Objective-C OOP Spring 2003. OOP Conceptually the same as C++, Java, and all other object oriented languages The syntax, however… …is, well, different

Objective-COOPSpring 2003

OOP

Conceptually the same as C++, Java, and all other object oriented languages

The syntax, however…

…is, well, different

Apple’s documentation on Objective-C classes:

http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/DefiningClasses/DefiningClasses.html

http://developer.apple.com/library/ios/%23documentation/cocoa/conceptual/ProgrammingWithObjectiveC/DefiningClasses/DefiningClasses.html




OOP

Examples in these slides are available as a compressed Xcode project (intro_classes.xcodeproj) at:

http://www.ithaca.edu/barr/Student/CS345/Examples/intro_classes.zip








Methods & MessagesMethods of class instances are called by passing messagesThe syntax is square brackets

The instance is the first item in the backetsThe message or method name is the second item

#import <Foundation/Foundation.h>#import "alien.h"

int main(int argc, const char * argv[]){

@autoreleasepool {

alien *george; double timeToHome; george = [[alien alloc] init]; timeToHome = [george calcTimeToHome];

// continued on next slide

Variable holding object

Method being called (note: no parameters)

All variables that hold objects must be pointers

Brackets around the call

If you try to use a static variable to hold an object will get the error “interface type cannot be statically allocated”

Methods & MessagesThe syntax is square brackets

The instance is the first item in the backetsThe message or method name is the second itemThe argument is the third item



@autoreleasepool {

alien *george; double timeToHome; double timeToPlace; george = [[alien alloc] init]; timeToPlace =[george timeToPlace: 22000];



Method being called (note: 1 parameter)

Argument

If you try to use a static variable to hold an object will get the error “interface type cannot be statically allocated”

Methods & MessagesThe syntax is square brackets

The instance is the first item in the backetsThe message or method name is the second item followed by a colonThe first argument is the third itemThe next part of the method name is the fourth item followed by colon



@autoreleasepool {

alien *george; double timeToHome; double timeToPlace; george = [[alien alloc] init]; timeToPlace =[george timeWithSpeed: 1000.4 atDistance: 1230000.5];



Method being called (note: 2 parameters)

First argument

Second argument

PropertiesChanging properties

Setters and getters are created automatically by the @synthesize directiveTwo ways of using setters and getters: brackets and dot-notation



@autoreleasepool { alien *george; double timeToHome = 0.0; george = [[alien alloc] init]; george.planet = @"Mars"; george.numEyes = 8; george.distanceToHome = 10000; // light years george.speedSpaceShip = 10.4; // hours/light year } return 0;}

Normal bracket notation

Dot notation

It’s good to initialize variables when you declare them. This isn’t necessary for object pointers, because the compiler will automatically set the variable to nil if you don’t specify any other initial value:

Creating Classes

Interface and implementation are separate (like C++)Can put both in same file

Or in separate .h and .m files

Interface (.h file):

#import <Foundation/Foundation.h>

@interface alien : NSObject { //properties double distanceToHome;}// methods- (double) calcTimeToHome;

@end

Even console applications use the Foundation framesworks

The interface starts with @interface

The interface ends with @end

All classes inherit from NSObject Indicate inheritance with colon: NSObject

Methods starting with a dash are instance methodsMethods starting with a plus sign are class methods

Return type

Implementation fileImplementation file (.m file):

#import "alien.h"

@implementation alien

@synthesize numEyes, planet, distanceToHome, speedSpaceShip;

- (double) calcTimeToHome{ double theTime; theTime = distanceToHome / speedSpaceShip; return theTime;}@end

Must import the .h file

The implementation starts with @implementation nameOfClass

The implementation ends with @end

Use the @synthesize directive to create setters and getters automatically (see later slide)

Signature is the same as in the interface file.

Return value

Method with a parameter

Implementation file (.m file):

#import "alien.h"



- (double) timeToPlace: (double) dist{ double theTime = dist / speedSpaceShip; return theTime;}

@end

Each argument must have a selector and a formal parameter


Return value

Method with two parameters

Implementation file (.m file):

#import "alien.h"



- (double) timeWithSpeed: (double) theSpeed atDistance: (double) theDist{ double theTime = theDist / theSpeed; return theTime;}

@end

Each argument must have a selector and a formal parameter

Signature is the same as in the interface file.Return

value

init methodImplementation file (.m file):

#import "alien.h"



- (id) init{ if (self = [super init])

{ numEyes = 4;

planet = @"Neptune"; distanceToHome = 1000000; speedSpaceShip = 1000; return (self);}return nil;

}@end

First call the init of the superclass. It may return a new instance of you and it may fail (in that case it returns nil and the if statement fails)


Must return self or nil for george = [[alien alloc] init] to work.

In objective-C self is a built-in variable that allows code to refer to itself.

init method with parameters

#import "alien.h"



- (id) initWithNum: (int) a andDistance: (double) b andPlanet: (NSString *) c{ if (self = [super init]) { numEyes = 4;

planet = c; distanceToHome = b; speedSpaceShip = a;

return (self); } return nil; }

@end

Use the @synthesize directive to create setters and getters automatically (see next slide)


First call the init of the superclass. It may return a new instance of you and it may fail (in that case it returns nil and the if statement fails)

Must return self or nil for george = [[alien alloc] initWithNum: 4 andDistnce: 22000] to work.

