`CXX_STD = CXX11` causes CRAN note on devel

[rorynolan]

CXX_STD = CXX11 causes a CRAN note. The current CRAN default is C++17 and it doesn't like you to specify an older version.
The easy fix is to change the docs 11 -> 17 but that too will probably be too old someday. Perhaps the docs could just say that at least C++11 is required for RcppParallel to work but not do anything in Makevars?
Up to you how to tackle this, just wanted to make sure you were aware.

[eddelbuettel]

Duplicate of #194

[kevinushey]

I think there are some places we need to update:

• The skeleton function, here:

  RcppParallel/R/skeleton.R

  Lines 108 to 130 in 649340b

  	# src/Makevars
  	message(" >> added src/Makevars")
  	cat(
  	c(
  	'CXX_STD = CXX11',
  	'# We also need importFrom(RcppParallel,RcppParallelLibs) in NAMESPACE',
  	'PKG_LIBS += $(shell ${R_HOME}/bin/Rscript -e "RcppParallel::RcppParallelLibs()")'
  	),
  	file = "src/Makevars",
  	sep = "\n"
  	)
  	# src/Makevars.win
  	message(" >> added src/Makevars.win")
  	cat(
  	c(
  	'CXX_STD = CXX11',
  	'PKG_CXXFLAGS += -DRCPP_PARALLEL_USE_TBB=1',
  	'PKG_LIBS += $(shell "${R_HOME}/bin${R_ARCH_BIN}/Rscript.exe" -e "RcppParallel::RcppParallelLibs()")'
  	),
  	file = "src/Makevars.win",
  	sep = "\n"
  	)

• The documentation for the GitHub pages site here:

  RcppParallel/index.Rmd

  Lines 80 to 96 in 16876a2

  	```make
  	CXX_STD = CXX11
  	PKG_CXXFLAGS += -DRCPP_PARALLEL_USE_TBB=1
  	PKG_LIBS += $(shell "${R_HOME}/bin${R_ARCH_BIN}/Rscript.exe" \
  	-e "RcppParallel::RcppParallelLibs()")
  	```
  	Note that the Windows variation (Makevars.win) requires an extra `PKG_CXXFLAGS` entry that enables the use of TBB. This is because TBB is not used by default on Windows (for backward compatibility with a previous version of RcppParallel which lacked support for TBB on Windows).
  	TBB uses a subset of C++11 features -- for example, `long long` is not defined in the C++98 standard. Although most compilers make this type available as a compiler extension, they may emit warnings / errors when using these types while compiling in C++98 mode. To ensure that your package builds on CRAN without these warnings, you can enforce compilation using C++11, as done by `CXX_STD = CXX11`.
  	After you've added the above to the package you can simply include the main RcppParallel package header in source files that need to use it:
  	```cpp
  	#include <RcppParallel.h>
  	```

I'll fix those up.

[eddelbuettel]

One thing I have been wondering, though, is whether or not we should set a CXX_STD. "Way back when" we had to as the then-default compiler was insisting on C++98. Now we have modern C++ by default, and adding this feels like adding a constraint. Should we just leave it open? (IIRC) R 4.0.0 turned on C++11, R 4.1.0 turned to C++14, R 4.3.0 will turn on C++17. Good enough?

Edit: Doh. And I wrote that before I saw your commit. And you did the Right Thing (TM) ❤️