[Dune] 'auto&&' or 'const auto&' ?

[Bernd Flemisch]

Hi Steffen,

I just tried at one occasion with 'auto&&' and gcc-5. It worked and 
didn't trigger the error, although it was inside a function template. 
Does that mean that it will also work for other users/compiler versions 
or when I make changes in the future?

Kind regards
Bernd

On 10/12/2015 06:03 PM, Steffen Müthing wrote:
> Hi Atgeirr,
>
>> Am 12.10.2015 um 17:16 schrieb Atgeirr Rasmussen <Atgeirr.Rasmussen at sintef.no>:
>>
>> Hi Dune,
>>
>> I am curious about why you require auto&& (starting with Dune 2.4 according to the web site)?
>> Are the iterators using proxy objects?
> It’s not actually proxy objects, but some grids (typically structured ones like YaspGrid) might just create
> a temporary entity every time they are dereferenced. There were some discussions about this topic
> (whether to have grid iterators always return a temporary or always return a const ref to an internally
> cached entity). In the end we decided to allow for both (one is good for unstructured grids, one is good for
> structured grids).
>
>> For general range-for loops I am not sure how I could get const-ness of these (const auto&& would
>> not be a universal const-reference would it)? I usually prefer to make my objects const if possible
>> (to prevent myself from making errors), but the auto&& will enable me to modify the range, unless
>> the range itself is const.
> You’re right, const auto&& is something else. auto&& always deduces to the right thing: It creates a variable
> of type const Entity& if the iterator returns a const ref, and a variable of type Entity if the returned value is
> a temporary (that variable will typically directly be initialized inside the operator* due to RVO and copy elision).
>
> As I said, in our case we are fine w.r.t. to constness, as we either require the iterator to return a const ref or
> a temporary. Using const auto& has some subtle performance implications in certain cases, see the discussion
> in FS1511 for all the gory details…
>
> Steffen
>
>> Atgeirr
>>
>>
>>
>>> 12. okt. 2015 kl. 16.22 skrev Steffen Müthing <steffen.muething at iwr.uni-heidelberg.de>:
>>>
>>> Hi Bernd,
>>>
>>>> Am 12.10.2015 um 15:50 schrieb Bernd Flemisch <bernd at iws.uni-stuttgart.de>:
>>>>
>>>> Dear Dune,
>>>>
>>>> should I use 'auto&&' or 'const auto&' in a range-based for-loop over the entities of a Dune GridView?
>>>>
>>>> On the one hand, there is
>>>> https://www.dune-project.org/doc/doxygen/html/group__GIIteration.html#_details
>>>> which says "**always** use 'auto&&'"
>>>>
>>>> On the other hand, there is
>>>> http://conan2.iwr.uni-heidelberg.de/git/pdelab/dune-pdelab/commit/d97c563807cd4f9aaa872dfee57c668eda12db59
>>>> together with
>>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63506
>>>> which indicates that a GCC bug is triggered when using 'auto&&‘.
>>> yes, it’s all quite a mess: Basically, the standard (and important people like Stroustrup and Sutter) say that you
>>> should *always* use auto&& in a range-based for loop, but GCC has this bug that sometimes causes a compilation
>>> failure if the range-based for appears inside a function template, so you have to use const auto&.
>>>
>>> So, the (rather unfortunate) advice is: You should use auto&&, but you can’t if you target current GCC due to a bug in
>>> GCC. In that case, you have to use const auto&.
>>>
>>> We should probably update the Doxygen documentation to explain the issue (I forgot about that when I discovered the compiler
>>> b) because we’ll be stuck with the problem for the foreseeable future - the bug is still present in the GCC 6 snapshot from
>>> the beginning of September that I have available on my machine, and the GCC Bugzilla bug hasn’t seen any attention in a long
>>> time. Apart from that, the bug is in GCC 5, which we will have to support for a few years to come…
>>>
>>> So I think the documentation needs to explain this issue.
>>>
>>> Steffen
>>>
>>>> Kind regards
>>>> Bernd
>>>>
>>>> --
>>>> _______________________________________________________________
>>>>
>>>> Bernd Flemisch                         phone: +49 711 685 69162
>>>> IWS, Universität Stuttgart             fax:   +49 711 685 60430
>>>> Pfaffenwaldring 61            email:bernd at iws.uni-stuttgart.de
>>>> D-70569 Stuttgart            url:www.hydrosys.uni-stuttgart.de
>>>> _______________________________________________________________
>>>>
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-- 
_______________________________________________________________

Bernd Flemisch                         phone: +49 711 685 69162
IWS, Universität Stuttgart             fax:   +49 711 685 60430
Pfaffenwaldring 61            email: bernd at iws.uni-stuttgart.de
D-70569 Stuttgart            url: www.hydrosys.uni-stuttgart.de
_______________________________________________________________

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