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

[Howard Hinnant]

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)?

Howard