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

[David Blaikie]

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.