Define a custom print method for exposed C++ classes

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When writing an R package wrapping some C++ data structures, using
Rcpp Modules
is a convenient option. After exposing a class to R, it can be used to
easily create new instances of that class.

As an example, let us look at the Uniform class of the Rcpp Modules vignette:

// cf Rcpp Modules vignette

#include 

using namespace Rcpp;

class Uniform {
  public:
    Uniform(double min_, double max_) : min(min_), max(max_) {}
    NumericVector draw(int n) const {
      RNGScope scope;
      return runif( n, min, max );
    }
    double min, max;
};

double uniformRange( Uniform* w) {
  return w->max - w->min;
}

RCPP_MODULE(unif_module) {
  class_( "Uniform" )
  
  .constructor()

  .field( "min", &Uniform::min )
  .field( "max", &Uniform::max )

  .method( "draw", &Uniform::draw )
  .method( "range", &uniformRange )
  ;
}

After sourcing the file the Uniform class can be used:

library(methods)    ## needed for S4 

## Create new instance myuniform:
myuniform <- new(Uniform, 0, 10)

# Print the new uniform isntance:
myuniform
C++ object <0x55fd63177450> of class 'Uniform' <0x55fd63788a20>

What happens now, is that the uniform instance calls its default print
method – which results in a fairly uninformative display.
It would be nice if the printer of the instances could be customized
to provide more information about the specific object.

Customize the Printer

It is possible to make use of the underlying
S4 structure of the exposed
C++ classes:

# Check exposed S4 structure:
isS4(myuniform)
[1] TRUE
# Get class name:
class(myuniform)
[1] "Rcpp_Uniform"
attr(,"package")
[1] ".GlobalEnv"

For the Uniform class this is Rcpp_Uniform. To obtain a custom printer the
last step now is to set the method show and define the function which should
be used as printer:

# Define the printer:
ignoreMe <- setMethod("show", "Rcpp_Uniform", function (object) {
  cat("\n Hi, I am an uniform object!\n")
  cat("\n I was initialized with a minimum value of", object$min)
  cat("\n and a maximum value of ", object$max, ".\n", sep = "")
  cat("\n Therefore my range is ", object$range(), ".", sep = "")
  cat("\n\n")
})

# Test the printer:
myuniform
 Hi, I am an uniform object!

 I was initialized with a minimum value of 0
 and a maximum value of 10.

 Therefore my range is 10.

This works very nicely. Now it is possible to provide some more informations about
a new Uniform instance. One thing to note is that setMethod returns the
method name as string, which we assign to an otherwise unused variable.

Use in Packages

To get that print method as a default printer after exposing the C++ class to R
within a package, it is sufficient to create a R file (e.g.
R/uniform_printer.R) and put the following code in there:

ignoreMe <- setMethod("show", "Rcpp_Uniform", function (object) {
  cat("\n Hi, I am an uniform object!\n")
  cat("\n I was initialized with a minimum value of", object$min)
  cat("\n and a maximum value of ", object$max, ".\n", sep = "")
  cat("\n Therefore my range is ", object$range(), ".", sep = "")
  cat("\n\n")
})

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