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NSObject Protocol Reference

Overview

The NSObject protocol groups methods that are fundamental to all Objective-C objects.

If an object conforms to this protocol, it can be considered a first-class object. Such an object can be asked about its:

• Class, and the place of its class in the inheritance hierarchy

• Conformance to protocols

• Ability to respond to a particular message

In addition, objects that conform to this protocol—with its retain, release, and autorelease methods—can also integrate with the object management and deallocation scheme defined in Foundation (for more information see, for example, Memory Management Programming Guide for Cocoa). Thus, an object that conforms to the NSObject protocol can be managed by container objects like those defined by NSArray and NSDictionary.

The Cocoa root class, NSObject, adopts this protocol, so all objects inheriting from NSObject have the features described by this protocol.

Tasks

Identifying Classes

• – class
• – superclass

Identifying and Comparing Objects

• – isEqual:
• – hash
• – self

Managing Reference Counts

• – retain
• – release
• – autorelease
• – retainCount

Testing Object Inheritance, Behavior, and Conformance

• – isKindOfClass:
• – isMemberOfClass:
• – respondsToSelector:
• – conformsToProtocol:

Describing Objects

• – description

Sending Messages

• – performSelector:
• – performSelector:withObject:
• – performSelector:withObject:withObject:

Determining Allocation Zones

• – zone

Identifying Proxies

• – isProxy

Instance Methods

autorelease

Adds the receiver to the current autorelease pool.

- (id)autorelease

Return Value

self.

Discussion

You add an object to an autorelease pool so it will receive a release message—and thus might be deallocated—when the pool is destroyed. For more information on the autorelease mechanism, see Memory Management Programming Guide for Cocoa.

Special Considerations

If garbage collection is enabled, this method is a no-op.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – retain

Related Sample Code

• CoreRecipes
• GridCalendar
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• Mountains
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Declared In

class

Returns the class object for the receiver’s class.

- (Class)class

Return Value

The class object for the receiver’s class.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• + class (NSObject class)

Declared In

conformsToProtocol:

Returns a Boolean value that indicates whether the receiver conforms to a given protocol.

- (BOOL)conformsToProtocol:(Protocol *)aProtocol

Parameters

aProtocol

A protocol object that represents a particular protocol.

Return Value

YES if the receiver conforms to aProtocol, otherwise NO.

Discussion

This method works identically to the conformsToProtocol: class method declared in NSObject. It’s provided as a convenience so that you don’t need to get the class object to find out whether an instance can respond to a given set of messages.

Availability

• Available in Mac OS X v10.0 and later.

Declared In

description

Returns a string that describes the contents of the receiver.

- (NSString *)description

Return Value

A string that describes the contents of the receiver.

Discussion

The debugger’s print-object command indirectly invokes this method to produce a textual description of an object.

Availability

• Available in Mac OS X v10.0 and later.

Declared In

hash

Returns an integer that can be used as a table address in a hash table structure.

- (NSUInteger)hash

Return Value

An integer that can be used as a table address in a hash table structure.

Discussion

If two objects are equal (as determined by the isEqual: method), they must have the same hash value. This last point is particularly important if you define hash in a subclass and intend to put instances of that subclass into a collection.

If a mutable object is added to a collection that uses hash values to determine the object’s position in the collection, the value returned by the hash method of the object must not change while the object is in the collection. Therefore, either the hash method must not rely on any of the object’s internal state information or you must make sure the object’s internal state information does not change while the object is in the collection. Thus, for example, a mutable dictionary can be put in a hash table but you must not change it while it is in there. (Note that it can be difficult to know whether or not a given object is in a collection.)

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – isEqual:

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Declared In

isEqual:

Returns a Boolean value that indicates whether the receiver and a given object are equal.

- (BOOL)isEqual:(id)anObject

Parameters

anObject

The object to be compared to the receiver.

Return Value

YES if the receiver and anObject are equal, otherwise NO.

Discussion

This method defines what it means for instances to be equal. For example, a container object might define two containers as equal if their corresponding objects all respond YES to an isEqual: request. See the NSData, NSDictionary, NSArray, and NSString class specifications for examples of the use of this method.

If two objects are equal, they must have the same hash value. This last point is particularly important if you define isEqual: in a subclass and intend to put instances of that subclass into a collection. Make sure you also define hash in your subclass.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – hash

Related Sample Code

• IdentitySample
• Quartz Composer WWDC 2005 TextEdit
• Sketch-112
• TextEditPlus

Declared In

isKindOfClass:

Returns a Boolean value that indicates whether the receiver is an instance of given class or an instance of any class that inherits from that class.

