[R-pkg-devel] C++ library USING_R

[Dirk Eddelbuettel]

On 14 October 2020 at 15:56, Joseph Park wrote:
| Dear R-devel,
| 
| The Writing R Extensions manual states:
| 
| 6.14 Platform and version information
| The header files define USING_R, which can be used to test if the code 
| is indeed being used with R.
| 
| I suppose this only applies to building R itself?  Or, perhaps when Rcpp 
| is used for the interface, or if one includes R headers explicitly.
| 
| I have a C++ library that is built with no R-dependencies, then wrapped 
| with an Rcpp interface. This same library is wrapped with pybind11 in 
| the Python world, and directly used in C++ applications.
| 
| How can the C++ library code "know" that it's R building the library? Is 
| there a platform-independent environment variable or macro?

So if I have this right you have

  - a library LIB using only standard C++ components

  - that you wrapped into a Python package via pybind11, and

  - that you want to build for R and 'let it know it used for R'

Well I think you would then use a (wrapper) R package R_LIB and could set up
a -DR_BUILD in the src/Makevars for R_LIB. This presume that you actually
recompile LIB as part of the build; if you were to link with a static or
shared build then you could not do this _for lack of a subsequent
compilation_ and can only rely on run-time information.

Hope this helps, and that I didn't misunderstand the question.

Dirk

-- 
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