On 6/15/2016 1:15 PM, Eric Fiselier wrote:
On Wed, Jun 15, 2016 at 11:45 AM, Craig, Ben via cfe-commits
<cfe-commits@lists.llvm.org <mailto:cfe-commits@lists.llvm.org>> wrote:
Does this change (and the paper) permit declarations like the
following?
lock_guard<> guard();
If that syntax is allowed, then this is also likely allowed...
lock_guard<>(guard);
I would really like the prior two examples to not compile. Here
is a common bug that I see in the wild...
unique_guard<mutex>(some_member_mutex);
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.
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.
It's also strong rationale for deduced constructor templates.
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0091r0.html)
auto guard = unique_guard(some_member_mutex);
You don't need to repeat types there, and it's very difficult to forget
to name the guard variable.
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?
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.
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.
There is actually no recursion in the variadic lock_guard
implementation, so the change is trivial.
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).
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.
I'm also going to guess that throwing inline namespaces at the problem
won't help, as that would probably cause compile-time ambiguity.
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?
--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project
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