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

Romain Francois romain at r-enthusiasts.com
Mon May 10 14:13:45 CEST 2010

Le 08/05/10 16:21, Douglas Bates a écrit :
> 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.

Hmm. The compiler error is really weird. It is not related to an 
underlying SEXP because there is no underlying SEXP. Rcpp::Dimension 
stores its data in a std::vector<int>

If I simplfy the code to this :

cdef <- '
class simple {
     Rcpp::Dimension dd;
     simple() : dd(1, 3) {}
     int nrow() const { return dd[0] ; }
cpp <- '
simple ss;
return wrap( ss.nrow() ) ;
ff <- cppfunction(signature(), cpp, includes = cdef)
ff( )

I get this compiler error:

Le chargement a nécessité le package : inline
Le chargement a nécessité le package : methods
file10d63af1.cpp: In member function ‘int simple::nrow() const’:
file10d63af1.cpp:8: error: invalid use of incomplete type ‘struct SEXPREC’
error: forward declaration of ‘struct SEXPREC’
file10d63af1.cpp:8: error: cannot convert ‘SEXPREC’ to ‘int’ in return
make: *** [file10d63af1.o] Error 1

which is __really__ weird... (time passes)... I think it comes from the 
operator SEXP. The compiler decided, for some reason to convert the 
Rcpp::Dimension to a SEXP using operator SEXP, then add 0 and dereference:

so dd[0] becomes *( dd + 0 ) which becomes *( (SEXP)dd + 0 ) and I guess 
this is where it gets unhappy.

If I change the code to explicitely call the operator[]:

     int nrow() const { return dd.operator[](0) ; }

I then get a much more reasonnable compiler error:

file10d63af1.cpp: In member function ‘int simple::nrow() const’:
file10d63af1.cpp:8: error: passing ‘const Rcpp::Dimension’ as ‘this’ 
argument of ‘int& Rcpp::Dimension::operator[](int)’ discards qualifiers
make: *** [file10d63af1.o] Error 1

I am not sure why the compiler does not want to call the operator[], so 
there is probably something we miss in Rcpp::Dimension.

Also, Rcpp::Dimension objects are often used temporarily (in matrix 
constructors, etc ...) so maybe we don't need them to return references 
and can return int instead. If you want to play with changing semantics 
of Rcpp::Dimension so that it returns "int" instead of "int&", that's 
fine by me. at the moment I don't think we do anything like:

Rcpp::Dimension x(3,3) ;
x[0] = 4 ;

so we don't need references here.

We might still need two versions of operator[] to deal with the 
eccentric decision from the compiler.


Romain Francois
Professional R Enthusiast
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