[Rcpp-devel] Creating an R interface to non-copyable C++ class

[Dirk Eddelbuettel]

On 11 April 2018 at 23:54, Cris Luengo wrote:
| > The way R thinks about this is that _it_ owns everything, and Rcpp makes
| > getting things back and forth _using the R memory system and its lifetime /
| > reference count control_ fairly easy.
| 
| Yes, that makes sense. But somehow it is not easy to transfer ownership
| of an object to R. There needs to be a new object created that it can own?

Yes, in general that works just fine. Ie when we do

   Rcpp::NumericVector x(10);

we use the R malloc functions, and to R this _is_ the same as a REAL with 10
elements.  Because everything has to be a SEXP, our C++ objects are really
proxies to underlying SEXPs. And R is happy. And controls everything and it
all works as if you used the C API of R.  Which we extend (by a lot, you
could say) to make things easier.

A unique_ptr with _control on the C++ side_ does not fit that mold at all so
we have to do something different here.  The singleton / static instance is
one approach _so that it survives across multiple (different) calls_ from R.

I am sure there are other approaches, but they still have to match the basic
constraint of 'SEXP in, SEXP out'.

Hth, Dirk

-- 
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