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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 - PowerPoint PPT Presentation
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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”
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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
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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
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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
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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
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Dynamic Language
• Almost everything is done at runtime
• Uses dynamic typing, linking, and binding
• This allows for greater flexibility
• Minimizes RAM and CPU usage
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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
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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 ]
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Basic syntax structure
C++ syntax
Objective-C syntax
void function(int x, int y, char z);
Object.function(x, y, z);
-(void) function:(int)x, (int)y, (char)z;
[Object function:x, y, z];
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Keyword: id
• 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…
extern id Pen;
id myPen;
myPen = [Pen new ];
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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
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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.
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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).
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Class Declaration (Interface)
@interface Node : NSObject {Node *link;int contents;
}+(id)new;-(void)setContent:(int)number;-(void)setLink:(Node*)next;-(int)getContent;-(Node*)getLink;@end
node.h
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Class Definition (Implementation)#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
node.m
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C++ VS. Objective-C
• 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
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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
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linkList class
#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.m
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Stack class
#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;}@endstack.h stack.m
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Example: main.c#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;
}
$ gcc -x objective-c node.m linkList.m stack.m main.c -framework Cocoa -o stackTest
$./stackTest
2 3 1
main.c
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C and Objective-C Programming
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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
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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 !
$
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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”);
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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);
•
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#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 ;}
$How many students does ITU have ?: 20000 (enter)ITU has 20000 students.$
Example !
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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 !
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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;
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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;}
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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;}
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More arrays
• Stringschar 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 arraysint points[3][4];points [1][3] = 12; /* NOT points[3,4] */printf(“%d”, points[1][3]);
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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’ characterchar name[6] = {‘C’,’S’,’4’,’1’,’4’,’\0’};
/* ’\0’= end of string */printf(“%s”, name); /* print until ‘\0’ */
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Memory layout and addresses
5 10 12.5 9. 8 c d
int x = 5, y = 10;float f = 12.5, g = 9.8;char c = ‘c’, d = ‘d’;
4300 4304 4308 4312 4316 4317
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Pointers made easy - 1
• Pointer = variable containing address of another variable float f; /* data variable */
float *f_addr; /* pointer variable */
f_addr = &f; /* & = address operator */
? ?
f f_addr
4300 4304
?
any float
any address
? 4300
f f_addr
4300 4304
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Pointers made easy - 2 *f_addr = 3.2; /* indirection operator */
float g=*f_addr; /* indirection:g is now 3.2 */
f = 1.3;
f f_addr
4300 4304
3.2 4300
f f_addr
4300 4304
1.3 4300
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#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 <- ??? */}
Pointer Example
40
#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 */
}
Dynamic Memory allocation• 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
Functions - why and how ?
• 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
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 */}
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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 */}
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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;}
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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;}
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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; }
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Memory layout of programs
Header info
Code
Data - Heap
0
100
400
560
1010
1200
Dynamic memory
Local memory+ function callstack
all normal vars
all malloc()s
Data - stack
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Program with multiple files
• Library headers– Standard– User-defined
void myproc(void);int mydata;
#include <stdio.h>#include “mypgm.h”
void myproc(void){ mydata=2; . . . /* some code */}
#include <stdio.h>#include “mypgm.h”
void main(void){ myproc();}
hw.c mypgm.c
mypgm.h
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Externs#include <stdio.h>
extern char user2line [20]; /* global variable defined in another file */
char user1line[30]; /* global for this file */void dummy(void); void main(void) { char user1line[20]; /* different from earlier
user1line[30] */ . . . /* restricted to this func */}
void dummy(){ extern char user1line[]; /* the global user1line[30] */ . . .}
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#include <stdio.h>
struct birthday{ int month; int day; int year; };
main() { struct birthday mybday; /* - no ‘new’ needed ! */ /* then, it’s just like Java ! */ mybday.day=1; mybday.month=1; mybday.year=1977; printf(“I was born on %d/%d/%d”, birth.day, birth.month, birth.year);}
Structures• Equivalent of Java’s classes with only data
(no methods)
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More on Structuresstruct person{ char name[41]; int age; float height; struct { /* embedded structure */ int month; int day; int year; } birth; };
struct person me; me.birth.year=1977;………
struct person class[60]; /* array of info about everyone in class */
class[0].name=“Gun”; class[0].birth.year=1971;……
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Passing/Returning a structure
/* pass struct by value */void display_year_1(struct birthday mybday) { printf(“I was born in %d\n”, mybday.year);} /* - inefficient: why ? */. . . .
/* pass struct by reference */void display_year_2(struct birthday *pmybday) { printf(“I was born in %d\n”, pmybday->year);
/* warning ! ‘->’, not ‘.’, after a struct pointer*/ }. . . .