- (BOOL)isKindOfClass:(Class)aClass

Parameters

aClass

A class object representing the Objective-C class to be tested.

Return Value

YES if the receiver is an instance of aClass or an instance of any class that inherits from aClass, otherwise NO.

Discussion

For example, in this code, isKindOfClass: would return YES because, in Foundation, the NSArchiver class inherits from NSCoder:

NSMutableData *myData = [NSMutableData dataWithCapacity:30];

id anArchiver = [[NSArchiver alloc] initForWritingWithMutableData:myData];

if ( [anArchiver isKindOfClass:[NSCoder class]] )

    ...

Be careful when using this method on objects represented by a class cluster. Because of the nature of class clusters, the object you get back may not always be the type you expected. If you call a method that returns a class cluster, the exact type returned by the method is the best indicator of what you can do with that object. For example, if a method returns a pointer to an NSArray object, you should not use this method to see if the array is mutable, as shown in the following code:

// DO NOT DO THIS!

if ([myArray isKindOfClass:[NSMutableArray class]])

{

    // Modify the object

}

If you use such constructs in your code, you might think it is alright to modify an object that in reality should not be modified. Doing so might then create problems for other code that expected the object to remain unchanged.

If the receiver is a class object, this method returns YES if aClass is a Class object of the same type, NO otherwise.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – isMemberOfClass:

Declared In

isMemberOfClass:

Returns a Boolean value that indicates whether the receiver is an instance of a given class.

- (BOOL)isMemberOfClass:(Class)aClass

Parameters

aClass

A class object representing the Objective-C class to be tested.

Return Value

YES if the receiver is an instance of aClass, otherwise NO.

Discussion

For example, in this code, isMemberOfClass: would return NO:

NSMutableData *myData = [NSMutableData dataWithCapacity:30];

id anArchiver = [[NSArchiver alloc] initForWritingWithMutableData:myData];

if ([anArchiver isMemberOfClass:[NSCoder class]])

    ...

Class objects may be compiler-created objects but they still support the concept of membership. Thus, you can use this method to verify that the receiver is a specific Class object.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – isKindOfClass:

Declared In

isProxy

Returns a Boolean value that indicates whether the receiver does not descend from NSObject.

- (BOOL)isProxy

Return Value

NO if the receiver really descends from NSObject, otherwise YES.

Discussion

This method is necessary because sending isKindOfClass: or isMemberOfClass: to an NSProxy object will test the object the proxy stands in for, not the proxy itself. Use this method to test if the receiver is a proxy (or a member of some other root class).

Availability

• Available in Mac OS X v10.0 and later.

Declared In

performSelector:

Sends a specified message to the receiver and returns the result of the message.

- (id)performSelector:(SEL)aSelector

Parameters

aSelector

A selector identifying the message to send. If aSelector is NULL, an NSInvalidArgumentException is raised.

Return Value

An object that is the result of the message.

Discussion

The performSelector: method is equivalent to sending an aSelector message directly to the receiver. For example, all three of the following messages do the same thing:

id myClone = [anObject copy];

id myClone = [anObject performSelector:@selector(copy)];

id myClone = [anObject performSelector:sel_getUid("copy")];

However, the performSelector: method allows you to send messages that aren’t determined until runtime. A variable selector can be passed as the argument:

SEL myMethod = findTheAppropriateSelectorForTheCurrentSituation();

[anObject performSelector:myMethod];

The aSelector argument should identify a method that takes no arguments. For methods that return anything other than an object, use NSInvocation.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – performSelector:withObject:
• – performSelector:withObject:withObject:

Declared In

performSelector:withObject:

Sends a message to the receiver with an object as the argument.

- (id)performSelector:(SEL)aSelector withObject:(id)anObject

Parameters

aSelector

A selector identifying the message to send. If aSelector is NULL, an NSInvalidArgumentException is raised.

anObject

An object that is the sole argument of the message.

Return Value

An object that is the result of the message.

Discussion

This method is the same as performSelector: except that you can supply an argument for aSelector. aSelector should identify a method that takes a single argument of type id. For methods with other argument types and return values, use NSInvocation.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – performSelector:withObject:withObject:
• – methodForSelector: (NSObject class)

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• SearchField
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Declared In

performSelector:withObject:withObject:

Sends a message to the receiver with two objects as as arguments.

- (id)performSelector:(SEL)aSelector withObject:(id)anObject withObject:(id)anotherObject

Parameters

aSelector

A selector identifying the message to send. If aSelector is NULL, an NSInvalidArgumentException is raised.

anObject

An object that is the first argument of the message.

anotherObject

An object that is the second argument of the message

Return Value

An object that is the result of the message.

