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Subject: Re: [boost] [Fit] formal review starts today
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2016-03-04 13:59:03

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:
>> On 03/03/2016 05:43 PM, Louis Dionne wrote:
>>> Steven Watanabe <watanabesj <at>> 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?


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