Objective C

Question 1

How is Objective C described?

Answer 1

• C plus objects
• A strict superset of C
• fabricated in C to build larger-scale assemblies

Question 2

What are the two object-oriented frameworks in objective C?

Answer 2

• Foundation: classes for basic data structures and for interacting with OS
• AppKit: classes for developing applications, windows, buttons, etc

Question 3

What did Objective C 2.0 add?

Answer 3

• Garbage collection
• Syntax enhancements
• Runtime performance improvements
• 64-bit support
• Features such as properties and fast enumerators

Question 4

What is the difference between #import and #include?

Answer 4

In a C header link file, the #include guard is required for any # statements. In an Objective C header link file, #import guard is an implicit #include guard

Question 5

What are the functions, methods and messages calls in all C types?

Answer 5

C function call:
myFunction(parameter1,parameter2);

C++ method call:
MyObject *obj = new MyObject;
obj->myMethod(parameter1,parameter2);

Objective-C messaging:
MyObject *obj = [MyObject new];
[obj callWith:parameter1 and:parameter2];

Question 6

What wrapper classes does the foundation framework provide?

Answer 6

• NSNumber (int, float, double, boolean)
• NSString and NSMutableString (Unicode strings (usually UTF-16))
• NSArray and NSMutableArray (dynamic arrays)

Question 7

Where is memory for objects allocated?

Answer 7

It is always allocated in the heap and never in the stack

Question 8

What is literal syntax?

Answer 8

It makes creating new objects easier
Without literal syntax:
BOOL b = NO;
NSNumber *num1 = [[NSNumber alloc] initWithInteger:43];
NSNumber *num2 = [[NSNumber alloc] initWithChar:’A’];
NSNumber *num3 = [[NSNumber alloc] initWithFloat:91.6];
NSNumber *num4 = [[NSNumber alloc] initWithBool:b];

With literal syntax:
BOOL b = YES;
NSNumber *num1 = @43;
NSNumber *num2 = @’A’;
NSNumber *num3 = @91.6;
NSNumber *num4 = @b;

This also works with expressions

Question 9

How are header files and implementation files structured?

Answer 9

header file: MyClass.h

<import>
@interface <classname> : <superclass>
{
<attribute>
}
<method>
@end

Method definitions are like function prototypes

implementation file: MyClass.m
<import>
@implementation <classname>
<method>
@end
</method></classname></import></method></attribute></superclass></classname></import>

Question 10

How do you access NSObject?

Answer 10

import <Foundation/Foundation.h>

Question 11

What should all classes extend and why?

Answer 11

NSObject as the class inherits alloc, init and new

Question 12

What makes a class method different to an instance method?

Answer 12

A class method cannot access attributes

Question 13

How are Objective C constructors called?

Answer 13

Method name always starts with init and is called after alloc
NSNumber *anum = [[NSNumber alloc] initWithInteger:@43];
NSNumber *pi = [[NSNumber alloc] initWithFloat:@3.14159];

Question 14

What is id?

Answer 14

A datatype that is a pointer to any type of object. It supports dynamic referencing which means that the object type is defined at runtime.

Question 15

How are destructors called?

Answer 15

• Always called dealloc
• Always called automatically by the runtime
• Never explicitly called

Question 16

What is MMR?

Answer 16

Manual Retain-Release:
- You explicitly manage memory by keeping track of how many owners an object has
- Implemented using a model called reference counting which is provided by NSObject and the Objective-C runtime

Question 17

What is reference counting?

Answer 17

• Create a new object and reference counter is set to 1
• You claim ownership of an object and reference count is incremented by 1
• You relinquish ownership of an object and reference count is decremented by 1
  An object exists while reference count is greater than zero. An object is destroyed when the reference count becomes zero.

Question 18

What are the reference counting methods?

Answer 18

• alloc: Create an object and claim ownership of it. Set reference count to 1.
• retain: Claim ownership of an existing object. Increase reference count by 1.
• copy:Copy an object and claim ownership of it. Set reference count to 1.
• release: Relinquish ownership of an object. Decrement its reference count by 1.
• autorelease: Relinquish ownership of an object but defer its destruction.
  Each alloc/retain/copy must have an associated release/autorelease to
  avoid memory leaks

Question 19

When is the best time to use autorelease?

Answer 19

You use autorelease when you need to send a deferred release message, typically when returning an object from a method

Question 20

What is ARC?

Answer 20

Automatic reference counting:
- needs to be switched on in Xcode
- works the same way as manual reference counting, except it automatically inserts appropriate retain and release calls
- means you must not explicitly call retain, release or autorelease
- means you usually do not need a custom dealloc method