Topic: storage class specifier of range-based for's
Author: Richard Smith <richard@metafoo.co.uk>
Date: Mon, 10 Oct 2016 13:36:08 -0700
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On Mon, Oct 10, 2016 at 12:16 PM, <szollosi.lorand@gmail.com> wrote:
> Hi,
>
> Range-based for declares a range and two iterator variables as auto&& and
> auto, respectively. This is unexpected if you declare your loop variable
> static or thread_local.
>
But you can't. [stmt.ranged]/2: "In the decl-specifier-seq of a
for-range-declaration, each decl-specifier shall be either a type-specifier
or constexpr."
It would also be a really strange thing to want to do: the value of the
loop variable would only be initialized by the first iteration of the loop,
the first time the loop is reached (per-thread or per-process).
> This also means that a stackless coroutine cannot yield from and return to
> the middle of the loop - which would be the basic use case for generators.
>
Any reasonable stackless coroutine proposal is able to persist the values
of automatic storage duration variables between suspension and resumption.
That is rather the point of the feature.
> I'd suggest to have the range and the two iterator variables have
> identical storage class specifier as the loop variable (i.e., range
> declaration). This would allow one to switch() / goto into the body of the
> loop.
>
This would still mean that each range-for statement only gets one range for
the entire process/thread.
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<div dir=3D"ltr">On Mon, Oct 10, 2016 at 12:16 PM, <span dir=3D"ltr"><<=
a href=3D"mailto:szollosi.lorand@gmail.com" target=3D"_blank">szollosi.lora=
nd@gmail.com</a>></span> wrote:<div class=3D"gmail_extra"><div class=3D"=
gmail_quote"><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px =
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"=
ltr">Hi,<br><br>Range-based for declares a range and two iterator variables=
as auto&& and auto, respectively. This is unexpected if you declar=
e your loop variable static or thread_local.</div></blockquote><div><br></d=
iv><div>But you can't. [stmt.ranged]/2: "In the decl-specifier-seq=
of a for-range-declaration, each decl-specifier shall be either a type-spe=
cifier or constexpr."</div><div><br></div><div>It would also be a real=
ly strange thing to want to do: the value of the loop variable would only b=
e initialized by the first iteration of the loop, the first time the loop i=
s reached (per-thread or per-process).</div><div>=C2=A0</div><blockquote cl=
ass=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid=
rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">This also means that a=
stackless coroutine cannot yield from and return to the middle of the loop=
- which would be the basic use case for generators.<br></div></blockquote>=
<div><br></div><div>Any reasonable stackless coroutine proposal is able to =
persist the values of automatic storage duration variables between suspensi=
on and resumption. That is rather the point of the feature.</div><div>=C2=
=A0</div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8e=
x;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr"=
>I'd suggest to have the range and the two iterator variables have iden=
tical storage class specifier as the loop variable (i.e., range declaration=
). This would allow one to switch() / goto into the body of the loop.</div>=
</blockquote><div><br></div><div>This would still mean that each range-for =
statement only gets one range for the entire process/thread.</div></div></d=
iv></div>
<p></p>
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Author: szollosi.lorand@gmail.com
Date: Mon, 10 Oct 2016 15:09:10 -0700 (PDT)
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Hi,
2016. okt=C3=B3ber 10., h=C3=A9tf=C5=91 22:36:14 UTC+2 id=C5=91pontban Rich=
ard Smith a=20
k=C3=B6vetkez=C5=91t =C3=ADrta:
>
> On Mon, Oct 10, 2016 at 12:16 PM, <szollos...@gmail.com <javascript:>>=20
> wrote:
>
>> Hi,
>>
>> Range-based for declares a range and two iterator variables as auto&& an=
d=20
>> auto, respectively. This is unexpected if you declare your loop variable=
=20
>> static or thread_local.
>>
>
> But you can't. [stmt.ranged]/2: "In the decl-specifier-seq of a=20
> for-range-declaration, each decl-specifier shall be either a type-specifi=
er=20
> or constexpr."=20
>
It would also be a really strange thing to want to do: the value of the=20
> loop variable would only be initialized by the first iteration of the loo=
p,=20
> the first time the loop is reached (per-thread or per-process).
>
Yep, that I've missed. A reasonable meaning would be, of course, to declare=
=20
the variable and then reassign (or rather destruct & placement new into)=20
it. Nevertheless, it's not an issue then, sorry for bothering here - it's=
=20
an issue with gcc then (which accepts it and does what you've explained).
This also means that a stackless coroutine cannot yield from and return to=
=20
>> the middle of the loop - which would be the basic use case for generator=
s.
>>
>
> Any reasonable stackless coroutine proposal is able to persist the values=
=20
> of automatic storage duration variables between suspension and resumption=
..=20
> That is rather the point of the feature.=20
>
... but *until* we have that accepted, we use a switch (or switch - goto)=20
and member / static variables.
I'd suggest to have the range and the two iterator variables have identical=
=20
>> storage class specifier as the loop variable (i.e., range declaration).=
=20
>> This would allow one to switch() / goto into the body of the loop.
