Is clang's c++11 support reliable?

问题:

I ran into an interesting issue when trying to mix clang (Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn, Target: x86_64-apple-darwin14.0.0), c++11 and CGAL (via MacPorts).

It seems that whether or not I call std::vector<>::reserve will determine whether my program will even compile.

I've trimmed down the problem into a minimal example (as minimal as CGAL examples get):

#include <vector>
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/AABB_tree.h>
#include <CGAL/AABB_traits.h>
#include <CGAL/AABB_triangle_primitive.h>

// CGAL::Epeck works fine, suggesting the problem is in CGAL::Epick
typedef CGAL::Epick Kernel;
typedef CGAL::Triangle_3<Kernel> Triangle_3; 
typedef typename std::vector<Triangle_3>::iterator Iterator;
typedef CGAL::AABB_triangle_primitive<Kernel, Iterator> Primitive;
typedef CGAL::AABB_traits<Kernel, Primitive> AABB_triangle_traits;
typedef CGAL::AABB_tree<AABB_triangle_traits> Tree;
typedef typename Tree::Point_and_primitive_id Point_and_primitive_id;
typedef CGAL::Point_3<Kernel>    Point_3;

template <typename BKernel>
void A()
{
  const CGAL::AABB_tree<
    CGAL::AABB_traits<BKernel, 
      CGAL::AABB_triangle_primitive<BKernel, 
        typename std::vector<CGAL::Triangle_3<BKernel> >::iterator
      >
    >
  > tree;
  Point_and_primitive_id pp = tree.closest_point_and_primitive(Point_3());
}

void B()
{
  std::vector<Triangle_3> T;
#ifdef MAGIC
  T.reserve(0);
#endif
  return A<Kernel>();
}

Issuing:

clang++ -std=c++11 -c example.cpp -I/opt/local/include

This fails to compile. Giving errors like:

    In file included from example.cpp:1:
    In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:265:
    In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__bit_reference:15:
    In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:626:
    In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/utility:157:
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__tuple:228:60: error: 
          no member named 'value' in 'std::__1::is_convertible<const CGAL::Point_3<CGAL::Epick> &,
          CGAL::Point_3<CGAL::Epick> >'
                                   is_convertible<_Tp0, _Up0>::value &&
                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__tuple:242:14: note: 
          in instantiation of template class 'std::__1::__tuple_convertible_imp<true, std::__1::__tuple_types<const
          CGAL::Point_3<CGAL::Epick> &, const CGAL::Vector_3<CGAL::Epick> &>,
          std::__1::__tuple_types<CGAL::Point_3<CGAL::Epick>, CGAL::Vector_3<CGAL::Epick> > >' requested here
        : public __tuple_convertible_imp<tuple_size<typename remove_reference<_Tp>::type>::value ==

However, this does compile if I make this magic call to std::vector::reserve, issuing:

clang++ -std=c++11 -c example.cpp -I/opt/local/include -DMAGIC

or by disabling c++11

clang++ -c example.cpp -I/opt/local/include

1. Is this a bug in CGAL or clang?
2. What explanation can there be for such erratic compiler behavior?
3. Is there a clean way of avoiding this (hopefully without really changing the templating or function prototype set up as I need the solution to fit my larger project).

回答1:

Since Apple's GCC is outdated (latest GPL v2 version from 2007, GCC 4.2.1) and not C++11 feature complete (hence the libstdc++ provided with it), you can install a more modern version of GCC through MacPorts (sudo port install gcc48 or sudo port install gcc49) and that will provide you a more modern version of libstdc++. I tested your code with:

/opt/local/bin/g++-mp-4.8 -std=c++11 -c example.cpp -I/opt/local/include

and it compiled succesfully.

If you prefer this solution and want a cleaner compiler call; you can set MacPorts' GCC as the default using gcc_select with the command (in my case for gcc48):

sudo port select --set gcc mp-gcc48

only once. Then, you can compile it with just

g++ -std=c++11 -c example.cpp -I/opt/local/include

in a new terminal session.