[R-pkg-devel] How to update "SystemRequirements: C++11"?

[Winston Chang]

On Mon, Feb 6, 2023 at 6:44 PM Duncan Murdoch <murdoch.duncan using gmail.com>
wrote:

> On 06/02/2023 3:46 p.m., Winston Chang wrote:
> > I recently submitted a package to CRAN with "SystemRequirements: C++11".
> > This raises the following NOTE on R-devel, and I was asked to fix and
> > resubmit:
> >
> > * checking C++ specification ... NOTE
> >    Specified C++11: please update to current default of C++17
> >
> > If I understand correctly, I have two options, neither of which will
> work:
> > 1. Remove "SystemRequirements: C++11" entirely. The problem with this is
> > that on systems with older versions of R (3.6 and below, I believe), it
> may
> > try to compile the package with an older C++ standard, which will fail.
> > 2. Update it to "SystemRequirements: C++17". The problem here is that on
> > systems that don't have a C++17 compiler, the package won't build -- even
> > though the package only actually requires a C++11 compiler.
> >
> > How should I deal with this?
>
> After reading the discussion here and the manuals, I think this is how
> to do it.  I'm assuming your code is compatible with both C++11 and
> C++17, but not some earlier standards.
>
> 1.  In SystemRequirements, state that it needs C++11 or higher.  That
> field is supposed to be human readable (though I think there are scripts
> that try to read it; maybe they will suggest a particular wording to use).
>
> 2.  Have a configure script (e.g. like the one Ivan posted) that
> specifies CXX_STD=CXX11 in Makevars if the R version is less than 4.0.0,
> and gives no specification otherwise (because from 4.0.0 onwards, if C++
> is available, it will be C++11 or higher).
>
> I think it wouldn't hurt to look through the available versions and pick
> from them, but I don't think it's supposed to be necessary.
>
> If it eventually turns out that your code is not compatible with some
> later standard, I'm sure CRAN will let you know.


Now that I think about it some more, I think I can do the following:

1. In the DESCRIPTION file, remove "SystemRequirements: C++11". I believe
this field is used only for R CMD check, not for actually installing
packages. CRAN only runs R CMD check on r-oldrel, r-release, and r-devel,
and all of those versions (4.1, 4.2, and 4.3) have C++11. For older
versions of R, the package might not pass R CMD check, but it will still
install (provided there's a C++11 compiler).

2. In my case, I think I can skip the configure script. This is because I
also have GNU Make as a requirement, and it allows you to conditionally set
the CXX_STD variable only when R < 4.0, by putting this in the Makevars
file:

R_VERSION=$(shell $(R_HOME)/bin/Rscript --vanilla -e 'if (getRversion()<4)
cat("lt4") else cat("ge4")')
ifeq ($(R_VERSION), lt4)
CXX_STD=CXX11
endif


-Winston

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