Introduction to
Objective-C Programming
(Level: Beginner)
Lecture 1
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Introduction
• Objective-C is implemented as set of extensions
to the C language.
• It's designed to give C a full capability for objectoriented programming, and to do so in a simple
and straightforward way.
• Its additions to C are few and are mostly based
on Smalltalk, one of the first object-oriented
programming languages.
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Why Objective C
• Objective-C incorporates C, you get all the benefits of C
when working within Objective-C.
• You can choose when to do something in an objectoriented way (define a new class, for example) and when
to stick to procedural programming techniques (define a
struct and some functions instead of a class).
• Objective-C is a simple language. Its syntax is small,
unambiguous, and easy to learn
• Objective-C is the most dynamic of the object-oriented
languages based on C. Most decisions are made at run
time.
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Object-Oriented Programming
• The insight of object-oriented programming is to
combine state and behavior, data and
operations on data, in a high-level unit, an
object, and to give it language support.
• An object is a group of related functions and a
data structure that serves those functions. The
functions are known as the object's methods,
and the fields of its data structure are its
instance variables.
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The Objective-C Language
• The Objective-C language is fully
compatible with ANSI standard C
• Objective-C can also be used as an
extension to C++.
• Although C++ itself is a Object-Oriented
Language, there are difference in the
dynamic binding from Objective-C
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Objective-C Language (cont.)
• Objective-C source files have a “.m”
extension
• “.h” file is the interface file
• For example:
– main.m
– List.h (Interface of List class.)
– List.m (Implementation of List class.)
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ID
• “id” is a data type used by Objective-C to
define a pointer of an object (a pointer to
the object’s data) (sort of like * in C++)
• Any type of object, as long as it is an
object, we can use the id data type.
• For example, we can define an object by:
id anObject;
• nil is the reserved word for null object
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Dynamic Typing
• “id” data type has no information about the
object
• Every object carries with it an isa instance
variable that identifies the object's class,
that is, what kind of object it is.
• Objects are thus dynamically typed at run
time. Whenever it needs to, the run-time
system can find the exact class that an
object belongs to, just by asking the object
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Messages
• To get an object to do something, you
send it a message telling it to apply a
method. In Objective-C, message
expressions are enclosed in square
brackets
[receiver message]
• The receiver is an object. The message is
simply the name of a method and any
arguments that are passed to it
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Messages (cont.)
• For example, this message tells the myRect
object to perform its display method, which
causes the rectangle to display itself
[myRect display];
C++ equiv: myRect.display();
[myRect setOrigin:30.0 :50.0];
C++ equiv: myRect.setOrigin(30.0, 50.0);
• The method setOrigin::, has two colons, one for
each of its arguments. The arguments are inserted
after the colons, breaking the name apart
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Polymorphism
• Each object has define its own method but for
different class, they can have the same method
name which has totally different meaning
• The two different object can respond differently
to the same message
• Together with dynamic binding, it permits you to
write code that might apply to any number of
different kinds of objects, without having to
choose at the time you write the code what kinds
of objects they might be
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Inheritance
• Root class is NSObject
• Inheritance is
cumulative. A Square
object has the methods
and instance variables
defined for Rectangle,
Shape, Graphic, and
NSObject, as well as
those defined
specifically for Square
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Inheritance (cont.)
• Instance Variables: The new object contains not
only the instance variables that were defined for its
class, but also the instance variables defined for
its super class, all the way back to the root class
• Methods: An object has access not only to the
methods that were defined for its class, but also to
methods defined for its super class
• Method Overriding: Implement a new method with
the same name as one defined in a class farther
up the hierarchy. The new method overrides the
original; instances of the new class will perform it
rather than the original
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Class Objects
• Compiler creates one class object to
contain the information for the name of
class and super class
• To start an object in a class:
id myRectx; myRect = [[Rectangle alloc] init];
• The alloc method returns a new instance
and that instance performs an init method
to set its initial state.
