c++ - Clang, std::shared_ptr and std::less/operator<

Question

Having the following code

#include <memory>

int main() {
    std::shared_ptr<int> ptr0( new int );
    std::shared_ptr<int> ptr1( new int );

    bool result = ptr0 < ptr1;
}

produces the following error when being compiled with clang (version 3.1, LLVM 3.1, Debian GNU/Linux Sid)

/usr/bin/../lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/shared_ptr.h:364:14: error: no matching function for call to object of type 'std::less<_CT>'
      return std::less<_CT>()(__a.get(), __b.get());
             ^~~~~~~~~~~~~~~~
foo.cpp:9:21: note: in instantiation of function template specialization 'std::operator<<int, int>' requested here
        bool result = ptr0 < ptr1;
                           ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/stl_function.h:236:7: note: candidate function not viable: no known conversion from 'int *' to 'int *&&&' for
      1st argument;
      operator()(const _Tp& __x, const _Tp& __y) const
      ^

Compiling the same code with GCC (version 4.7.0) doesn't throw any error messages. Is there a reason why operator<() doesn't work for shared pointers in clang?

Answer 1

clang++ and libstdc++ doesn't match perfectly yet. You could do one of the followings:

• Switch to libc++ (by using clang++ -stdlib=libc++ -std=c++11 ...)

• Apply the following patch to /usr/include/c++/4.7.0/type_traits (as documented in http://clang.llvm.org/cxx_status.html):

  Index: include/std/type_traits
  ===================================================================
  --- include/std/type_traits (revision 185724)
  +++ include/std/type_traits (working copy)
  @@ -1746,7 +1746,7 @@
  
     template<typename _Tp, typename _Up>
       struct common_type<_Tp, _Up>
  -    { typedef decltype(true ? declval<_Tp>() : declval<_Up>()) type; };
  +    { typedef typename decay<decltype(true ? declval<_Tp>() : declval<_Up>())>::type type; };
  
     template<typename _Tp, typename _Up, typename... _Vp>
       struct common_type<_Tp, _Up, _Vp...>
  

If you check bits/shared_ptr.h you did find a std::common_type, and the clang developers claim that it's actually a bug of libstdc++, although I don't believe a bug of libstdc++ alone would cause the non-existent type int*&&& to appear.