How allocate STRSXP outside of gc

-----Original Message-----
From: r-devel-bounces@stat.math.ethz.ch 
[mailto:r-devel-bounces@stat.math.ethz.ch] On Behalf Of Jan T. Kim
Sent: Wednesday, April 13, 2005 3:44 AM
To: r-devel@stat.math.ethz.ch
Subject: Re: [Rd] How allocate STRSXP outside of gc

On Tue, Apr 12, 2005 at 12:31:03PM -0700, Vadim Ogranovich wrote:

Hi,
 
I am trying to figure a way to allocate a string SEXP so that gc() 
won't ever collect it.
 
Here is a little bit of a background. Suppose I want to write a 
.Call-callable function that upon each call returns the same value, 
say
mkChar("foo"):
 
SEXP getFoo() {
   return mkChar("foo");
}
 
The above implementation doesn't take advantage of the fact that
mkChar("foo") could be pre-computed only once, and then the 

function 

would return the pre-computed value. So the question is how 

to create 

this precomputed value.
 
 
The closest thing I could find in the sources is 

R_NaString, but I was 

not able to trace down how it comes about.

For being unaffected by R's memory management, it may be the 
best to not use a SEXP for storing the pre-computed result at 
all. Rather, use a static variable "private" to your code, as in

    SEXP getFoo()
    {
      static char *foo = NULL;

      if (foo == NULL)
      {
        foo = the_difficult_to_compute_value_of_foo();
      }
      return mkChar(foo);
    }

This way, getFoo indeed invokes mkChar each time, but in your 
scenario, that might be an overhead which is negligible 
compared to the actual computation of the foo value.

Best regards, Jan
--
 +- Jan T. Kim 
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