[RFC] Relax the rules on const auto&? (was Re: r203179 - [C++11] Replacing iterators redecls_begin() and redecls_end() ...)

[Aaron Ballman]

On Fri, Mar 7, 2014 at 12:15 PM, David Blaikie <dblaikie at gmail.com> wrote:
> On Fri, Mar 7, 2014 at 8:58 AM, Duncan P. N. Exon Smith
> <dexonsmith at apple.com> wrote:
>> +llvmdev
>>
>> On Mar 7, 2014, at 5:15 AM, Tobias Grosser <tobias at grosser.es> wrote:
>>
>>> On 03/07/2014 01:40 PM, Aaron Ballman wrote:
>>>> On Thu, Mar 6, 2014 at 7:12 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
>>>>> On Thu, Mar 6, 2014 at 7:10 PM, Tobias Grosser <tobias at grosser.es> wrote:
>>>>>> I wonder if you could use 'auto const &' in some of these cases?
>>>>>>
>>>>>> http://llvm.org/docs/CodingStandards.html#beware-unnecessary-copies-with-auto
>>>>>
>>>>> I'll look into it once I've gotten the build bots green again. Thank
>>>>> you for the reminder. :-)
>>>>
>>>> I looked into this a bit more, and I'm not certain there's any real
>>>> benefit. Everything returned from these containers is already a
>>>> pointer, so this would change them from const Decl * to const Decl *&,
>>>> which doesn't strike me as any better (or worse). So I am thinking I
>>>> will continue without the const auto & in these cases (and as I move
>>>> other pointer-based ranges forward), but thank you for the reminder!
>>>
>>> I follow your reasoning. It seems the coding standards are formulated a little to strictly. Duncan, would it make sense to put an exception
>>> for cases like this into the standard. It seems to be a very common case in fact.
>>
>> Unless the pointers themselves are changing, the only benefit here is
>> consistency.  But ranges of iterators should use const&, since many
>> iterators are expensive to copy.
>
> To clarify, this isn't about copying iterators (iterators are
> generally cheap to copy) but copy the elements being iterated over.
>
>> Accidental copies can be a huge headache, since they lead to subtle
>> bugs that are hard to track down (performance implications aside).
>>
>> I still favour applying the rule generally, since consistency matters.
>> But we could text like:
>>
>>     Pointer ranges are a common exception.
>>
>> Any other opinions?
>
> If you write the exception as "auto *x" then it's unambiguous and easy
> to still look at "auto x" as being questionable/to-be-avoided.

Thanks for the clarity on this -- I'll start switching over to using *
instead of just auto as I range-ify things.

~Aaron