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Defining a Class
• In Objective-C, classes are defined in two
parts:
– An interface (.h) that declares the methods
and instance variables of the class and
names its super class
– An implementation (.m) that actually defines
the class (contains the code that implements
its methods)
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The Interface
• The declaration of a class interface begins
with the compiler directive @interface and
ends with the directive @end
@interface ClassName : ItsSuperclass
{
instance variable declarations
}
method declarations
@end
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Declaration
Instance Variables:
float width;
float height;
BOOL filled;
NSColor *fillColor;
Methods:
• names of methods that can be used by class objects,
class methods, are preceded by a plus sign
+ alloc
• methods that instances of a class can use, instance
methods, are marked with a minus sign:
- (void) display;
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Declaration (cont.)
• Importing the Interface: The interface is
usually included with the #import directive
#import "Rectangle.h"
• To reflect the fact that a class definition builds on
the definitions of inherited classes, an interface
file begins by importing the interface for its super
class
• Referring to Other Classes: If the interface
mentions classes not in this hierarchy, it must
declare them with the @class directive:
@class Rectangle, Circle;
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GCC and Objective-C
• Objective-C is layered on top of the C language
• iPhone & iPad “native” applications are written in
Objective C
• The Apple dev kit for Objective-C is called
“Cocoa”
• Can be written on any computer that has GCC
plus “GNUstep” plug-in
• Apple computers have all of this pre-installed,
and also have an iPhone simulator in the XCode
IDE
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Tools
• Apple: pre-installed with the Cocoa frameworks
– XCode or GCC in terminal window
• Ubuntu: GnuStep is a free clone of Cocoa
– sudo aptget
– install buildessentials
– gobjc gnustep
– gnustepmake
– gnustepcommon
– libgnustepbasedev
• Windows: http://www.gnustep.org/
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File Overview
• Source code files have a .m extension
• Header files have a .h extension
• You can use gcc to compile in the same
way as C code
• But, you will need to add the Cocoa or
GNUStep frameworks to the build.
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Uses ARC – Compile with Xcode
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[])
{
@autoreleasepool {
// insert code here...
NSLog(@"Hello, World!"); }
return 0;
}
NSLog() is equivalent to a C printf()
Very similar to C, C programs are valid in Objective-C
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No ARC – Compile with GCC
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Compiling Hello World in Xcode
(Mac)
• File->New Project
– Mac OS X category – Select Application
– Command Line Tool
– Type - “Foundation”
• Name project helloworld and choose folder
• Save.
• Note that Foundation.framework has been included
for us already
• “Build and run”.
• The debugger console should show “Hello World”
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Compiling Hello World on Terminal
(Mac)
• Write hello.m using a plain text editor. Compile:
gcc –framework Foundation hello.m –o hello
or use: “clang”
• Foundation brings in the Foundation framework
(Cocoa) which bundles together a set of
dependencies (header files and libraries). You
will need this every time we compile Objective C
code.
• Type: ./hello will run the program in the terminal
• Note: this approach is fine for early examples
but the iPhone applications will be much easier
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to build in XCode.
Intro to HelloWorld
• Objective-C uses a string class similar to the
std::string or Java string. It is called
NSString.
• Constant NSStrings are preceded by @ for
example: @”Hello World”
• You will notice that NSLog() also outputs
time and date and various extra information.
NSAutoreleasePool* pool
=[[NSAutoreleasePool alloc] init]; allocates a
lump of memory
• Memory must be freed with [pool drain];
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Example 2
#import <Foundation/Foundation.h>
int main(int argc, const char* argv[]) {
NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
NSLog (@”Hello World”);
int undergrads = 120;
int postgrads = 50;
int students = undergrads + postgrads;
NSLog (@”Now featuring...\n %i iOS students”, students);
[pool drain];
return 0;
}
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Some things to note
• No line break needed at end of NSLog
statements.
• NSString constants use C-style variables.
• Test this program to see results.
