[Rcpp-devel] pass-by-reference for reference objects

[Greg Minshall]

hi.  i have been trying to understand how to pass objects back and forth
by reference (in order to avoid copying).  this works smoothly if one is
calling a method on the object.  but, i was having trouble figuring this
out for "free functions" (i.e., non-member functions, i.e.,
non-methods).  the term "free function" itself i discovered in
Rcpp_modules.pdf.

the answer is to define a function which has the desired
pass-by-reference class as its first parameter, as a pointer, and then
list that function as a *.method* in the class_ field of the
RCPP_MODULE() call.  i.e. (see that attached for the gory details),
given a class
----
class space2d ...
----

one defines a function or two:
----
space2d* psps(space2d* arg, int val) {
    arg->set(1, val);
    return arg;
}

void vps(space2d* arg, int val) {
    arg->set(1, val);
}
----

and then does some module stuff:
----
RCPP_EXPOSED_CLASS(space2d)

using namespace Rcpp;
RCPP_MODULE(test6_module) {
    class_<space2d>("space2d")
        .constructor()
        .method("psps", &psps)
        .method("vps", &vps);
}
----

in the above, changes made to arg in both psps() and vps() will be seen
by the caller.  in addition, the receiver of the return value of psps()
will *also* hold a shared copy of the reference object.

hth.

(it might be a Nice Addition to remove the limitations of only one
pass-by-reference, location in the formal parameters, etc.  someday...)

cheers, Greg
----
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