Make cpp functions visible to other packages

[NicChr]

Hello, is there a way to make my C++ functions accessible through cpp11 (or other means) to other R packages that use C/C++?

The goal is to be able to call package A's C++ functions directly in package B's C++ code though I'm not sure how to accomplish this given that much of the registration is automatically generated.

Any help would be appreciated, thanks!

[DavisVaughan]

It is possible, though it is not at all easy, but systemfonts does this

It utilizes [[cpp11::init]] to register functions as "callable" at the C level
https://github.com/r-lib/systemfonts/blob/02b567086379edaca1a9b3620ad6776e6bb876a7/src/font_fallback.cpp#L78
https://github.com/r-lib/systemfonts/blob/02b567086379edaca1a9b3620ad6776e6bb876a7/src/font_fallback.h#L15

Then exposes a header file that is used by other packages
https://github.com/r-lib/systemfonts/blob/02b567086379edaca1a9b3620ad6776e6bb876a7/inst/include/systemfonts.h

For example, ragg uses it by LinkingTo systemfonts
https://github.com/r-lib/ragg/blob/6e8bfd1264dfaa36aa6f92592e13a1169986e7b9/DESCRIPTION#L35

Note that you have to be very careful about the types you expose. This basically goes through a "pure C" api path, so you can't really expose C++ types that easily, and you have to "rebuild" any custom types like this one
https://github.com/r-lib/systemfonts/blob/02b567086379edaca1a9b3620ad6776e6bb876a7/inst/include/systemfonts.h#L18