220 5855 <CA+cyFgvGeOJQSfXAQg3z2TKXvFY-OYvhp6_EGMx_mxoskys7LQ@mail.gmail.com> article
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From: Geoffrey Romer <gromer@google.com>
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Subject: Re: Remove vector<bool>?
Date: Mon, 26 Aug 2013 17:16:55 -0700
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On Mon, Aug 26, 2013 at 4:22 PM, Howard Hinnant <howard.hinnant@gmail.com>wrote:

>
> On Aug 26, 2013, at 7:18 PM, Zhihao Yuan <zy@miator.net> wrote:
>
> > So you mean it's Okey for bitvector's begin() to produce an
> > object which can be used in any STL algorithms but such an object
> > fits into no iterator categories?  Don't you think that's an anti-pattern
> > of STL design?
>
> Question for Zhihao:
>
> Why does forward iterator's operator*() have to return a value_type& ?


> I'm well aware that this is what C++98/03/11 requires.  That's not what
> I'm asking.  I'm asking what is the benefit of this requirement in the
> C++1y context?  What problem does this requirement solve?
>


I'm not Zhihao, but...

One problem this solves is that code like this is guaranteed to work:

template <typename iter_t>
value_type& GetValue(iter_t iter) {
  return *iter;
}

If operator* can return a proxy with an implicit conversion to value_type,
GetValue is at serious risk of undefined behavior. I've seen multiple real
bugs in the wild arising from this problem, and they are truly heinous to
diagnose (they've led to a concerted effort to purge Boost.ptr_container, a
repeat offender in this respect, from our codebase).

This is not a knock-down objection, but it's an important cost to bear in
mind.


> Howard
>
> --
>
> ---
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>

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<div dir=3D"ltr"><br><div class=3D"gmail_extra"><div class=3D"gmail_quote">=
On Mon, Aug 26, 2013 at 4:22 PM, Howard Hinnant <span dir=3D"ltr">&lt;<a hr=
ef=3D"mailto:howard.hinnant@gmail.com" target=3D"_blank">howard.hinnant@gma=
il.com</a>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-=
left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;p=
adding-left:1ex"><div class=3D"im"><br>
On Aug 26, 2013, at 7:18 PM, Zhihao Yuan &lt;<a href=3D"mailto:zy@miator.ne=
t">zy@miator.net</a>&gt; wrote:<br>
<br>
&gt; So you mean it&#39;s Okey for bitvector&#39;s begin() to produce an<br=
>
&gt; object which can be used in any STL algorithms but such an object<br>
&gt; fits into no iterator categories? =A0Don&#39;t you think that&#39;s an=
 anti-pattern<br>
&gt; of STL design?<br>
<br>
</div>Question for Zhihao:<br>
<br>
Why does forward iterator&#39;s operator*() have to return a value_type&amp=
; ?</blockquote><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0=
px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-le=
ft-style:solid;padding-left:1ex">

<br>
I&#39;m well aware that this is what C++98/03/11 requires. =A0That&#39;s no=
t what I&#39;m asking. =A0I&#39;m asking what is the benefit of this requir=
ement in the C++1y context? =A0What problem does this requirement solve?<br=
>
</blockquote><div><br></div><div><div><br class=3D"">I&#39;m not=A0Zhihao, =
but...</div><div><br></div><div>One problem this solves is that code like t=
his is guaranteed to work:</div><div><br></div><div>template &lt;typename i=
ter_t&gt;</div>
<div>value_type&amp; GetValue(iter_t iter) {</div><div>=A0 return *iter;</d=
iv><div>}</div><div><br></div><div>If operator* can return a proxy with an =
implicit conversion to value_type, GetValue is at serious risk of undefined=
 behavior. I&#39;ve seen multiple real bugs in the wild arising from this p=
roblem, and they are truly heinous to diagnose (they&#39;ve led to a concer=
ted effort to purge Boost.ptr_container, a repeat offender in this respect,=
 from our codebase).</div>
<div><br></div><div>This is not a knock-down objection, but it&#39;s an imp=
ortant cost to bear in mind.</div><div><br></div></div><blockquote class=3D=
"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left-width:1px;borde=
r-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">

<span class=3D""><font color=3D"#888888"><br>
Howard<br>
</font></span><div class=3D""><div class=3D"h5"><br>
--<br>
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std-proposals/" target=3D"_blank">http://groups.google.com/a/isocpp.org/gro=
up/std-proposals/</a>.<br>
</div></div></blockquote></div><br></div></div>

<p></p>

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