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@interface
@interface ClassName : ParentClassName
{
declare member variable here;
declare another member variable here;
}
declare method functions here;
@end
•
•
•
•
Equivalent to C class declaration
Goes in header (.h) file
Functions outside curly braces
Don’t forget the @end tag
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@implementation
#import <Foundation/Foundation.h>
#include “Example.h“
@implementation ClassName
define method function here;
define another method function here;
define yet another method function here;
@end
● Equivalent to C class function definitions
● Goes in source (.m) file
● Don't forget the @end tag
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Example 3: Student.h
@interface Student : NSObject
{
int mUndergrads;
int mPostgrads;
}
-(void) print;
-(void) setUndergrads: (int) undergrads;
-(void) setPostgrads: (int) postgrads;
@end
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Some things to note
• NSObject is the “root” object in Objective-C
• No direct access to instance variables so we write
some get/set “instance methods”
• “Instance methods” (affect internal state of class)
are preceded with a minus sign
• “Class methods” (higher level functions) are
preceded with a plus sign e.g. create new class
• Method return type in parentheses
• Semicolon before list of parameters
• Parameter type in parenthesis, followed by name
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Example 3: Student.m
#include “Student.h”
@implementation Example
-(void) print {
int totalStudents = mUndergrads + mPostgrads;
NSLog (@”Total students in CompSci = %i”, totalStudents);
}
-(void) setUndergrads: (int) undergrads {
mUndergrads = undergrads;
}
-(void) setPostgrads: (int) postgrads {
mPostgrads = postgrads;
}
@end
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Some things to note
• Note prefix minus sign
• Very similar to C
• Same format as with the interface
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Example 3: main.m
#import <Foundation/Foundation.h>
#include “Student.h”
int main(int argc, char* argv[]) {
NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
id myStudent; myStudent = [[Student alloc] init]; //
allocate and initialize all in one!
Student* iOSstudent; // pointer
iOSstudent = [Student alloc];
// allocate memory
iOSstudent = [iOSstudent init]; // initialize
[iOSstudent setUndergrads: 120]; // apply method to instance
[iOSstudent setPostgrads: 2000];
[iOSstudent print];
[myStudent print];
[iOSstudent release];
[myStudent release];
[pool drain];
return 0;
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}
Some things to note
• Applying methods to instance format:
[receiver message];
• Semi-colon for variables or values passed
to methods
• [pool drain] etc. are also applying methods.
• Built-in messages; alloc, init, release.
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Some other things to note
alloc is equivalent to C++ new but it also zeros
all variables
init should be applied to an instance before use
These are often combined in shorthand:
Student* iOSstudent = [[Student alloc] init];
There is no garbage collection on iOS, so we
should release all of our instance memory.
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To compile example 3
• Create a project in Xcode
(don’t use “Auto Reference Counting” if
you cut-n-paste from this lecture)
• Use ARC with the example code provided
separately.
• Note: You will probably use “ARC” for all of
your other projects
• Compile and run!
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Data Structures
• Objective-C arrays are similar to C arrays, but
you can initialize whole array in a list.
• Or just a few indices of the array. The rest are
set to 0. Or mix and match:
int values[3] = { 3, 4, 2 };
char letters[3] = { 'a', 'c', 'x' };
float grades[100] = {10.0,11.2,1.1};
int array[] = {[3]=11,[2]=1,[7]=0};
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Multi-dimensional Arrays
• Can be initialized by using subset notation with
braces. Note no comma after second subset.
• Subset braces are optional, the compiler will fill in
blanks where it can as with 1D arrays.
int array[2][3] = {
{ 0, 3, 4},
{ 1, 1, 1}
};
int array[2][3] = {0, 3, 4, 1, 1, 1};
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Arrays and Functions
• Arrays can be passed as arguments to functions.
• This function will print every integer in an array but it needs
to also be told how long the array is (arrayLength).
• Statements with hard-coded indices such as array[4] are
potentially dangerous.
void function(int array[], int arrayLength) {
int fourthValue = array[4];
for (int i = 0; i < arrayLength; i++) {
NSLog(@”%i”, array[i]; }
}
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Structs
• Similarly, the Objective C “struct” can be
initialized in one go.
• Use the '.' member notation or just use
values in the correct order or use some
values. 'unknown.'
• In the following example we explicitly
define the last member but the others are
undefined.
