CXX_STD and configure.ac in packages
On 03/31/2014 04:30 AM, Romain Fran?ois wrote:
Le 31 mars 2014 ? 12:20, Martyn Plummer <plummerm at iarc.fr> a ?crit :
On Mon, 2014-03-31 at 07:09 +0000, Martyn Plummer wrote:
Hi Martin, Thanks for the patch. I have applied it. I also added CXX1X and friends to the list of approved variables for R CMD config. So you can now query the existence of C++11 support with `R CMD config CXX1X` (It is empty if C++11 support is not available) and then take appropriate action in your configure script if, in Dirk's words, you want to do the configure dance.
Thanks, this is what I was looking for.
The philosophy underlying C++ support in R is that there are only two standards - C++98 and C++11 - and that you should write to one of those standards.
A should add a clarification. The way I wrote this makes it sound like an even-handed choice, but only C++98 has cross-platform support. If you use C++11 then many users will not currently be able to use your code.
Yes, the Writing R Extensions section at first seduced me into thinking that I could get broad support for C++11 with a simple macro, but obviously that can only come from the underlying compilers and R is making no guarantees about these.
OTOH, if nobody goes there, the need for C++11 might not be perceived as important by people who take care of cross platform support. Probably not Martin?s fight. One can do the gymnastics to get an unordered_map with C++98 (through boost, tr1, etc ...), but C++11 brings a great combination of new features that make it a better language, and I agree that it is almost a new language. And once you start using it, it is hard to look back.
Nobody should be writing new code that uses TR1 extensions now: they are superseded by the new standard.
For me unordered_map is a small part of a large mostly C code base; using it instead of map has substantial benefits, but restricting package use to C++11 isn't really on the table in this particular case. I'll take Martyn's philosophical statement that for R there are only two standards -- C++98 and C++11, with attendant trade-offs -- as a guiding principle and as a pragmatic solution avoid my complicated unordered_map configure dance for now. Thanks all for the various inputs. Martin Morgan
The map and unordered_map classes are a corner case, as they offer the same functionality but latter has much better complexity guarantees, so it is tempting to use it when available. But from a global perspective you should think of C++98 and C++11 as two different languages. Martyn
________________________________________
From: r-devel-bounces at r-project.org [r-devel-bounces at r-project.org] on behalf of Romain Francois [romain at r-enthusiasts.com]
Sent: 31 March 2014 08:22
To: Martin Morgan
Cc: R-devel
Subject: Re: [Rd] CXX_STD and configure.ac in packages
Hi,
My advice would be to use SystemRequirements: C++11
As <unordered_map> is definitely a part of C++11, assuming this version of the standard gives it to you. Your package may not compile on platforms where a C++11 compiler is not available, but perhaps if this becomes a pattern, then such compilers will start to be available, as in the current version of OSX and recent enough versions of various linux distributions.
The subset of feature that the version of gcc gives you with Rtools might be enough.
Alternatively, if you use Rcpp, you can use the RCPP_UNORDERED_MAP macro which will expand to either unordered_map or tr1::unordered_map, all the condition compiling is done in Rcpp.
Romain
Le 30 mars 2014 ? 21:50, Martin Morgan <mtmorgan at fhcrc.org> a ?crit :
In C++ code for use in a R-3.1.0 package, my specific problem is that I would like to use <unordered_map> if it is available, or <tr1/unordered_map> if not, or <map> if all else fails.
I (think I) can accomplish this with configure.ac as
AC_INIT("DESCRIPTION")
CXX=`"${R_HOME}/bin/R" CMD config CXX`
CXXFLAGS=`"${R_HOME}/bin/R" CMD config CXXFLAGS`
AC_CONFIG_HEADERS([src/config.h])
AC_LANG(C++)
AC_CHECK_HEADERS([unordered_map tr1/unordered_map])
AC_OUTPUT
Use of configure.ac does not seem to be entirely consistent with section 1.2.4 of Writing R Extensions, where one is advised that to use C++(11? see below) code one should
CXX_STD = CXX11
in Makevars(.win). My code does not require a compiler that supports the full C++11 feature set. In addition, I do not understand the logic of setting a variable that influences compiler flags in Makevars -- configure.ac will see a compiler with inaccurate flags.
Is use of configure.ac orthogonal to setting CXX_STD=CXX11?
Some minor typos:
/R-3-1-branch$ svn diff
Index: doc/manual/R-exts.texi
===================================================================
--- doc/manual/R-exts.texi (revision 65339)
+++ doc/manual/R-exts.texi (working copy)
@@ -2250,7 +2250,7 @@
@subsection Using C++11 code
@R{} can be built without a C++ compiler although one is available
-(but not necessarily installed) or all known @R{} platforms.
+(but not necessarily installed) on all known @R{} platforms.
For full portability across platforms, all
that can be assumed is approximate support for the C++98 standard (the
widely used @command{g++} deviates considerably from the standard).
@@ -2272,7 +2272,7 @@
support a flag @option{-std=c++0x}, but the latter only provides partial
support for the C++11 standard.
-In order to use C++ code in a package, the package's @file{Makevars}
+In order to use C++11 code in a package, the package's @file{Makevars}
file (or @file{Makevars.win} on Windows) should include the line
@example
@@ -2329,7 +2329,7 @@
anything other than the GNU version of C++98 and GNU extensions (which
include TR1). The default compiler on Windows is GCC 4.6.x and supports
the @option{-std=c++0x} flag and some C++11 features (see
- at uref{http://gcc.gnu.org/gcc-4.6/cxx0x_status.html}. On these
+ at uref{http://gcc.gnu.org/gcc-4.6/cxx0x_status.html}). On these
platforms, it is necessary to select a different compiler for C++11, as
described above, @emph{via} personal @file{Makevars} files. For
example, on OS X 10.7 or later one could select @command{clang++}.
--
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