Xcode 4.5: To return “self” the method name must begin with “initWith” where only the ‘W’ is capitalized.

@synthesizeIn Xcode 4.5, the @synthesize statement in the .m file is now optional, and properties are synthesized automatically.

Automatically generated IBOutlet properties therefore no longer add @synthesize because it's no longer required.

When properties are synthesized automatically (without an explicit @synthesize statement) the corresponding instance variable is prepended with an underscore.

This is so the instance variable and property names don't overlap.

Accessing instance variables and properties has not changed. All that has changed is the default name of the instance variable. E.g.:

_foo = @"Bar"; // Setting an instance variable directly.

self.foo = @"Bar"; // Setting an instance variable via a property accessor method.

The underscore is merely a matter of style, so that it's more clear that you're accessing the instance variable rather than the property.

You can add a @synthesize statement yourself, and that will force the name of the corresponding instance variable to be whatever you want it to be.

Likewise, if you add your own property accessor methods then that will prevent the instance variable from being generated automatically.

@synthesize

Example of the use of properties:

[george setPlanet:@"Mars"];

george.numEyes = 8;

george.distanceToHome = 10000; // light years

george.speedSpaceShip = 10.4;

theSpeed = [george speedSpaceShip];

theDistance = george.distanceToHome;

NSLog(@"George's speed is %.2f and his distance to home is %.2f\n", theSpeed, theDistance);

@synthesize automatically gives a setter:[object setVariableName: value]

Where “set” is lowercase, variable name is same as property except first letter is always capitalizedOr: can use the dot notation. Do not change variable name.

@synthesize automatically gives a getter:[object variableName]

Where “variable name is same as Or: can use the dot notation. Do not change variable name.

description methodThe description method. Override the method from the superclass. Do not have to declare in the interface.

#import "alien.h"



- (NSString *) description{ NSString *aboutMe; aboutMe = [NSString stringWithFormat:@"I am an alien that lives on %@ with %d eyes!", planet, numEyes ];

return aboutMe;} // description}

@end

This method is not in the .h file. It is called when you print an instance of this class.The method is used to allow a class to print out a string describing itself.

stringWithFormat is a class method. So we don’t use a variable as the first object, but the name of the class. Note that this particular method (stringWithFormat) works like printf instead of like normal methods!

description methodThe description method. Override the method from the superclass. Do not have to declare in the interface.

#import "alien.h"



sally = [alien new];

NSLog(@"Here's the description: %@", sally);

@end

you can use new if there are no arguments to the constructor; this does both alloc and init

Arrays of objectsTo put objects in arrays use the id class

#import "alien.h"



id theAliens[5]; // automatically an array of pointers

// insert code to create two aliens in variables george and sally

theAliens[0] = george; // automatically stores address theAliens[1] = sally;

NSLog(@"Here's george's description: %@", theAliens[0]);

@endFrom Apple’s documentation: As mentioned earlier, you need to use a pointer to keep track of an object in memory. Because of Objective-C’s dynamic nature, it doesn’t matter what specific class type you use for that pointer—the correct method will always be called on the relevant object when you send it a message.

The id type defines a generic object pointer. It’s possible to use id when declaring a variable, but you lose compile-time information about the object.

id is Objective-C’s root object (it’s a NSObject ).

Determining Variable type

To determine the type of a variable holding a class, use class methods from the NSObject class

if ([george isKindOfClass:[alien class]]) NSLog(@"george is an alien");else NSLog(@"george is NOT an alien (despite what you think)");

NSLog(@"george is of type %@", [alien class]);

The class name

The variable that holds an instance of a class.

The class method returns a NSString

Pass-by-referenceCommon technique in many languages

Example:

void f(int &x){

x = 55;}int main(int argc, char*argv[]){ int x = 42; f(y); printf(“y = %d\n”, y); // prints 55}

Does not exist in Objective-C, even if you’re using a pure C function in the .m file!

Virtual methods (not)The term "virtual function" has no direct counterpart in Objective-C.

In Objective-C you don't call functions on objects, you send messages to them.

The object itself then decides how to respond to the message, typically by looking up the message in its class object, finding the associated method and invoking it.

This all happens at run time, not compile time.

The mapping between messages ("selectors" is the technical term) and methods is built entirely from the @implementation.

The method declarations in the @interface are only there to give the compiler the information it needs to warn you that you may have forgotten a method implementation.

And it is only a warning because you can't tell until run time whether whether the object really does respond to the message or not.

For example, somebody else could add a category to an existing class that provides implementations for missing methods, or a class could override forwardingTargetForSelector: to forward messages it doesn't respond to elsewhere.

This is from JeremyP on stackOverflow: http://stackoverflow.com/questions/4374677/what-is-the-equivalent-of-a-c-pure-virtual-function-in-objective-c Not clear this is true for

the most recent version of Objective-C

C++ vs Objective C

Virtual vs ??

Abstract vs protocol

?? Vs categories

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Objective-C OOP Spring 2003. OOP Conceptually the same as C++, Java, and all other object oriented languages The syntax, however… …is, well, different