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Structs
struct Student {
int BSYears;
int PhDYears;
int WellSpentYears;
};
Student myCareer = {
.BSYears = 3, .PhDYears = 3, .WellSpentYears = 6
};
Student poorLifeDecisions = { 3, 8, 3 };
Student unknown = { 3 };
Student andFurtherMore = { .WellSpentYears = 0 };
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Fitting into limited memory: Unions
• Union data structures allow ambiguity.
• Useful for allowing one storage area to hold different
variable types.
• Can only hold one of the variables at a time.
• Must be careful to ensure retrieval type matches last type
stored.
• Might come in handy for storing lists of “data” that might be
of different types.
• The next example gives ones potentially practical use;
• A series of recordings store ‘Data.’
• The struct has a char to indicate the type of data recorded
in Data.
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Memory Management
Memory management in Objective-C is semiautomatic:
The programmer must allocate memory for
objects either
a) explicitly (alloc) or
b) indirectly using a constructor
No need to deallocate
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Allocation
• Allocation happens through the class method alloc.
• The message ‘alloc’ is sent to the class of the
requested object. Alloc is inherited from NSObject.
Every alloc creates a new instance (=object)
[HelloWorld alloc];
• The class creates the object with all zeros in it and
returns a pointer to it.
HelloWorld *p = [HelloWorld alloc];
• The pointer p now points to the new instance. Now
we send messages to the instance through p.
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The reference counter
• Every instance has a so called reference
counter. It counts how many references are
retaining the object. The counter is 1 after
allocation. It does not count how many
references exist to the object
• Sending the retain message to the object
increases the reference counter by 1.
• Sending the release message decreases the
reference counter by 1.
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reference counter = retain counter
• When the reference counter reaches zero,
the object is automatically de-allocated.
The programmer does not deallocate.
• The programmer only does:
alloc
retain
release
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Rules for memory management
• The method that does an alloc or a retain
must also do a release, it must maintain
the balance between:
(alloc or retain) and (release)
• If a method does alloc and returns a
pointer to the created object then the
method must do an autorelease instead
of release. The calling code can do
(retain – release) but this is not a must.
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Autorelease pool – if no ARC
• For autorelease to work the programmer
must create an autorelease pool, using:
NSAutoreleasePool *pool
= [[NSAutoreleasePool alloc] init];
• When Cocoa is used then the autorelease
pool is created automatically, the
programmer does not need to do it.
• To release the pool and all objects in it, do:
[pool release];
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Constructors
• This is a class method that allocates and
initializes an object. The programmer is
neither doing alloc nor init.
• Example:
+(id)studentWithName :(NSString*)name AndGpa:(float)gpa
{ id newInst = [[self alloc]initStudent:name :gpa];
return [newInst autorelease];
}
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Constructors
• Essential: the method sends alloc to ‘self’
which is the Student class object
• Essential: the method autoreleases the
instance, because it returns a pointer to
the created instance
• Not essential: This example uses an
existing initializer, it could use something
else or initialize the Student data directly
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Constructors
Calling the constructor:
id stud
= [Student studentWithName:@"Johnnie" AndGpa: 3.8];
The message is sent to the Student class
object and returns a pointer to it, the
pointer is assigned to stud
The calling code does neither alloc nor init
An autorelease pool must be in place
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Something to note!
• You can program for either iOS or OSX or
both in the latest XCode.
• If you are going to do the examples for
both simultaneously be aware that there
are two sets of .h and .m files (one for
each).
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Frameworks
UIKit framework for developing standard iOS GUIs (buttons
etc.)
UITextField (user-entered text that brings up the keyboard)
UIFont
UIView
UITableView
UIImageView
UIImage
UIButton (a click-able button)
UILabel (a string of text on-screen)
UIWindow (main Window on iPhone)
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iOS programming
• Event driven framework
• Interface Designer has some extra macros for
the code that act like hooks to variables;
– IBAction - trigger events like mouse clicks
– IBOutlet - captures outputs from events
• These tags are not compiled (don't affect the
code) but sit as an extra before variables that
the Interface Designer can see.
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End of Lecture 1
• Next, we will be doing more Object-C
programming and building more
applications.
• Make sure to practice writing Object-C
source code so you can create highly
functioning apps.
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