[Rcpp-devel] Passing an exposed function to another C++ function as a parameter

[Romain Francois]

Hi,

This looks very similar to a problem Dirk faced in rewriting the DEOptim 
package to use Rcpp (the RcppDE package is on the Rcpp svn repo).

Essentially the problem is that the Rcpp::Function is a C++ object that 
calls back to R, with additional protection, which eventually calls C++.

You should be able to bypass this entirely and access directly the 
pointer to the C++ function from the R variable. This way, you stay in 
the C++ world. This would allow you to branch your code depending on 
whether you loop over a pure R function or a C++ function.

I'll cook an example.

Romain

Le 03/08/11 08:22, Manuel Castejón Limas a écrit :
>
>> Dear all,
>>
>> I'm rewriting the AMORE package using Rcpp --- in fact it's more like
>> I'm having a lot of fun while rewriting the AMORE package thanks to Rcpp.
>> Nevertheless, I'm facing this little problem that I hope it would be
>> pretty easy for you to solve.
>>
>> Let's consider the C++ function Tanh_f0
>>
>> double
>>
>> Tanh_f0(double inducedLocalField)
>>
>>   {
>>
>>     return tanh(inducedLocalField);
>>
>>   }
>>
>>
>> After compilation using inline, Tanh_f0 is exposed to R using Module
>> and accessible through actMod$Tanh_f0
>>
>>
>> actMod$Tanh_f0(0.1)
>>
>> # [1] 0.09966799
>>
>>
>> Now, I want to pass actMod$Tanh_f0 as a parameter of a function in
>> order to use the original C++ function Tanh_f0.
>>
>>
>>
>> testCode <- ' Rcpp::Function fx (myfun);
>>
>> double result = as<double>(fx(x));
>>
>> return wrap( result );
>>
>> '
>>
>> usingTanhf0 <- cfunction(sig=signature(myfun="function", x="numeric"),
>> body=testCode,…)
>>
>>
>> And that indeed works, but when compared to using a simple tanh
>> function it shows quite a bad performance.
>>
>>
>>
>> testCode <- ' double value = as<double>(x);
>>
>> double result = tanh(value);
>>
>> return wrap( result );
>>
>> '
>>
>>
>> usingTanh <- cfunction(sig=signature(x="numeric"), body=testCode, … )
>>
>> benchmark(usingTanhf0(myfun=actMod$Tanh_f0, x=0.1) , usingTanh(
>> x=0.1), columns=c("test", "replications", "elapsed", "relative"),
>> order="relative", replications=1000)
>>
>> #                                           test replications elapsed
>> relative
>>
>> # 2                           usingTanh(x = 0.1)         1000   0.004
>>       1
>>
>> # 1 usingTanhf0(myfun = actMod$Tanh_f0, x = 0.1)         1000   0.080
>>       20
>>
>>
>> Looks like having to go the R way to get access to Tanh_f0 has a high
>> cost. In order to be faster, it would be great if I could have access
>> to the original Tanh_f0 which I guess is pointed to by
>>  actMod$Tanh_f0, may be at the address 0x100153d30 ?
>>
>>
>> actMod$Tanh_f0
>>
>> # internal C++ function <0x100153d30>
>>
>> #     signature : double Tanh_f0(double)
>>
>>
>> After having had a look at RppDE sources, I guess the solution would
>> be to pass the function as an external pointer; something like :
>>
>> testCode <- '
>>
>> typedef double (*funPtr)(double) ;
>>
>> Rcpp::XPtr<  funPtr > fx (myfun);
>>
>> double result = (*fx)(0.1);
>>
>> return wrap( result );
>>
>> '
>>
>> testCodefun <- cfunction(sig=signature(myfun="C++Function"),
>> body=testCode,…)
>>
>> result <- testCodefun(myfun=actMod$Tanh_f0)
>>
>>
>> But this does NOT work and crashes R.
>>
>> Any hints?
>>
>> Thank you for your patience in reading such long message!
>> Manuel
>>
>>
>>
>
>
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Romain Francois
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