A while back, I created a fork of the RDCOMClient package to keep it working with R 3.6 (https://github.com/dkyleward/RDCOMClient). People are now running into issues again because it won't work with R 4.0. The problem doesn't seem as easy to fix, and I'm hoping for some help.
If I flip Rstudio back to R 3.6 (and rtools35), I can use the package after installing with devtools::install_github(). When I try in R 4.0 (and rtools40), the package builds and I can connect over COM to an application. The first line of code below works, and xl is a COM pointer; however, trying to do anything with it (like set Excel to visible) will crash R.
xl <- RDCOMClient::COMCreate("Excel.Application")
xl[["Visible"]] <- TRUE
Again, the above works in R 3.6.
Is there is a way to continue building with the previous rtools? I came across https://github.com/r-windows/rtools-backports#readme, which talks about using rtools35 to keep building packages, so I have hope, but I don't understand how to make it happen.
Alternatively, if there are minor changes I can make to the R or cpp code that will solve my problem, I'm all ears. I'm a cpp novice, though.
Rik's answer was incredibly helpful and got a version working; however, after spending a day on it, I was able to improve on it. I want to put that here in case I have to do it again. The main improvement is being able to build a working package for both 32- and 64-bit architectures. By default, R installs both, and this makes things easier when installing dependent packages.
The first two steps are the same:
Install R-4.0.0 (https://cran.r-project.org/bin/windows/base/old/4.0.0/R-4.0.0-win.exe)
Install Rtools35 (https://cran.r-project.org/bin/windows/Rtools/Rtools35.exe) in directory c:\Rtools
If (like me) you had already installed rtools40, a system environment variable named RTOOLS40_HOME is created. The first step is to change that to:
C:\rtools
If you don't have rtools40 installed, then create the RTOOLS40_HOME system environment variable.
Two changes are still needed in the make files. These are found in your R installation directory.
In etc\x64\Makeconf, add underscores to match the rtools35 directory structure by setting these values:
MINGW_PREFIX = /mingw_$(WIN)
BINPREF ?= "$(RTOOLS40_ROOT)/mingw_64/bin/"
Do the same in etc\i386\Makeconf:
MINGW_PREFIX = /mingw_$(WIN)
BINPREF ?= "$(RTOOLS40_ROOT)/mingw_32/bin/"
Do not set BINPREF as an environment variable, or this will overwrite the makefile changes (like RTOOLS40_HOME does). With these complete, finish off with the same steps that Rik outlined:
Open windows command prompt and change to the directory that contains the RDCOMClient subdirectory and type:
R CMD INSTALL RDCOMClient –-build RDCOMClient.zip
This installs RDCOMClient in the local installation of R-4.0.0 and additionally creates the file RDCOMClient_0.94-0.zip that can be installed on other systems using the following command:
install.packages("RDCOMClient_0.94-0.zip", repos = NULL, type = "win.binary")