Objective-C: a gentle introduction

Objective-C
a gentle introduction

Gabriele Petronella

Outline

• From C to Objective-C
• OO in Objective-C
• Memory management
• Demo iOS

2

History, in brief
• Created in the early ‘80s by Brad Cox and
Tom Love
• First Obj-C runtime in 1992
• Used by NeXT in the 90’s, later acquired by
Apple
• Apple introduced Obj-C 2.0 in 2006
3

A C superset

• Obj-C adds Smalltalk-style messaging to C,
used for OO operations
• C code is completely supported and used
for any non OO operation
• Different philosophy w.r.t. C++

4

Obj-C vs C++
• C++ adds OO-programming, generic
programming and metaprogramming to the
C language
• Obj-C adds OO-programming, dynamic
typing and reﬂection
• Bottom line C++ is geared toward
compile-time features, whereas Obj-C is
geared toward run-time features

5

A remarkable quote

“I made up the term ‘object-oriented’ and I
can tell you I did not have C++ in mind”
(Alan Kay, 1997)

6

Obj-C vs C (strings)
• C is fully supported, but some constructs are
seldom used
• Strings are one example
C string: “string”
Obj-C string: @“string”
• Notice the @ sign

7

Pointers,
pointers everywhere
• Typical C pointer usage
int* intPtr;
/* stuff */
int anInt = *intPtr //dereferencing
• Usually pointers are dereferenced and we
speak about the actual objects they point

8

Pointers,
pointers everywhere
• In Obj-C that’s not the case: we declare
pointers and we treat them as actual objects
• Anything you’ll want to do in Obj-C with an
object will expect a pointer
• Obj-C itself will take care of accessing the
actual objects under the hood.

9

Pointers,
pointers everywhere
NSString * s = @”Hi there”
• That’s convenient and that’s why we’ll tend
to say that “s is an NSString” when it is
actually a pointer to it
• But, please, never forget that a pointer is a
pointer!

10

Pointers may trick you
o1 o2 o1 o2

ptr1 = ptr2
ptr1 ptr2 ptr1 ptr2

• A typical beginners’ mistake is to think that
the above assignment will provide a copy of
o2, which is obvious not true since we’re
assigning a pointer

11

Obj-C Objects Syntax

• The Obj-C syntax derives from Smalltalk
• Messages are sent to objects with a square
brackets syntax, like for example
[myObject doThis]
an instance a message

12

Message parameters
• A message can have parameters, of course
[myObj doWithPar:par1
otherPar:par2]
• The corresponding method’s signature will
be
doWithPar:otherPar:

13

Overloading
• Obj-C does not support overloading
• That’s not a big deal since methods use the
inﬁx notation
• The name of the method is mixed with it’s
arguments
• This increases verbosity but also clarity
14

Writing vs Reading

Peter Hallam (Microsoft): What Do Programmers Really Do Anyway?
http://blogs.msdn.com/b/peterhal/archive/2006/01/04/509302.aspx

15

Java vs Inﬁx notation
• A real method call from an Android app
PendingIntent.getActivity(context, 0, new
Intent(), 0);

• In Obj-C it would look something like
[PendingIntent activityWithContext:context
requestCode:0
intent:[Intent new]
ﬂags:0];
16

Intent(), 0);
dafuq is this?!
requestCode:0
intent:[Intent new]
ﬂags:0];
16

Intent(), 0);
dafuq is this?!

