[Rd] Is it possible to shrink an R object in place?

[Kevin Ushey]

Suppose I generate an integer vector with e.g.

    SEXP iv = PROTECT(allocVector(INTSXP, 100));

and later want to shrink the object, e.g.

    shrink(iv, 50);

would simply re-set the length to 50, and allow R to reclaim the
memory that was previously used.

Is it possible to do this while respecting how R manages memory?

The motivation: there are many operations where the length of the
output is not known ahead of time, and in such cases one typically
uses a data structure that can grow efficiently. Unfortunately, IIUC
SEXPRECs cannot do this; however, an alternative possibility would
involve reserving extra memory, and then shrinking to fit after the
operation is complete.

There have been some discussions previously that defaulted to answers
of the form "you should probably just copy", e.g.
https://stat.ethz.ch/pipermail/r-devel/2008-March/048593.html, but I
wanted to ping and see if others had ideas, or if perhaps there was
code in the R sources that might be relevant.

Another reason why this is interesting is due to C++11 and
multi-threading: if I can pre-allocate SEXPs that will contain results
in the main thread, and then fill these SEXPs asynchronously (without
touching R, and hence not getting in the way of the GC or otherwise),
I can then fill these SEXPs in place and shrink-to-fit after the
computations have been completed. With C++11 support coming with R
3.1.0, functionality like this is very attractive.

The obvious alternatives are to 1) determine the length of the output
first and hence generate SEXPs of appropriate size right off the bat
(potentially expensive), and 2) fill thread-safe containers and copy
to an R object (definitely expensive).

I am probably missing something subtle (or obvious) as to why this may
not work, or be recommended, so I appreciate any comments.

Thanks,
Kevin