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

Subject: Re: [boost] [Fit] formal review starts today
From: Paul Fultz II (pfultz2_at_[hidden])
Date: 2016-03-04 12:26:16

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.
 
Paul