[Rd] R_MakeExternalPtr

[Prof Brian Ripley]

This is one of those undocumented things (AFAIK): LENGTH applies only to
'vectors'.  So don't use t and definitely don't set it for others.  The 
macro expands to

#define LENGTH(x)       (((VECSEXP) (x))->vecsxp.length)

What is a vector here?  Something which gets actually allocated by 
allocVector() AFAICS, that is an atomic or generic vector or an 
expression.  OTOH, an EXTPTRSXP is stored in a cons cell.

There is also a function length(), which is safer.

On Mon, 24 Oct 2005, TB wrote:

> Hi,
>
> I'm using R_MakeExternalPtr() to store handles to (COM) objects in a SEXP.
>
> The code basically is
>
> 	sexp = R_MakeExternalPtr(handle,R_NilValue,R_NilValue);
> 	R_RegisterCFinalizerEx(...);
>
> After creating the sexp, LENGTH(sexp) returns some quite large integer
> value. It seems like an "unitialized" value.
>
> Can I safely assume, that an SEXP of type EXTPTRSXP can only contain a
> single pointer value

It stores a pointer plus other things including a tag.

> and never represent a vector (an array) of pointers? Or
> should I maybe explicitly add something like
>
> 	SETLENGTH(sexp,1);
>
> Best wishes from sunny Austria,
>
> Thomas Baier
>
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>
>

-- 
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)
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