16

Intent(), 0);
dafuq is this?!
requestCode:0
intent:[Intent new]
oh, a request code, I see...
ﬂags:0];
16

Nesting calls

• Of course calls can be nested
[politeObject sayHiTo:[anOtherObj name]];
[[MyClass alloc] initWithName:[foo name]];

17

The nil case
• A non-valid object pointer has value nil
• Almost the same as a NULL pointer
• It is a form of zero, therefore the following
code is (ugly but) legal
obj = nil;
if(obj) { /% do stuff %/ }

18

Talking to nil
• Any Java programmer here? Do you love
NullPointerExceptions?
• In Obj-C there no such thing! That’s because
sending a message to nil is legal

• What’s the value of obj2 after this code?
obj1 = nil;
obj2 = [obj1 doSomething];

19

Talking to nil

• Whether this is a good or a bad thing is a
quasi-religious issue
• Cons: may cause silent failures difﬁcult to
track down
• Pros: allows ﬂexibility

20

A touch of Class

• Classes are generally divided into two
chunks of code
@interface MyClass deﬁned in
@end MyClass.h
@implementation MyClass deﬁned in
@end MyClass.m

21

Inheritance

• The @interface declaration allows to
specify a parent class
@interface MyClass : NSObject
• NSObject is the Cocoa- base class- but it is
(actually another one exists NSProxy
not of our interest)

22

Methods declaration
• Methods are declared as follows
@interface MyClass : NSObject
+ (MyClass *)myClassInstance;
- (NSString *)sayHelloClass;
@end
• Class methods starts with the + sign
• Instance methods with the - sign
23

Methods deﬁnition
• Methods are then deﬁned in the
@implementation section of the class
@implementation MyClass
- (NSString *)sayHelloClass {
return @”Hello Class!”;
}
...
@end

24

Instance variables

• Instance vars are traditionally declared in
the @interface section.
• However since iOS 5 it is allowed to
declare them in the @implementation

• Consider that the @interface section is
usually visible to other classes

25

Instance variables

• Here’s an example of ivars declaration
@interface MyClass : NSObject {
NSInteger anInteger;
NSString * aString;
}
@end

26

Properties
• A property is a syntactical feature of Obj-C
2.0, i.e. syntactic sugar for calling an accessor
method. Example:
NSString * name = [aPerson name];
[aPerson setName:@”Mary”];
equivalent to
NSString * name = aPerson.name;
aPerson.name = @”Mary”;
27

Properties declaration
• A property is generally declared in the
@interface section as follows (< iOS4
style)
@property(nonatomic, retain)this changed *
NSString
* since iOS5
name;

• The above line declares the accessor methods for
the name variable.
• The options in the parenthesis deﬁne the
28

Properties deﬁnition
• The actual implementation of accessor
methods is achieved like follows:
@implementation MyClass
@synthesize name;
...
• The @synthesize keyword provides the
implementation of the two methods
- (NSString *)name
- (void)setName:(NSString *)aName
29

Memory Management

30

Memory Management
alloc
1

30

Memory Management
alloc retain count
1

30

Memory Management
alloc retain count I care!
1 2

30

Memory Management
1 2

I care!

I care! 5

I care!

30

Memory Management
1 2

I care! I don’t care any longer

I care! me neither 1
5

I care! neither do I neither do I

30

Memory Management
1 2


5


I don’t care...
0

30

Memory Management
1 2


5


I don’t care... dealloc
0

30

The NARC Rule
• Every new, alloc, retain, copy (and
mutableCopy) call MUST be balanced
with a release
• This still holds even under ARC (Automatic
Retain Count). The only difference is that
the rule is automatically respected by the
compiler for you.

31

Accessor methods and
memory management
- (void)setName:(NSString *)newName {
if (newName != self->name) {
/* release the old object*/
[self->name release];
/* retain the new one */
self->name = [newName retain];
}
}

32

Memory policies
• strong/retain: ‘normal’ reference that
increases the retainCount
• copy: same as retain, but clones the object
• weak: ARC speciﬁc. Does not retain the
object. Automatically nullify the pointer
• assign: pre-ARC. Same as weak but dåoes
not automatically nullify the pointer!

33

Other options

• nonatomic/atomic: thread-safe or not.
Default is atomic.
• getter/setter: deﬁnes the getter/setter
names
• readonly/readwrite: whether produce the
setter or not. Default is readwrite.

34

References

• Programming iOS 5 by Matt Neuborg
• Apple ofﬁcial documentation

35

Objective-C: a gentle introduction

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