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<p>Makes sense. Here's hoping parameter deduction for constructors
makes it in!</p>
<p>(better link)
<a class="moz-txt-link-freetext" href="http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0091r2.html">http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0091r2.html</a><br>
</p>
<br>
<div class="moz-cite-prefix">On 6/15/2016 1:54 PM, Eric Fiselier
wrote:<br>
</div>
<blockquote
cite="mid:CAB=TDAUiOuB8qXGYtb253EJ-f6rDgSqEmxHjk6WfBbKVrsL2Uw@mail.gmail.com"
type="cite">
<div dir="ltr"><br>
<div>I've had a change of heart. I think that lock_guard<>
has some utility in generic code, and I'm not sure removing it
is a good idea. For example a function like:
<div><br>
</div>
<div>template <class Func, class ...Locks></div>
<div>void ExecuteUnderLocks(Func&& fn, Locks&...
locks) {</div>
<div> lock_guard<Locks...> g(locks...);</div>
<div> fn();</div>
<div>}</div>
<div><br>
</div>
<div>I checked the proposal and it's clear that
"lock_guard<>" is expected to compile and be default
constructable. For this reason I'm not going to remove
"lock_guard<>", at least not without further
discussion.</div>
<div> </div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Wed, Jun 15, 2016 at 12:47 PM,
Craig, Ben <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:ben.craig@codeaurora.org" target="_blank">ben.craig@codeaurora.org</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"><span class=""> On
6/15/2016 1:15 PM, Eric Fiselier wrote:<br>
<blockquote type="cite">
<div dir="ltr">On Wed, Jun 15, 2016 at 11:45 AM,
Craig, Ben via cfe-commits <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:cfe-commits@lists.llvm.org"
target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:cfe-commits@lists.llvm.org">cfe-commits@lists.llvm.org</a></a>></span>
wrote:<br>
<div class="gmail_extra">
<div class="gmail_quote">
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px
#ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<p>Does this change (and the paper) permit
declarations like the following?</p>
<p> lock_guard<> guard();</p>
<p>If that syntax is allowed, then this is
also likely allowed...</p>
<p> lock_guard<>(guard);</p>
<p>I would really like the prior two
examples to not compile. Here is a
common bug that I see in the wild...</p>
<p>
unique_guard<mutex>(some_member_mutex);</p>
<p>That defines a new, default constructed
unique_guard named "some_member_mutex",
that likely shadows the member variable
some_member_mutex. It is almost never
what users want.<br>
</p>
</div>
</blockquote>
<div><br>
</div>
<div>I had no idea that syntax did that. I
would have assumed it created an unnamed
temporary. I can see how that would cause
bugs.</div>
</div>
</div>
</div>
</blockquote>
</span> It's also strong rationale for deduced
constructor templates. (<a moz-do-not-send="true"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0091r0.html"
target="_blank">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0091r0.html</a>)<br>
auto guard = unique_guard(some_member_mutex); <br>
You don't need to repeat types there, and it's very
difficult to forget to name the guard variable.<span
class=""><br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div class="gmail_extra">
<div class="gmail_quote">
<div> </div>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px
#ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<p> </p>
<p>Is it possible to have the empty
template remain undefined, and let the
one element lock_guard be the base case
of the recursion? Does that help any
with the mangling?<br>
</p>
</div>
</blockquote>
<div>Nothing in the spec says the empty
template should be undefined. The default
constructor on the empty template is
technically implementing
"lock_guard(MutexTypes...)" for an empty
pack.</div>
<div>However your example provides ample
motivation to make it undefined. I'll go
ahead and make that change and I'll file a
LWG defect to change the standard.</div>
<div><br>
</div>
<div>There is actually no recursion in the
variadic lock_guard implementation, so the
change is trivial.</div>
<div><br>
</div>
<div>As for mangling I'm not sure what you
mean? It definitely doesn't change the fact
that this change is ABI breaking. (Note this
change is not enabled by default for that
reason).</div>
</div>
</div>
</div>
</blockquote>
</span> My thought regarding the mangling was that you
could still provide a one argument lock_guard, as well
as a variadic lock_guard. The one argument lock_guard
would have the same mangling as before. I think some of
your other comments have convinced me that that won't
work, as I think the variadic lock_guard has to be made
the primary template, and I think the primary template
dictates the mangling.<br>
</div>
</blockquote>
<div><br>
</div>
<div>Exactly.</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"> <br>
I'm also going to guess that throwing inline namespaces
at the problem won't help, as that would probably cause
compile-time ambiguity.<br>
<br>
If I'm not mistaken, this only breaks ABI for those
foolish enough to pass a lock_guard reference or pointer
as a parameter across a libcxx version boundary. Does
that sound accurate?</div>
</blockquote>
<div><br>
</div>
<div>It breaks the ABI any time "lock_guard<Mutex>"
participates in the mangling of some function or type. In
addition to your example this will also break any time
"lock_guard<Mutex>" is used as a template parameter:
ie</div>
<div><br>
</div>
<div>using T = MyType<lock_guard<Mutex>>;</div>
<div>MyFunction<lock_guard<Mutex>>();</div>
<div><br>
</div>
<div>The two different implementations are still layout
compatible, so if mangling were not an issue I think this
change would have been safe.</div>
<div><br>
</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"> <span class="">
<pre cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
</pre>
</span></div>
</blockquote>
</div>
<br>
</div>
</div>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
</pre>
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