[R-pkg-devel] C++11 requirements for package dependencies

[Dirk Eddelbuettel]

On 30 November 2020 at 17:06, Mark Clements wrote:
| [Apologies for cross-posting]
| 
| A colleague uses a package I maintain (rstpm2) as a dependency in their
| package (rsimsum) with testing using GitHub Actions. They found that
| testing failed against R versions 3.3, 3.4 and 3.5 because recent
| versions of RcppArmadillo (which is a dependency in rstpm2) require
| C++11. As a dependency diagram:
| 
| rsimsum --> rstpm2 --> RcppArmadillo
| 
| Should I update rstpm2 to include "CXX_STD = CXX11" in the Makevars and
| Makevars.win files and add "SystemRequirements: C++11" to the
| DESCRIPTION, or is there a simple way in GitHub Actions to use C++11 for
| older versions of R?

Yes.  Writing R Extension has the goods, as usual:

-----------------------------------------------------------------------------

1.2.4 Using C++11 code
----------------------

R can be built without a C++ compiler although one is available (but not
necessarily installed) on all known R platforms.  As from R 4.0.0 a C++
compiler will be selected only if it conforms to the 2011 standard
('C++11').  A minor update(1) ('C++14') was published in December 2014.
A revision ('C++17') was published in December 2017, and a further
revision ('C++20', with many new features) is scheduled for publication
in May 2020.

   What standard a C++ compiler aims to support can be hard to
determine: the value(2) of '__cplusplus' may help but some compilers use
it to denote a standard which is partially supported and some the latest
standard which is (almost) fully supported.

   The webpage <http://en.cppreference.com/w/cpp/compiler_support> gives
some information on which compilers are known to support recent C++
features.  'g++' claims full C++11 support from version 4.8.1.

   As from version 3.6.2(3), R selects a default C++ compiler with
options that conform as far as possible(4) to C++11.  Packages which do
not specify 'R (>= 4.0)' in their 'DESCRIPTION' files need to explicitly
require C++11, hence the rest of this section.

   In order to specify C++11 code in a package to be used with R
versions from 3.1.0 but before 3.6.2, the package's 'Makevars' file (or
'Makevars.win' on Windows) should include the line

     CXX_STD = CXX11
Compilation and linking will then be done with the C++11 compiler (if
any).

   Packages without a 'src/Makevars' or 'src/Makefile' file may specify
that they require C++11 for code in the 'src' directory by including
'C++11' in the 'SystemRequirements' field of the 'DESCRIPTION' file,
e.g.

     SystemRequirements: C++11

   If a package does have a 'src/Makevars[.win]' file then setting the
make variable 'CXX_STD' is preferred, as it allows 'R CMD SHLIB' to work
correctly in the package's 'src' directory.

[... rest omitted ...]

-----------------------------------------------------------------------------
 
| Moreover, as a principle, should a package need to change the Makevars
| and DESCRIPTION files to suit the most recent updates of their
| dependencies? I would have thought that such a need would break many
| packages.

I don't understand what you are trying to say.  R, as shown in the quote,
supports C++11 and even defaults to it.  What would break anywhere?

Dirk

-- 
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