[Rcpp-devel] Odd-seeming behaviour of [[ and $

[North, Ben]

Hi,

I recently came across some behaviour of `[[` and `$` which surprised
me.  I was trying to access a method of a class implemented using Rcpp,
choosing the method name at runtime, but the example below uses a fixed
method name to keep it short.

The full C++ code:

- - - - 8< - - - -

#include <Rcpp.h>

struct Constant
{
    Constant(double x) : x(x) {}
    double value() const { return x; }
    const double x;
};

RCPP_MODULE(Constant)
{
    Rcpp::class_<Constant>("Constant")
        .constructor<double>()
        .method("value", &Constant::value);
}

- - - - 8< - - - -

and the R script with (summarised) results embedded as comments:

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## Versions:
## R 2.15.1
## Rcpp 0.9.14

require(methods)
require(Rcpp)

dll.info <- dyn.load("Constant")
mod <- Module("Constant", dll.info, mustStart = TRUE)
Constant <- mod$Constant

x <- new(Constant, 2.2)

x[["value"]]
## NULL

x$value
## Class method definition for method value()
## [...]

x[["value"]]
## Class method definition for method value()
## [...]

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The fact that 'x[["value"]]' finds the method only if I've first done
'x$value' seemed odd.

Thanks,

Ben.


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