[Rd] level of mutability for the type of a SEXP
Prof Brian Ripley
ripley at stats.ox.ac.uk
Fri Feb 22 08:45:32 CET 2008
On Tue, 19 Feb 2008, Laurent Gautier wrote:
> Dear list,
> I am writing C code to interface with R, and I would like to know the
> level of mutability for the type of a SEXP.
> I see that there is a macro/function TYPEOF(), and that it can be used
> as an l-value, as well as a macro/function SET_TYPEOF().
The macros are not available to user code, so TYPEOF is not available to
use as an l-value.
> My question is "should the type be considered immutable, or it can it
> change after the SEXP has been created and used for a while ?".
> I understand that a call to TYPEOF will not perform any conversion,
> and my question is only about being sure that once the type of a given
> SEXP is set it cannot change.
It is SEXPRECs that have types, and their type can be changed and in a few
cases is in R's internal code. You should only change type between
SEXPREC types with the same structure, e.g. from VECSXP to EXPRSXP or
DOTSXP to LISTSXP. I am unaware of any examples where this happens that
involve a SEXPREC pointed to by a SEXP not in the internal code concerned.
Since a SEXP is a pointer, the type of the SEXPREC it points to can change
(as which SEXPREC it is can) and frequently does -- that is one way
arguments are coerced.
The R Internals manual has the details of the SEXPRECs and their
I don't see the value of knowing though. Anyone who needs to ask about R
internals should regard the arguments passed to .Call/.External calls as
read-only and also assume that there is no simultaneous evaluation going
on that might change them.
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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