Discussion

This method is the same as performSelector: except that you can supply two arguments for aSelector. aSelector should identify a method that can take two arguments of type id. For methods with other argument types and return values, use NSInvocation.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – performSelector:withObject:
• – methodForSelector: (NSObject class)

Declared In

release

Decrements the receiver’s reference count.

- (oneway void)release

Discussion

The receiver is sent a dealloc message when its reference count reaches 0.

You would only implement this method to define your own reference-counting scheme. Such implementations should not invoke the inherited method; that is, they should not include a release message to super.

For more information on the reference counting mechanism, see Memory Management Programming Guide for Cocoa.

Special Considerations

If garbage collection is enabled, this method is a no-op.

You must complete the object initialization (using an init method) before invoking release. For example, the following code shows an error:

id anObject = [MyObject alloc];

[anObject release];

You may call release from within an init method if initialization fails for some reason provided that you have at least called superclass's designated initializer.

Availability

• Available in Mac OS X v10.0 and later.

Related Sample Code

• CoreRecipes
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Declared In

respondsToSelector:

Returns a Boolean value that indicates whether the receiver implements or inherits a method that can respond to a specified message.

- (BOOL)respondsToSelector:(SEL)aSelector

Parameters

aSelector

A selector that identifies a message.

Return Value

YES if the receiver implements or inherits a method that can respond to aSelector, otherwise NO.

Discussion

The application is responsible for determining whether a NO response should be considered an error.

You cannot test whether an object inherits a method from its superclass by sending respondsToSelector: to the object using the super keyword. This method will still be testing the object as a whole, not just the superclass’s implementation. Therefore, sending respondsToSelector: to super is equivalent to sending it to self. Instead, you must invoke the NSObject class method instancesRespondToSelector: directly on the object’s superclass, as illustrated in the following code fragment.

if( [MySuperclass instancesRespondToSelector:@selector(aMethod)] ) {

    // invoke the inherited method

    [super aMethod];

}

You cannot simply use [[self superclass] instancesRespondToSelector:@selector(aMethod)] since this may cause the method to fail if it is invoked by a subclass.

Note that if the receiver is able to forward aSelector messages to another object, it will be able to respond to the message, albeit indirectly, even though this method returns NO.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – forwardInvocation: (NSObject class)
• + instancesRespondToSelector: (NSObject class)

Declared In

retain

Increments the receiver’s reference count.

- (id)retain

Return Value

self.

Discussion

You send an object a retain message when you want to prevent it from being deallocated without your express permission.

An object is deallocated automatically when its reference count reaches 0. retain messages increment the reference count, and release messages decrement it. For more information on this mechanism, see Memory Management Programming Guide for Cocoa.

As a convenience, retain returns self because it is often used in nested expressions:

NSString *systemApps = [[NSString

    stringWithCString:"/Applications"] retain];

You would implement this method only if you were defining your own reference-counting scheme. Such implementations must return self and should not invoke the inherited method by sending a retain message to super.

Special Considerations

If garbage collection is enabled, this method is a no-op.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – autorelease
• – release

Related Sample Code

• CIAnnotation
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• Reducer
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Declared In

retainCount

Returns the receiver’s reference count.

- (NSUInteger)retainCount

Return Value

The receiver’s reference count.

Discussion

You might override this method in a class to implement your own reference-counting scheme. For objects that never get released (that is, their release method does nothing), this method should return UINT_MAX, as defined in <limits.h>.

The retainCount method does not account for any pending autorelease messages sent to the receiver.

Special Considerations

If garbage collection is enabled, the return value is undefined.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – autorelease
• – retain

Related Sample Code

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Declared In

self

Returns the receiver.

- (id)self

Return Value

The receiver.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• – class

Related Sample Code

• GridCalendar
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Declared In

superclass

Returns the class object for the receiver’s superclass.

- (Class)superclass

Return Value

The class object for the receiver’s superclass.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• + superclass (NSObject class)

Declared In

zone

Returns a pointer to the zone from which the receiver was allocated.

- (NSZone *)zone

Return Value

A pointer to the zone from which the receiver was allocated.

Discussion

Objects created without specifying a zone are allocated from the default zone.

Availability

• Available in Mac OS X v10.0 and later.

See Also

• + allocWithZone: (NSObject class)

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• QTCoreVideo103
• QTCoreVideo201
• QTCoreVideo202
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Declared In

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