[Rd] R 3.1.0 and C++11

Dirk Eddelbuettel edd at debian.org
Mon Oct 7 23:47:58 CEST 2013

Hi Martyn,

On 7 October 2013 at 21:18, Martyn Plummer wrote:
| I don't see any harm in allowing optional C++11 support,

That would be a nice step forward.

| and it is no trouble to update the documentation to acknowledge the
| existence of C++11 conforming compilers.


| However, the questions of what is possible, what is recommended, and what
|  is required for CRAN submissions are distinct. 

You may be aware of the difficulties we as R package developers have with
discussions involving CRAN maintainers.  
| I have a couple of comments on the macro:
| a) Your version implies mandatory C++11 support. One needs
| AX_CXX_COMPILE_STDCXX_11(noext,optional) for optional support. 

I used an existing macros from the GNU autoconf archive. It can certainly be
tweaked. R's stack of configure logic is an impressive piece of work and I
wasn't expecting this to flow through. It was meant to start a discussion.

My principal points are that 

   i)  we do have compilers now that can support this, and 

   ii) we can test for their capabilities when R itself is compiled.

| b) I find it unhelpful that the macro picks up the partial C++11 support in
| gcc 4.7 via the -std=c++0x flag, so I would edit (and rename) the macro to
| remove this. 

Of course. All this can and should be discussed. I just wanted to get the
ball rolling and had a choice between just emailing Kurt (as the configure
and m4 point man) and emailing here.

To the extent that c++0x support is also widely available, I do not see why
one could not allow it either.  But that is a minor issue: I would really
like us to (eventually) move beyond what is going to become a more and more
constraining C++ standard.  

Optional support for deployments where C++11 is indeed available seems like a
step in the right direction.

Thanks for your feedback!


| Martyn
| ________________________________________
| From: r-devel-bounces at r-project.org [r-devel-bounces at r-project.org] on behalf of Dirk Eddelbuettel [edd at debian.org]
| Sent: 07 October 2013 01:54
| To: R-devel org
| Subject: [Rd] R 3.1.0 and C++11
| I would like to bring up two issues concerning C++11.
| First, the R-devel manuals contain incorrect statements regarding C++11:
|   i)   R-exts.texi:
|        Although there is a 2011 version of the C++ standard, it is not yet
|        fully implemented (nor is it likely to be widely available for some
|        years) and portable C++ code needs to follow the 1998 standard
|        (and not use features from C99).
|   ii)  R-ints.texi:
|        The type `R_xlen_t' is made available to packages in C header
|        `Rinternals.h': this should be fine in C code since C99 is
|        required.  People do try to use R internals in C++, but C++98
|        compilers are not required to support these types (and there are
|        currently no C++11 compilers).
| But since the summer we have g++ and clang with working C++11 implementations:
|   iii) g++ implements C++11:
|        http://isocpp.org/blog/2013/05/gcc-4.8.1-released-c11-feature-complete
|   iv)  llvm/clang++ implements C++11:
|        http://isocpp.org/blog/2013/06/llvm-3.3-is-released
| I would suggest to change the wording prior to the release of R 3.1.0 next
| year as it is likely that even Microsoft will by then have a fully-conformant
| compiler (per Herb Sutter at a recent talk in Chicago). If it helped, I would
| be glad to provide minimal patches to the two .texi files.
| Moreover, the C++ Standards Group is working towards closing the delta
| between standards being adopted, and compilers being released. They expect
| corresponding compilers for C++14 (a "patch" release for C++11 expected to be
| ready next spring) to be available within a year---possibly during 2014.
| Second, the current R Policy regarding C++11 is unnecessarily strict. I would
| propose to treat the availability of C++11 extensions more like the
| availability of OpenMP: something which configure can probe at build time,
| and which can be deployed later via suitable #ifdef tests.
| As a proof of concept, I added this macro from the autoconf archive to the
| m4/ directory of R-devel:
|   http://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html
| and made a one-line change to configure.ac (indented two spaces just for email)
|   edd at max:~/svn/r-devel$ svn di configure.ac
|   Index: configure.ac
|   ===================================================================
|   --- configure.ac      (revision 64031)
|   +++ configure.ac      (working copy)
|   @@ -906,6 +906,7 @@
|    AC_LANG_PUSH(C++)
|   +AX_CXX_COMPILE_STDCXX_11(noext)
|    AC_LANG_POP(C++)
|    ### *** ObjC compiler
|   edd at max:~/svn/r-devel$
| After running 'aclocal -Im4; autoheader; autoconf', the configure test then
| properly detected C++11 (or, in one case, C++0x) on four different compilers:
|   [ g++-4.7 case, Ubuntu 13.04 ]
|   checking whether g++ supports C++11 features by default... no
|   checking whether g++ supports C++11 features with -std=c++11... no
|   checking whether g++ supports C++11 features with -std=c++0x... yes
|   [ CC=clang CXX=clang++ (3.1), Ubuntu 13.04 ]
|   checking whether clang++ accepts -M for generating dependencies... yes
|   checking for clang++ option to support OpenMP... unsupported
|   checking whether clang++ supports C++11 features by default... no
|   checking whether clang++ supports C++11 features with -std=c++11... yes
|   [ g++-4.8 case, Debian testing ]
|   checking whether g++ supports C++11 features by default... no
|   checking whether g++ supports C++11 features with -std=c++11... yes
|   [ CC=clang CXX=clang++ (3.2), Debian testing ]
|   checking whether clang++ supports C++11 features by default... no
|   checking whether clang++ supports C++11 features with -std=c++11... yes
| It would be easy to another #define to config.h.in.
| And of course, I understand that R Core is comprised primarily of C
| programmers.  But to those of us who lean more towards C++ than C, the step
| towards C++11 is a big one, and a very exciting one.  More and more upstream
| authors are considering right now whether to switch to C++11-only.  I expect
| such switches to become more common as time pass. C++11 provides a lot -- and
| preventing programmers from using these tools cannot be in our interest.
| I think that the timing of the next R release will be a good opportunity to
| permit use of C++11 where compilers support it -- as a wide range of sites
| will already be capable of deploying it.
| Thanks, Dirk
| --
| Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
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Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com

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