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

[Michael van der Westhuizen]

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?

Thanks,
Michael