/* return struct by value */struct birthday get_bday(void){ struct birthday newbday; newbday.year=1971; /* ‘.’ after a struct */ return newbday;} /* - also inefficient: why ? */
56
enum - enumerated data types#include <stdio.h>enum month{
JANUARY, /* like #define JANUARY 0 */FEBRUARY, /* like #define FEBRUARY 1 */MARCH /* … */
};
/* JANUARY is the same as month.JANUARY */
/* alternatively, …. */
enum month{JANUARY=1, /* like #define JANUARY 1 */FEBRUARY, /* like #define FEBRUARY 2 */MARCH /* … */
};
57
typedef int Employees;
Employees my_company; /* same as int my_company; */
typedef struct person Person;
Person me; /* same as struct person me; */
typedef struct person *Personptr;
Personptr ptrtome; /* same as struct person *ptrtome;*/
Synonym for a data type
• Easier to remember
• Clean code
58
int month[12]; /* month is a pointer to base address 430*/
month[3] = 7; /* month address + 3 * int elements => int at address (430+3*4) is now 7 */
ptr = month + 2; /* ptr points to month[2], => ptr is now (430+2 * int elements)= 438 */
ptr[5] = 12; /* ptr address + 5 int elements
=> int at address (434+5*4) is now 12.Thus, month[7] is now 12 */
ptr++; /* ptr <- 438 + 1 * size of int = 442 */(ptr + 4)[2] = 12; /* accessing ptr[6] i.e., array[9] */
More pointers
• Now , month[6], *(month+6), (month+4)[2], ptr[3], *(ptr+3) are all the same integer variable.
59
2-D arrays
• 2-dimensional arrayint weekends[52][2];
weekends
• weekends[2][1] is same as *(weekends+2*2+1)– NOT *weekends+2*2+1 :this is an int !
[0][0] [0][1] [1][0] [1][1] [2][0] [2][1] [3][0] . . . .
60
#include <stdio.h> /* program called with cmd line parameters */
void main(int argc, char *argv[]) { int ctr;
for (ctr = 0; ctr < argc; ctr = ctr + 1) { printf(“Argument #%d is -> |%s|\n”, ctr, argv[ctr]); } /* ex., argv[0] == the name of the program */}
Pointer Example - argc and argv parameters
61
Strings#include <stdio.h>
main() { char msg[10]; /* array of 10 chars */ char *p; /* pointer to a char */ char msg2[]=“Hello”; /* msg2 = ‘H’’e’’l’’l’’o’’\0’ */
msg = “Bonjour”; /* ERROR. msg has a const address.*/ p = “Bonjour”; /* address of “Bonjour” goes into p */
msg = p; /* ERROR. Message has a constant address. */ /* cannot change it. */
p = msg; /* OK */ p[0] = ‘H’, p[1] = ‘i’,p[2]=‘\0’;
/* *p and msg are now “Hi” */}
62
Pointer to function
• Advantage ? more flexibility
int func(); /*function returning integer*/ int *func(); /*function returning pointer to integer*/ int (*func)(); /*pointer to function returning integer*/ int *(*func)(); /*pointer to func returning ptr to int*/
63
Pointer to function - Example#include <stdio.h>
void myproc (int d);void mycaller(void (* f)(int), int param);
void main(void) { myproc(10); /* call myproc with parameter 10*/mycaller(myproc, 10); /* and do the same again ! */
}
void mycaller(void (* f)(int), int param){(*f)(param); /* call function *f with param */
}
void myproc (int d){ . . . /* do something with d */}
64
Doing more complicated things…
To declare an array of N pointers to functions returning pointers to functions returning pointers to characters
1. char *(*(*a[N])())();
2. Build the declaration up in stages, using typedefs: typedef char *pc; /* pointer to char */ typedef pc fpc(); /* function returning pointer to char */ typedef fpc *pfpc; /* pointer to above */ typedef pfpc fpfpc(); /* function returning... */ typedef fpfpc *pfpfpc; /* pointer to... */ pfpfpc a[N]; /* array of... */
65
Before you go onto Objective-C
• Always initialize anything before using it (especially pointers)
• Don’t use pointers after freeing them• Don’t return a function’s local variables by reference• No exceptions – so check for errors everywhere• An array is also a pointer, but its value is immutable.
66
References
• Cox, Brad. Object Oriented Programming: an Evolutioary Approach
• Sebesta, Robert. Concepts of Programming Languages
• Apple Inc. Apple Developer Connectionhttp://developer.apple.com
• Stevenson, Scott. Theocacaohttp://theocacao.com/document.page/510