[SeanLuke]

> In other languages, like C++, this would crash your program, but in Objective-C, invoking a method on nil returns a zero value. This greatly simplifies expressions, as it obviates the need to check for nil before doing anything:

    // For example, this expression...
    if (name != nil && [name isEqualToString:@"Steve"]) { ... }

    // ...can be simplified to:
    if ([name isEqualToString:@"steve"]) { ... }

Wow, that is some bad, bad code. Consider this snippet instead:

    if ([name isDifferentFromString:@"bob"]) { ... }

[ajanuary]

You do raise an interesting point that a nil swallowing language forces you to write your methods in such a way as to logically return false for a nil receiver.

I'd argue this is the way most methods are naturally written, so it doesn't have too much of an impact.

I'm struggling to think of an example where logically returning true for a nil receiver is a much more natural than the other way around. Most of them, including your example of isEqualToString vs. isDifferenceFromString, are at best equal.

It's probably something that should be made more explicit in discussions about nil swallowing, such as the OP.

[to3m]

A container might have an isEmpty function, and if the pointer is nil, that is probably more empty than not empty (though personally I don't believe either answer makes sense). "[x isEmpty]" might be more appropriate than "[x count]==0" for a list-type container.

I always check for nil explicitly, personally. It sidesteps any question of whether the method makes sense in the nil case. And sending messages to nil can't work in general, because the return value could be a struct.

[ajanuary]

Cheers for the counter-example.

I agree there isn't really a sensible answer for [nil isEmpty].

There are two uses of isEmpty I can think of.

    if ([container isEmpty]) {
        [container addItem:item];
    }

We want to add an item to an empty container. If it's nil, we don't add an item to it; that seems okay.

    if (![container isEmpty]) {
        Item item = [container getItemAt:0];
        [item doSomething];
    }

We check there will be an item before we get a value out of it. This one would cause issues.

NSArray doesn't have an isEmpty method. I wonder if this is one of the reasons why, or if it's just something like wanting to keep the interface small.

[Edit] You can of course invert the logic. Using [container hasItems] in the two examples above would function just fine. While isEmpty seems to be the variant used everywhere, hasItems doesn't seem too unnatural.

[to3m]

NSArray has a constant-time size query, so you might as well do "[x count]==0". If you had a linked list type structure, though, you might just have next/prev pointers in your list object, making a size query an O(N) operation. (The C++ STL provides an "empty()" function in its containers for presumably this reason. It has some containers that have to be implemented as linked lists or trees, meaning there's the possibility that "x.empty()" to be much more efficient than "x.size()==0". Not sure how often implementations take advantage of this, mind.)

As for how you'd use it, I've pretty much always used it for early outs:

    if(items.empty()) return NULL;
    return &items[items.size()%rand()];

or for filling in caches:

    if(items.empty()) return NULL;

    if(cache.empty()) { /* fill cache */ }

    assert(cache[index].underlying_item==items[index]);
    return cache[index].some_other_data;

So it always ends up being tested positively. Or it does if you code like me, anyway :) (I make no statement about whether you should or not.)

[jfb]

I dunno. I think a nil container is neither empty nor full, it's nil. But trying to reason about 3VL in an environment that doesn't support it is … trying.

[judofyr]

`[a compare:b]` returns 0 both if they are equal and if a is nil.

[ajanuary]

Cheers for the counter-example.

I can't think of a compare method in the Objective-C core lib (the biggest nil-swallowing lib I know of). Does anyone know of one? Would be interesting to see how it's handled and if it causes issues.

You could return an enum, which would allow you to use non-obvious values (Lower = 1, Equal = 2, Higher = 3), though this would make it a bit of a pain to interface with C-style libs.

Obviously the presence of a work-around doesn't mean nil-swallowing is a good idea, but rather cements the idea that library authors need to put in extra effort to make sure nil receivers are handled appropriately.

[judofyr]

> I can't think of a compare method in the Objective-C core lib (the biggest nil-swallowing lib I know of). Does anyone know of one? Would be interesting to see how it's handled and if it causes issues.

Maybe I misunderstood you, but there is plenty of compare:'s in Cocoa: http://cl.ly/image/1a112f1B1W1g.

They all return an enum that's either -1, 0 or 1.

[chc]

judofyr's code is actually valid. Most "value" classes have compare: — NSString, NSDate, NSNumber, etc.

[Groxx]

I'd argue this is correct. If a is nil, it's not comparable. The alternative would be to throw an exception, which is pretty simple to emulate by checking.

Also, nils aren't allowed in the collection classes, so sorting with compare: will simply never encounter this case.

[tyler-dodge]

True, although none of the other return values make that much sense with nil either.

[tyler-dodge]

That seems like a naming decision, I think it'd make more sense to have an equals selector than a not equals selector in objective c. Plus isEqualToString: is already part of NSString

[efnx]

That's not bad code, you must be sarcastic. I've never heard of the method isDifferentFromString: and besides, the method name suggests it's the same thing as isEqualToString: approached from the other side. Are you referring to the capitalization mistake in @"steve"? When using sarcasm keep in mind there's no tone in text ;)

[alayne]

He's not being sarcastic. Maybe a better example of what he is trying to get at is

if (![missleLauncher isDisabled]) { /* declare thermonuclear war */ }

When missleLauncher is nil, thermonuclear war is still declared which may not be the programmer's intent.

[xsmasher]

Properties and methods are given "positive" names as a matter of convention, and to make reasoning about Nil easier.

In Objective-C, the question "should I name my method isEnabled or isDisabled?" has a reasonable answer - you consider what makes most sense (or any sense) when called on Nil.

This is probably a good rule for other languages too - "negative" booleans can lead to double-negatives and unreadable code.

[alayne]

Even with a positive naming convention, you will have conditionals for the negative case, e.g. if (![str isEqualToString:@"myvalue"]) and falling into its block is not desirable when str is nil. Personally I have had plenty of bugs around things being unexpectedly nil, but I can't think of a case like this where I executed a block unexpectedly.

[optymizer]

<sarcasm>Well, if you've never heard of it then it cannot exist and that invalidates his example, right? </sarcasm>

I'm not saying the example is correct, I'm pointing out that "I've never heard of it" / "Works on my computer" are dangerous attitudes to have.

[efnx]

I'm sorry, I should have been more clear. I meant "I've never heard of that method, so I looked it up in Xcode and it turns out it is not a method on NSString (and why should it be if !isEqualToString: achieves the same purpose?) which leads me to believe that this comment is sarcasm, if it isn't can you please clarify?"

[kstenerud]

Why is it bad code? If name is nil, isEqualToString will return false. And that's correct behavior because nil is not equal to the string @"steve".

[mpweiher]

You generally try to make tests positive, which then makes this work ( -isEqual: rather than -isDifferent:).

Of course, there is one place where you have to go the other way: -isNil: does not work, you need to use -isNotNil

Unless you use the runtime's (private) nil-hook:

    +(void)setNilHandler
    {
        installed=YES;
        _objc_setNilReceiver([self nsNil]);
    }

Then you can send messages to nil and they will be sent to your class. Of course, you want to mimic the nil-eating behavior as much as possible, so:

    static id idresult( id receiver, SEL selector, ... )  { return nil; }

    +(BOOL)resolveInstanceMethod:(SEL)selector
    {
        class_addMethod(self, selector, (IMP)idresult , "@@:@");
        return YES;
    }

[nicholassmith]

No snark: could you explain why it's bad code for you?