>>
>
> This would still mean that each range-for statement only gets one range=
=20
> for the entire process/thread.
>
If it's worth considering, one might say that the range gets declared and=
=20
assigned on the first entry; then destructed and placement new'ed into on=
=20
subsequent entries to the for itself. A goto / switch over the for()=20
statement is not considered 'entry'. I'm aware that this is slightly=20
different from what static means, but this can be emulated using a bool=20
flag.
Thanks,
-lorro
--=20
You received this message because you are subscribed to the Google Groups "=
ISO C++ Standard - Future Proposals" group.
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To post to this group, send email to std-proposals@isocpp.org.
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..org/d/msgid/std-proposals/704911f1-95fd-4b7f-8964-2efc2c782e70%40isocpp.or=
g.
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<div dir=3D"ltr">Hi,<br><br>2016. okt=C3=B3ber 10., h=C3=A9tf=C5=91 22:36:1=
4 UTC+2 id=C5=91pontban Richard Smith a k=C3=B6vetkez=C5=91t =C3=ADrta:<blo=
ckquote class=3D"gmail_quote" style=3D"margin: 0;margin-left: 0.8ex;border-=
left: 1px #ccc solid;padding-left: 1ex;"><div dir=3D"ltr">On Mon, Oct 10, 2=
016 at 12:16 PM, <span dir=3D"ltr"><<a href=3D"javascript:" target=3D"_=
blank" gdf-obfuscated-mailto=3D"YaxnX0jaAAAJ" rel=3D"nofollow" onmousedown=
=3D"this.href=3D'javascript:';return true;" onclick=3D"this.href=3D=
'javascript:';return true;">szollos...@gmail.com</a>></span> wro=
te:<div><div class=3D"gmail_quote"><blockquote class=3D"gmail_quote" style=
=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding=
-left:1ex"><div dir=3D"ltr">Hi,<br><br>Range-based for declares a range and=
two iterator variables as auto&& and auto, respectively. This is u=
nexpected if you declare your loop variable static or thread_local.</div></=
blockquote><div><br></div><div>But you can't. [stmt.ranged]/2: "In=
the decl-specifier-seq of a for-range-declaration, each decl-specifier sha=
ll be either a type-specifier or constexpr." <br></div></div></div></d=
iv></blockquote><blockquote class=3D"gmail_quote" style=3D"margin: 0;margin=
-left: 0.8ex;border-left: 1px #ccc solid;padding-left: 1ex;"><div dir=3D"lt=
r"><div><div class=3D"gmail_quote"><div>It would also be a really strange t=
hing to want to do: the value of the loop variable would only be initialize=
d by the first iteration of the loop, the first time the loop is reached (p=
er-thread or per-process).</div></div></div></div></blockquote><div>Yep, th=
at I've missed. A reasonable meaning would be, of course, to declare th=
e variable and then reassign (or rather destruct & placement new into) =
it. Nevertheless, it's not an issue then, sorry for bothering here - it=
's an issue with gcc then (which accepts it and does what you've ex=
plained).<br><br></div><blockquote class=3D"gmail_quote" style=3D"margin: 0=
;margin-left: 0.8ex;border-left: 1px #ccc solid;padding-left: 1ex;"><div di=
r=3D"ltr"><div><div class=3D"gmail_quote"><blockquote class=3D"gmail_quote"=
style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);p=
adding-left:1ex"><div dir=3D"ltr">This also means that a stackless coroutin=
e cannot yield from and return to the middle of the loop - which would be t=
he basic use case for generators.<br></div></blockquote><div><br></div><div=
>Any reasonable stackless coroutine proposal is able to persist the values =
of automatic storage duration variables between suspension and resumption. =
That is rather the point of the feature. <br></div></div></div></div></bloc=
kquote><div>.. but <i>until</i> we have that accepted, we use a switch (or =
switch - goto) and member / static variables.<br><br></div><blockquote clas=
s=3D"gmail_quote" style=3D"margin: 0;margin-left: 0.8ex;border-left: 1px #c=
cc solid;padding-left: 1ex;"><div dir=3D"ltr"><div><div class=3D"gmail_quot=
e"><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;bord=
er-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">I'=
;d suggest to have the range and the two iterator variables have identical =
storage class specifier as the loop variable (i.e., range declaration). Thi=
s would allow one to switch() / goto into the body of the loop.</div></bloc=
kquote><div><br></div><div>This would still mean that each range-for statem=
ent only gets one range for the entire process/thread.</div></div></div></d=
iv></blockquote><div>If it's worth considering, one might say that the =
range gets declared and assigned on the first entry; then destructed and pl=
acement new'ed into on subsequent entries to the for itself. A goto / s=
witch over the for() statement is not considered 'entry'. I'm a=
ware that this is slightly different from what static means, but this can b=
e emulated using a bool flag.<br><br>Thanks,<br>-lorro<br></div></div>
<p></p>
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