[cfe-dev] Inheriting from std::error_category with libc++

[Michael van der Westhuizen]

On 26 Sep 2012, at 5:02 PM, Howard Hinnant <hhinnant at apple.com> wrote:

> On Sep 26, 2012, at 9:49 AM, Michael van der Westhuizen <r1mikey at gmail.com> wrote:
> 
>> Hi All,
>> 
>> I've come across what I believe is a problem in libc++.
>> 
>> While working through Chris Kohlhoff's blog entries about system_error (http://blog.think-async.com/2010/04/system-error-support-in-c0x-part-4.html) I discovered that it's not possible to inherit from std::error_category in libc++.
>> 
>> The following small test case compiles correctly under G++ 4.6.3 on Linux with libstdc++, but fails under Mac OS X Mountain Lion with Xcode 4.5 (Apple clang version 4.0 (tags/Apple/clang-421.0.60) (based on LLVM 3.1svn)).
>> 
>> #include <system_error>
>> #include <string>
>> 
>> namespace {
>> class test_category_impl
>>   : public std::error_category
>> {
>> public:
>>   virtual const char * name() const noexcept
>>   {
>>       return "test_category";
>>   }
>>   virtual std::string message(int ev) const
>>   {
>>       return "default";
>>   }
>> };
>> }
>> 
>> const std::error_category & test_category()
>> {
>>   static test_category_impl inst;
>>   return inst;
>> }
>> 
>> 
>> 
>> With G++ on Linux I'm compiling with:
>> g++ -c -std=c++0x -o bugtest.o bugtest.cpp
>> 
>> This works as expected.
>> 
>> With clang on the Mac I'm compiling with:
>> clang++ -c -std=c++11 -stdlib=libc++ -o bugtest.o bugtest.cpp
>> 
>> This gives the following error:
>> $  clang++ -c -std=c++11 -stdlib=libc++ -o bugtest.o bugtest.cpp
>> bugtest.cpp:22:31: error: call to implicitly-deleted default constructor of '<anonymous>::test_category_impl'
>>   static test_category_impl inst;
>>                             ^
>> bugtest.cpp:5:7: note: 'test_category_impl' defined here
>> class test_category_impl
>>     ^
>> 1 error generated.
>> 
>> All literature I've come across indicates that I should be able to inherit from error_category and the wording in the draft of N3242 states "Classes may be derived from error_category to support categories of errors in addition to those defined in this International Standard."  The libc++ implementation seems quite deliberate in its intent to not allow the user to inherit this class (it uses a private default constructor and a private friend class called __do_message internally within the library).
>> 
>> So have things changed? Are we no longer intended to be able to create error_category subclasses, or is this a bug?
> 
> This looks like a standards bug.  I filed a LWG issue on it here:
> 
> http://cplusplus.github.com/LWG/lwg-active.html#2145
> 
> but the lwg has not yet commented on the issue.  Generally my policy is to wait until the lwg addresses the issue before changing the libc++ implementation.  But in this case I'm tempted to go ahead and fix this in anticipation of the lwg accepting the issue as proposed.
> 
> Does anyone have any /objections/ to me pushing this fix in now (as proposed in http://cplusplus.github.com/LWG/lwg-active.html#2145)?

This looks like exactly what I'm experiencing.  As an aside, the libstdc++ folks have declared the default constructor as protected, which might be a better choice.

Michael

> Howard
>