[Rd] Question about copying arguments in C.

[Simon Knapp]

Hi Simon,

Thanks for your advice, but I'm still not clear. In my case I don't
want to modify the result - the integer acts as a handle for indexing
an array in later calls back into my library.

As I understand it, returning result like

SEXP func(SEXP arg) {return arg;}

would not copy arg and hence I would have two pointers to the same
object immediately after the call... and (if this is the case) I'm not
sure whether this is OK.


Just to be clear, are you saying that the proper way to do things is
(at the end of my function in the original post):

SET_SLOT(ans, Rf_install("myInteger"), duplicate(thingysInteger));
return ans;

rather than

SET_SLOT(ans, Rf_install("myInteger"), thingysInteger);
return ans;

?

The last thing I'm not clear on is if it is OK to create a new SEXP
(with a call like duplicate) in a call to another function (as in the
first case above) or does this leave it unprotected?

Thanx again for your help,
Simon

On Sat, Sep 15, 2012 at 10:40 PM, Simon Urbanek
<simon.urbanek at r-project.org> wrote:
>
> On Sep 14, 2012, at 11:10 PM, Simon Knapp wrote:
>
>> Hi List,
>>
>> I'd imagine this is a question that has been answered before, but I
>> can't seem to track it down, sorry for the duplication if it has.
>>
>> I am writing an interface for a C library and want to return an S4
>> class from the 'constructing' method. One of the slots of the argument
>> to be returned will be filled with one of the arguments passed to the
>> function. My question is about whether I can directly pass arguments
>> to the function directly to slots of the returned object (or to a
>> return value more generally for that matter), or whether I have to
>> copy them. If it is the latter, then how may I do this. The question
>> is phrased in the following (simplified) code.
>>
>> int constructThingy(int thingysInteger);
>>
>> SEXP constructThingy(SEXP thingysInteger) {
>>    SEXP ans, TClass, ti;
>>    if(!isInteger(thingysInteger)) error("thingysIntegermust be an integer.");
>>    if(constructThingy(INTEGER(thingysInteger)[0])) error("error in
>> getting a thingy");
>>    TClass = MAKE_CLASS("thingy");
>>    PROTECT(ans = NEW_OBJECT(TClass));
>>
>>
>>    // *****QUESTION STARTS HERE*****
>>    // CAN I SAY:
>>    SET_SLOT(ans, Rf_install("myInteger"), thingysInteger);
>>
>>    // IF NOT, CAN I SAY
>>    SET_SLOT(ans, Rf_install("myInteger"), AS_INTEGER(thingysInteger));
>>
>>    // OR DO I NEED TO SAY
>>    PROTECT(ti = allocVector(INTSXP, 1)); INTEGER(pns)[0] =
>> INTEGER(thingysInteger)[0];
>>    SET_SLOT(ans, Rf_install("myInteger"), ti);
>>    // *****END OF QUESTION*****
>>
>>    UNPROTECT(1); // or UNPROTECT(2) in latter case.
>>    return ans;
>> }
>>
>>
>>
>> I think this is the same as asking whether the following is OK:
>>
>> SEXP func(SEXP arg) {
>>    return arg;
>> }
>>
>
> Yes.
>
> In fact if you wanted to duplicate (e.g, if you want to modify an incoming argument and return the modified result), the proper way would be to use duplicate() and not the contortions your were trying.
>
> Cheers,
> Simon
>
>
>>
>>
>>
>>
>> Thanks in advance,
>> Simon
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>>
>