Boost C++ Libraries: Ticket #2857: change return type of the c_array() member function

niels_dekker — Sun, 15 Mar 2009 13:44:27 GMT

Discussion on this subject: "[boost][array] change proposal for the c_array() function" http://thread.gmane.org/gmane.comp.lib.boost.devel/187487 http://lists.boost.org/Archives/boost/2009/03/149711.php

niels_dekker — Wed, 18 Mar 2009 17:32:24 GMT

David Abrahams and I just mailed about the issue and we agreed that it would be preferable to have such a function as a non-member. Because tr1::array doesn't have a c_array() member function and std::array probably won't have one either. (See LWG issue 930, Access to std::array data as built-in array type) Such a boost::get_c_array(arg) function could be overloaded to support both std::array and boost::array. So it could be implemented as follows:

  // Specific for boost::array: simply returns its elems data member.
  template <typename T, std::size_t N>
  T(&get_c_array(boost::array<T,N>& arg))[N]
  {
    return arg.elems;
  }
  // Const version.
  template <typename T, std::size_t N>
  const T(&get_c_array(const boost::array<T,N>& arg))[N]
  {
    return arg.elems;
  }
  // Overload for std::array, assuming that std::array will have
  // explicit conversion functions as discussed at the WG21 meeting
  // in Summit, March 2009.
  template <typename T, std::size_t N>
  T(&get_c_array(std::array<T,N>& arg))[N]
  {
    return static_cast<T(&)[N]>(arg);
  }
  // Const version.
  template <typename T, std::size_t N>
  const T(&get_c_array(const std::array<T,N>& arg))[N]
  {
    return static_cast<T(&)[N]>(arg);
  }

When rvalue references are supported, get_c_array could be overloaded for rvalue arrays as well.

niels_dekker — Thu, 06 Aug 2009 12:22:42 GMT

Update: std::array might still have c_array() member functions, as the proposed resolution of LWG #930 was reconsidered by the Standard Library Working Group in Frankfurt, Saturday, July 18. The proposed resolution currently has status Review.

niels_dekker — Wed, 21 Oct 2009 17:30:35 GMT

Update: Yesterday, the Standard Library Working Group has discussed my proposal to add c_array() member functions to std::array, in Santa Cruz. Unfortunately: There was not enough consensus that this was sufficiently useful. http://home.roadrunner.com/~hinnant/issue_review/lwg-closed.html#930 So now it is unlikely that std::array will offer an easy way to get a reference to its internal built-in array.

I wonder if it's safe to cast std::array<T,N> to a pointer-to-array, as follows:

static_cast<T(*)[N]>(static_cast<void*>(arr.data()));

owner changed

Marshall Clow — Sun, 28 Mar 2010 05:22:28 GMT

owner changed from No-Maintainer to Marshall Clow

status changed

Marshall Clow — Sun, 28 Mar 2010 05:23:40 GMT

status new → assigned

I will keep an eye on this issue; see what the standards committee decides.

niels_dekker — Sun, 28 Mar 2010 11:01:25 GMT

Replying to comment 6:

I will keep an eye on this issue; see what the standards committee decides.

Thanks, Marchall. Unfortunately the standard library issue, LWG #930, Access to std::array data as built-in array type, has been rejected. I do not entirely understand why.

Anyway, a non-member function like get_c_array(array<T,N>&) (as I suggested here before at comment 2) would also be fine to me.

milestone changed

Marshall Clow — Sun, 06 Jun 2010 16:06:40 GMT

milestone Boost 1.39.0 → To Be Determined

I've checked get_c_array into the trunk; waiting to see how the tests turn out.

status changed; resolution set

Marshall Clow — Fri, 13 Aug 2010 20:50:06 GMT

status assigned → closed
resolution → fixed

get_c_array is part of the 1.44 release.