[Rcpp-devel] Rcpp::Dimension initialized from an SEXP

[Dirk Eddelbuettel]

On 8 May 2010 at 11:32, Douglas Bates wrote:
| On Sat, May 8, 2010 at 10:31 AM, Dirk Eddelbuettel <edd at debian.org> wrote:
| >
| > On 8 May 2010 at 09:21, Douglas Bates wrote:
| > | The enclosed section of code using Rcpp::Dimension fails to compile
| > | because of the const qualifiers for the simple::nrow and simple::ncol
| > | method functions.  If you omit those const qualifiers then it will
| > | compile and behave as desired.  Of course, I would prefer to have
| > | those method functions use the const qualifier.  Does anyone have
| > | suggestions on how to modify this code so I can use the const
| > | qualifiers?
| > |
| > | My guess is that this is related to the Rcpp::Dimension::operator[]()
| > | returning an int& and not an int so somehow there is a delayed
| > | evaluation going on - but I haven't learned enough C++ to be able to
| > | decide exactly where the problem originates.
| >
| > My guess is that it is the underlying SEXP (and here we put the stress on
| > P for pointer). My half-informed guess is that the compiler simply cannot see
| > how something passed as a pointer could be guaranteed const
| 
| Yes, I was concentrating on the fact that I was returning a value, not
| a reference or a pointer, from the method function.  I think what you
| are saying is that for the compiler once it sees that I am doing
| something with the SEXP within the method function it figures that all
| bets are off as far as the const keyword goes.
| 
| I did get around the problem by the simple expedient of defining int
| members, d_nrow and d_ncol, assigning their values during construction
| and returning those integer values in the const member functions.

Nice one.  I had one quick try making nrow() and ncol() call a new sibbling
functions _nrow() and _ncol() that I declared private and non-const, but the
compiler saw through that. That additional layer gets taken away.

The trouble with the stateful dimension vars d_nrow and d_ncol is course that
one day you will forget to update them...  I think in my defensive
programming mood, I'd still stick with the nrow() and ncol() you had and I'd
clinch my teeth and remove the const qualifier.
 
| By the way, I have for a couple of weeks now been reading the C++
| Annotations book that you mentioned and find it very interesting.  I
| am trying to follow some of the suggestions for class design from that
| book, which is why there is a d_ prefix on the private data member
| names.

I still haven't read much more than a section here or there. I quite like it,
but it is loooong.

-- 
  Regards, Dirk