float *f_addr; /* pointer variable */ • f_addr = &f; /* & = address operator */

  38. Pointers made easy - 2 f f_addr 3.2 1.3 4300 4300 4300 4304 f f_addr 4300 4304 *f_addr = 3.2; /* indirection operator */ float g=*f_addr; /* indirection:g is now 3.2 */ f = 1.3;

  39. Pointer Example #include <stdio.h> void main(void) { int j; int *ptr; ptr=&j; /* initialize ptr before using it */ /* *ptr=4 does NOT initialize ptr */ *ptr=4; /* j <- 4 */ j=*ptr; /* j <- ??? */ }

  40. Dynamic Memory allocation #include <stdio.h> void main(void) { int *ptr; /* allocate space to hold an int */ ptr = malloc(sizeof(int)); /* do stuff with the space */ *ptr=4; free(ptr); /* free up the allocated space */ } • Explicit allocation and de-allocation

  41. Elementary file handling • #include <stdio.h> • void main(void) { • /* file handles */ • FILE *input_file=NULL; • /* open files for writing*/ • input_file = fopen(“cwork.dat”, “w”); • if(input_file == NULL) • exit(1); /* need to do explicit ERROR CHECKING */ • /* write some data into the file */ • fprintf(input_file, “Hello there”); • /* don’t forget to close file handles */ • fclose(input_file); • return; • }

  42. Error Handling • Moral from example: • unlike Java, no explicit exceptions • need to manually check for errors • Whenever using a function you’ve not written • Anywhere else errors might occur

  43. If a program is too long Modularization – easier to code debug Code reuse Passing arguments to functions By value By reference Returning values from functions By value By reference Functions - why and how ?

  44. Functions – basic example • #include <stdio.h> • int sum(int a, int b); • /* function prototype at start of file */ • void main(void){ • int total = sum(4,5); /* call to the function */ • printf(“The sum of 4 and 5 is %d”, total); • } • int sum(int a, int b){ /* the function itself • - arguments passed by value*/ • return (a+b); /* return by value */ • }

  45. Arguments by reference • #include <stdio.h> • int sum(int *pa, int *pb); • /* function prototype at start of file */ • void main(void){ • int a=4, b=5; • int *ptr = &b; • int total = sum(&a,ptr); /* call to the function */ • printf(“The sum of 4 and 5 is %d”, total); • } • int sum(int *pa, int *pb){ /* the function itself • - arguments passed by reference */ • return (*pa+*pb); /* return by value */ • }

  46. Why pointer arguments?! #include <stdio.h> void swap(int, int); main() { int num1 = 5, num2 = 10; swap(num1, num2); printf(“num1 = %d and num2 = %d\n”, num1, num2); } void swap(int n1, int n2) { /* passed by value */ int temp; temp = n1; n1 = n2; n2 = temp; }

  47. Why pointer arguments? This is why #include <stdio.h> void swap(int *, int *); main() { int num1 = 5, num2 = 10; swap(&num1, &num2); printf(“num1 = %d and num2 = %d\n”, num1, num2); } void swap(int *n1, int *n2) { /* passed and returned by reference */ int temp; temp = *n1; *n1 = *n2; *n2 = temp; }

  48. What’s wrong with this ? #include <stdio.h> void dosomething(int *ptr); main() { int *p; dosomething(p) printf(“%d”, *p); /* will this work ? */ } void dosomething(int *ptr){ /* passed and returned by reference */ int temp=32+12; ptr = &(temp); } /* compiles correctly, but gives run-time error */

  49. Passing and returning arrays #include <stdio.h> void init_array(int array[], int size) ; void main(void) { int list[5]; init_array(list, 5); for (i = 0; i < 5; i++) printf(“next:%d”, array[i]); } void init_array(int array[], int size) { /* why size ? */ /* arrays ALWAYS passed by reference */ int i; for (i = 0; i < size; i++) array[i] = 0; }

  50. Data - stack Memory layout of programs 0 100 400 560 1010 1200 Header info Code all malloc()s Data - Heap Dynamic memory Local memory + function call stack all normal vars

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