[cfe-dev] C++11 error about initializing explicit constructor with {} ?

[James Dennett]

On Fri, Jan 18, 2013 at 1:49 AM, Csaba Raduly <rcsaba at gmail.com> wrote:
> Hi Benjamin,
>
> On Thu, Jan 17, 2013 at 10:52 PM, Benjamin Redelings
> <benjamin.redelings at duke.edu> wrote:
>> Hi,
>>
>>     I'm trying to compile some of my C++11 code under clang-3.2; I
>> originally wrote it under g++.  This is a case where g++-4.8 (snapshot)
>> accepts my code, but clang-3.2 does not.  The error message is:
>>
>> In file included from ../../../master/src/mcmc/moves.C:20:
>> In file included from ../../../master/src/mcmc/sample.H:26:
>> ../../../master/src/mcmc/mcmc.H:390:94: error: chosen constructor is
>> explicit in copy-initialization
>>     std::set<int> get_affected_parameters(const
>> owned_ptr<Probability_Model>&) const {return {};}
>> ^~
>> /usr/include/c++/v1/set:378:14: note: constructor declared here
>>     explicit set(const value_compare& __comp = value_compare())
>>
>>
>> So, basically, this code is trying to initialize a std::set<int> with {}.
>> I'm not sure clang is right here, but I'm also not sure is wrong.  Can
>> someone tell me, so I can possibly submit a bug report?
>
> Maybe it's already fixed. The following code:
>
> #include <set>
> std::set<int> getset() { return {}; }
>
> is compiled without errors by
>
> $ clang++ -v --std=c++0x setinit.cc
> clang version 3.3 (trunk 172707)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix

You may be compiling against libstdc++, which has a C++11-compliance
bug in that its default constructor for set is not explicit (while the
standard specifies that it should be), which would make this compile
when it should not.

(On the other hand, I'm not sure that the standard _meant_ to specify
that, or if it was just an accident.  Turning {} into an empty set
seems like a rather pretty thing.)

-- James