> The "==" case that has been mentioned just omits the second step because self
> isn't modified by calling [super init]. And the equivalent statements for the
> other case (! / ==) do the same thing except that the test step is the
> inverse.
init is allowed to return a different "self", so if you skip the assignment
you're looking at a stale self.
________________________________________
From: cocoa-dev-bounces+lrucker=vmware....@lists.apple.com
[cocoa-dev-bounces+lrucker=vmware....@lists.apple.com] on behalf of Steve
Christensen [puns...@mac.com]
Sent: Sunday, May 31, 2015 6:45 PM
To: Alex Zavatone
Cc: Cocoa-Dev List
Subject: Re: Looking at self = [super init].
No, it doesn't make a difference. In both cases the compiler will generate a
"test and branch" to the method's epilogue. For the "=" case:
if (self = [super init]) ...
is equivalent to:
if ((self = [super init]) != nil) ...
is equivalent to:
self = [super init];
if (self) ...
is equivalent to:
self = [super init];
if (self != nil) …
They all:
• Call the superclass' -init method.
• Store the result in self.
• Test if self is not equal to nil/zero.
• Branch to method epilogue (or at least past {...}) if not.
The "==" case that has been mentioned just omits the second step because self
isn't modified by calling [super init]. And the equivalent statements for the
other case (! / ==) do the same thing except that the test step is the inverse.
In my opinion having a macro to replace the "self = [super init]" idiom saves
you a couple of seconds of typing — once; it obfuscates behavior since you need
to locate the macro to see what it does if you forget; and it is applicable
only to subclasses where you're calling a superclass' -init method. It doesn't
help for, e.g., -initWithCoder:, -initWithFrame:, etc., which then means you
need to come up with a bunch of other macros to handle those cases or you're
special-casing [super init].
Choosing to do an early return or not is up to you. Personally I prefer the "if
(self != nil) {...}" case, even if the method is long so that I can see
structure. To say more risks getting into a "religious" discussion that nobody
wins. :)
> On May 30, 2015, at 3:20 PM, Alex Zavatone <z...@mac.com> wrote:
>
> Actually, i was typing by habit and included == instead of = by mistake.
>
> So, while you answered the question, you may have answered the wrong question.
>
> The question is not for
>
> if ( self == [super init])
>
> It's
>
> if ( self = [super init])
>
> How does that change your answer?
>
> On May 30, 2015, at 6:08 PM, Michael David Crawford wrote:
>
>> While in principle machine code implementations of subroutines can
>> return from several different places, in practice they don't. Rather
>> the compiler's code generator emits a branch instruction to the end of
>> the subroutine, there there is an "epilog".
>>
>> There are many good reasons for returning from the middle in certain
>> specific cases; what if the only epilog you need is an "rts"?
>> Branching to the epilog could cause a cache miss.
>>
>> I expect the compiler developers know all about this but don't
>> typically avail themselves of it because writing compilers is
>> difficult.
>>
>> To be clear, the following source code:
>>
>> - (id) init
>> {
>> if ( self == [super init] ) return nil;
>>
>> // lots of code goes here
>>
>> return self;
>> }
>>
>> ... is implemented as something like this, but in machine code:
>>
>> - (id) init
>> {
>> id result;
>> if ( self == [super init] ){
>> result = nil;
>> goto epilog;
>> }
>>
>> // lots of code goes here
>> result = self;
>>
>> epilog:
>> return result;
>> }
>> Michael David Crawford, Consulting Software Engineer
>> mdcrawf...@gmail.com
>> https://urldefense.proofpoint.com/v2/url?u=http-3A__www.warplife.com_mdc_&d=BQIGaQ&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=ie7S-J__EKnfyVOBV7-jV2rZ--p47O6vkyTklpDM3h4&m=Z8vcdjmsMKlbvM2wCAfNohcqAEcoUBSmr2LdV39oZp8&s=0LcsucVzgw_rFosbryao4sTH3L4wj7OecTlM6LeQ3gU&e=
>>
>> Available for Software Development in the Portland, Oregon Metropolitan
>> Area.
>>
>>
>> On Fri, May 29, 2015 at 6:25 PM, Graham Cox <graham....@bigpond.com> wrote:
>>>
>>>> On 30 May 2015, at 3:22 am, Alex Zavatone <z...@mac.com> wrote:
>>>>
>>>> // We don't care if this gets long.
>>>
>>>
>>> My take is that you're rewriting a well-recognised idiom to solve a
>>> non-existent problem.
>>>
>>> The well-recognised idiom makes it easy to verify it's correct. Hiding a
>>> different construct inside a macro obscures that, making it harder to
>>> verify the code. It's not "wrong" exactly, just harder to see at a glance
>>> that it's right.
>>>
>>> The non-existent problem you're trying to solve is that the gap between a
>>> pair of braces could get large. So what? Early returns can be another
>>> source of bugs, so structural purists would tell you that you shouldn't do
>>> that. Sometimes I think it's justified, but not usually worthwhile. Another
>>> religious issue is whether matching braces should line up or not.
>>> Personally I prefer that they do, at the cost of an extra line. Because you
>>> aren't doing that, your long distance between braces is bothering you,
>>> because you're losing track of where it started (I assume that's why it's
>>> bothering you). If you line up the braces that is much less of an issue.
>>>
>>> Source code is for humans, so it should be as readable as you can possibly
>>> make it. Macros often hinder that. Unaligned braces hinder that. Multiple
>>> statements per line hinder that.
>>>
>>> Factoring code helps, so I'd suggest that's the better way to solve this.
>>> (and it's also beneficial when it comes to making sure that -initWithCoder:
>>> and other initializers that don't correctly follow the designated
>>> initializer rule can get access to your common initialization. While this
>>> is rarely a problem, I did notice that the recent change to encourage the
>>> use of -initWithCoder: for unpacking NSViews from a nib breaks this
>>> long-standing rule and so a common init method that both can call is a
>>> simple workaround).
>>>
>>> --Graham
>>>
>>>
>>>
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>>
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