Re: [boost] [Fit]Â formal review starts today

Subject: Re: [boost] [Fit] formal review starts today
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2016-03-04 14:05:21

Le 04/03/2016 19:59, Vicente J. Botet Escriba a écrit :
> Le 04/03/2016 18:26, Paul Fultz II a écrit :
>>
>> On Thursday, March 3, 2016 at 8:14:30 PM UTC-6, Steven Watanabe wrote:
>>> AMDG
>>>
>>> On 03/03/2016 05:43 PM, Louis Dionne wrote:
>>>> Steven Watanabe <watanabesj <at> gmail.com> writes:
>>>>
>>>>> On 03/03/2016 04:43 AM, Vicente J. Botet Escriba wrote:
>>>>> capture.hpp:
>>>>>
>>>>> 32: // Capture lvalues by reference and rvalues by value.
>>>>>      The default should be to capture everything by value.
>>>>>      Capturing by reference is potentially dangerous, and
>>>>>      switching based on whether the argument is an lvalue
>>>>>      is even more so.
>>>> I've implemented the same thing in Hana, so I had to make the same
>>> design
>>>> decision as Paul did at some point. I decided to be consistent with 
>>>> the
>>> rest
>>>> of Hana, which is to capture by value.
>>>>
>>>> However, I am not sure that my decision in Hana was a wise one, and 
>>>> I am
>>>> tempted to think that Paul's solution is actually the right one. 
>>>> Indeed,
>>>> in most cases, `fit::capture` will be used as a temporary:
>>>>
>>>>      some_alogrithm(..., fit::capture(variables...)(function));
>>>>
>>>> In this case, capturing by reference is harmless. <snip> Basically, my
>>> point is simply that
>>>> saying "you should always capture by value" is naive in this context.
>>>> Perhaps you're right, perhaps you're wrong, but in all cases this 
>>>> design
>>>> choice is not an obvious one, at least not to me. Just my .02.
>>>>
>>>    You have a point, and perhaps capturing by
>>> reference makes sense.  In fact, lambdas
>>> do capture by reference by default, so
>>> there is precedent for it.  I do maintain,
>>> however, that choosing whether to capture
>>> by value based on l/r-valueness is definitely
>>> wrong.
>>>
>> Why is that wrong? Furthermore the name was chosen not for it to be 
>> default
>> but based on the transformations it makes to the type categories.
>>
>> - capture_decay:
>>           Transform each category using decay
>>
>> - capture_forward:
>>           Transform each category to a reference. So that means 
>> rvalues become
>>      references.
>>
>> - capture:
>>
>>      Dont transform the type category. Lvalues are captured as 
>> lvalues and
>>      rvalues are captured as rvalues.
>>
>> Furthermore, capturing rvalues by rvalue instead of reference helps 
>> improve
>> safety, because rvalue references do not always extend the lifetime of a
>> temporary.
>>
> What is the rational to capture all the same way?
> Why not have something like std::ref to change the way?
>
>
If we consider that by default we capture by reference, can we capture a 
specific parameter by value?

Vicente