NSObject Protocol Reference

Discussion

For more information about autorelease pool blocks, see Advanced Memory Management Programming Guide.

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.

Discussion

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

NSObject implements this method by calling through to the description method. Thus, by default, an object’s debug description is the same as its description. However, you can override debugDescription if you want to decouple these.

Discussion

This method is used to create a textual representation of an object, for example in a formatted string:

ClassName *anObject = <#An object#>;

NSString *string = [NSString stringWithFormat:@"anObject is %@", anObject];

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.)

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.

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.

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.

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).

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.

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.

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.

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 Advanced Memory Management Programming Guide.

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.

Discussion

You send an object a retain message when you want to prevent it from being deallocated until you have finished using it.

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 Advanced Memory Management Programming Guide.

As a convenience, retain returns self because it may be used in nested expressions.

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.