iOS > “id” vs. NSObject [duplicate]

问题:

This question already has answers here:

Why use id when we can just use NSObject? (4 answers)

Closed 6 years ago.

Is there a difference between relating to an Object in a 'Polymorphic' way with the type id than as NSObject *?

In what way is:

NSString* aString = @"Hello";
id anObj = aString;

different than:

NSString* aString = @"Hello";
NSObject* anObj = aString;

回答1:

id is a special keyword used in Objective-C to mean “some kind of object.” It does not contain isa pointer(isa, gives the object access to its class and, through the class, to all the classes it inherits from), So you lose compile-time information about the object.

NSString* aString = @"Hello";
id anObj = aString;  

NSObject contain isa pointer.

NSString* aString = @"Hello";
NSObject* anObj = aString;

From Objective-C Is a Dynamic Language

Consider the following code:

id someObject = @"Hello, World!";
[someObject removeAllObjects];

In this case, someObject will point to an NSString instance, but the compiler knows nothing about that instance beyond the fact that it’s some kind of object. The removeAllObjects message is defined by some Cocoa or Cocoa Touch objects (such as NSMutableArray) so the compiler doesn’t complain, even though this code would generate an exception at runtime because an NSString object can’t respond to removeAllObjects.

Rewriting the code to use a static type:

NSString *someObject = @"Hello, World!";
[someObject removeAllObjects];

means that the compiler will now generate an error because removeAllObjects is not declared in any public NSString interface that it knows about.

回答2:

id is generic. By using id you're telling the compiler that you will fill in details about usage later. The compiler assumes that any code you have is correct and doesn't warn you about anything. At runtime checks are made to verify what you're trying to do and you will get an exception if your code is wrong.

NSObject is specific. By using NSObject you're telling the compiler exactly what the object is. When you try to call methods on it they will be checked against what NSObject understands. You will get compile time errors if you make a mistake.

All that said, you can just cast in both cases to get to another Class type.

Your concern is what you're going to do with the reference in the future. Generally, using NSObject doesn't have any benefits.

回答3:

1. id is a language keyword but the NSObject is the base class in objective c.
2. For id, you dont need to typecast the object. But for NSObject, you have to typecast it into NSObject.
3. An id object can be used to invoke any methods. But NSObject object can be use to invoke only particular methods that are defined in NSObject

回答4:

When using id, you can send it any known message and when using NSObject *, you can only send it messages declared by NSObject (not methods of any subclass) or else it will generate a warning.