Sending a message to nil in Objective-C

Question

As a Java developer who is reading Apple's Objective-C 2.0 documentation: I wonder what "sending a message to nil" means - let alone how it is actually useful. Taking an excerpt from the documentation:

There are several patterns in Cocoa that take advantage of this fact. The value returned from a message to nil may also be valid:

• If the method returns an object, any pointer type, any integer scalar of size less than or equal to sizeof(void*), a float, a double, a long double, or a long long, then a message sent to nil returns 0.
• If the method returns a struct, as defined by the Mac OS X ABI Function Call Guide to be returned in registers, then a message sent to nil returns 0.0 for every field in the data structure. Other struct data types will not be filled with zeros.
• If the method returns anything other than the aforementioned value types the return value of a message sent to nil is undefined.

Has Java rendered my brain incapable of grokking the explanation above? Or is there something that I am missing that would make this as clear as glass?

I do get the idea of messages/receivers in Objective-C, I am simply confused about a receiver that happens to be nil.

Answer 1

I don't think any of the other answers have mentioned this clearly: if you're used to Java, you should keep in mind that while Objective-C on Mac OS X has exception handling support, it's an optional language feature that can be turned on/off with a compiler flag. My guess is that this design of "sending messages to nil is safe" predates the inclusion of exception handling support in the language and was done with a similar goal in mind: methods can return nil to indicate errors, and since sending a message to nil usually returns nil in turn, this allows the error indication to propagate through your code so you don't have to check for it at every single message. You only have to check for it at points where it matters. I personally think exception propagation&handling is a better way to address this goal, but not everyone may agree with that. (On the other hand, I for example don't like Java's requirement on you having to declare what exceptions a method may throw, which often forces you to syntactically propagate exception declarations throughout your code; but that's another discussion.)

I've posted a similar, but longer, answer to the related question "Is asserting that every object creation succeeded necessary in Objective C?" if you want more details.

Answer 2

Don't think about "the receiver being nil"; I agree, that is pretty weird. If you're sending a message to nil, there is no receiver. You're just sending a message to nothing.

How to deal with that is a philosophical difference between Java and Objective-C: in Java, that's an error; in Objective-C, it is a no-op.

Answer 3

Well, I think it can be described using a very contrived example. Let's say you have a method in Java which prints out all of the elements in an ArrayList:

void foo(ArrayList list)
{
    for(int i = 0; i < list.size(); ++i){
        System.out.println(list.get(i).toString());
    }
}

Now, if you call that method like so: someObject.foo(NULL); you're going to probably get a NullPointerException when it tries to access list, in this case in the call to list.size(); Now, you'd probably never call someObject.foo(NULL) with the NULL value like that. However, you may have gotten your ArrayList from a method which returns NULL if it runs into some error generating the ArrayList like someObject.foo(otherObject.getArrayList());

Of course, you'll also have problems if you do something like this:

ArrayList list = NULL;
list.size();

Now, in Objective-C, we have the equivalent method:

- (void)foo:(NSArray*)anArray
{
    int i;
    for(i = 0; i < [anArray count]; ++i){
        NSLog(@"%@", [[anArray objectAtIndex:i] stringValue];
    }
}

Now, if we have the following code:

[someObject foo:nil];

we have the same situation in which Java will produce a NullPointerException. The nil object will be accessed first at [anArray count] However, instead of throwing a NullPointerException, Objective-C will simply return 0 in accordance with the rules above, so the loop will not run. However, if we set the loop to run a set number of times, then we're first sending a message to anArray at [anArray objectAtIndex:i]; This will also return 0, but since objectAtIndex: returns a pointer, and a pointer to 0 is nil/NULL, NSLog will be passed nil each time through the loop. (Although NSLog is a function and not a method, it prints out (null) if passed a nil NSString.

In some cases it's nicer to have a NullPointerException, since you can tell right away that something is wrong with the program, but unless you catch the exception, the program will crash. (In C, trying to dereference NULL in this way causes the program to crash.) In Objective-C, it instead just causes possibly incorrect run-time behavior. However, if you have a method that doesn't break if it returns 0/nil/NULL/a zeroed struct, then this saves you from having to check to make sure the object or parameters are nil.

Answer 4

ObjC messages which are sent to nil and whose return values have size larger than sizeof(void*) produce undefined values on PowerPC processors. In addition to that, these messages cause undefined values to be returned in fields of structs whose size is larger than 8 bytes on Intel processors as well. Vincent Gable has described this nicely in his blog post

Answer 5

It means often not having to check for nil objects everywhere for safety - particularly:

[someVariable release];

or, as noted, various count and length methods all return 0 when you've got a nil value, so you do not have to add extra checks for nil all over:

if ( [myString length] > 0 )

or this:

return [myArray count]; // say for number of rows in a table

Answer 6

A message to nil does nothing and returns nil, Nil, NULL, 0, or 0.0.