Re: [boost] [smart_ptr] make_shared usage

From: Daniel Frey (d.frey_at_[hidden])
Date: 2008-07-28 04:12:02

On Sun, 2008-07-27 at 18:25 +0300, Peter Dimov wrote:
> You should be able to make it work by making the appropriate make_shared 
> overload a friend:
> 
>  template< class T, class A1, class A2 >
>  friend
>  boost::shared_ptr< T > boost::make_shared( A1 const & a1, A2 const & a2 );

Good idea, thanks. It would also render create() useless, since the user
could call make_shared directly. Sadly, it doesn't work for me. I've
tried it with GCC 4.3:

#include <memory>

class A
{
private:
  A() {}

public:
  static std::shared_ptr< A > create()
  { return std::make_shared< A >(); }

  template< typename... Args >
  friend std::shared_ptr< A > std::make_shared( Args... );
};

int main()
{
  A::create();
}

which fails, because the implementation forwards the ctor call into some
nested functions. Here's the (terrible to read) output:

frey_at_fiasko:~/work/test/make_shared_2$ g++ -std=c++0x -pedantic -O3 t.cc
-o t && ./t
t.cc: In constructor ‘std::_Sp_counted_ptr_inplace<_Tp, _Alloc,
_Lp>::_Sp_counted_ptr_inplace(_Alloc) [with _Tp = A, _Alloc =
std::allocator<A>, __gnu_cxx::_Lock_policy _Lp = _S_mutex]’:
/home/frey/work/gcc-4.3.1-install/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../include/c++/4.3.1/bits/boost_sp_shared_count.h:293:   instantiated from ‘std::__shared_count<_Lp>::__shared_count(std::_Sp_make_shared_tag, _Tp*, _Alloc, _Args&& ...) [with _Tp = A, _Alloc = std::allocator<A>, _Args = , __gnu_cxx::_Lock_policy _Lp = _S_mutex]’
/home/frey/work/gcc-4.3.1-install/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../include/c++/4.3.1/tr1_impl/boost_shared_ptr.h:509:   instantiated from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_make_shared_tag, _Alloc, _Args&& ...) [with _Alloc = std::allocator<A>, _Args = , _Tp = A, __gnu_cxx::_Lock_policy _Lp = _S_mutex]’
/home/frey/work/gcc-4.3.1-install/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../include/c++/4.3.1/tr1_impl/boost_shared_ptr.h:926:   instantiated from ‘std::shared_ptr<_Tp>::shared_ptr(std::_Sp_make_shared_tag, _Alloc, _Args&& ...) [with _Alloc = std::allocator<A>, _Args = , _Tp = A]’
/home/frey/work/gcc-4.3.1-install/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../include/c++/4.3.1/tr1_impl/boost_shared_ptr.h:1104:   instantiated from ‘std::shared_ptr<_Tp> std::allocate_shared(_Alloc, _Args&& ...) [with _Tp = A, _Alloc = std::allocator<A>, _Args = ]’
/home/frey/work/gcc-4.3.1-install/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../include/c++/4.3.1/tr1_impl/boost_shared_ptr.h:1119:   instantiated from ‘std::shared_ptr<_Tp> std::make_shared(_Args&& ...) [with _Tp = A, _Args = ]’
t.cc:9:   instantiated from here
t.cc:6: error: ‘A::A()’ is private
/home/frey/work/gcc-4.3.1-install/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../include/c++/4.3.1/bits/boost_sp_shared_count.h:177: error: within this context
frey_at_fiasko:~/work/test/make_shared_2$ 

Is this a bug in GCC's implementation of make_shared and I should file a
bug report for it? Is it intended to standardize that the above code
should work?

Regards, Daniel

BTW: I just filed another bug for GCC's make_shared,
<http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36949>, but there is IMHO
nothing unclear about that... :)