The RcppBDT package interfaces Boost.Date_Time with R. Both systems have their own date representations—and this provides a nice example of custom as<>() and wrap() converters. Here, we show a simplified example.
We start with the forward declarations:
#include <RcppCommon.h>
#include <boost/date_time/gregorian/gregorian_types.hpp> // Gregorian calendar types, no I/O
namespace Rcpp {
// 'date' class boost::gregorian::date
//
// non-intrusive extension via template specialisation
template <> boost::gregorian::date as(SEXP dt);
//
// non-intrusive extension via template specialisation
template <> SEXP wrap(const boost::gregorian::date &d);
}
Given these forward declarations, we can now define the converters.
For as(), we first instantiate a date object and use it to obtain the year, month and day fields to create a boost::gregorian date.
Similarly, for the inverse operation, we construct an Rcpp date from these components.
#include <Rcpp.h>
// define template specialisations for as and wrap
namespace Rcpp {
template <> boost::gregorian::date as(SEXP dtsexp) {
Rcpp::Date dt(dtsexp);
return boost::gregorian::date(dt.getYear(), dt.getMonth(), dt.getDay());
}
template <> SEXP wrap(const boost::gregorian::date &d) {
boost::gregorian::date::ymd_type ymd = d.year_month_day(); // convert to y/m/d struct
return Rcpp::wrap( Rcpp::Date( ymd.year, ymd.month, ymd.day ));
}
}
With these converters, we can now use a Boost Date_Time function. As a simple example, we use the compute the first given weekday after a date function.
// [[Rcpp::export]]
Rcpp::Date getFirstDayOfWeekAfter(int weekday, SEXP date) {
boost::gregorian::first_day_of_the_week_after fdaf(weekday);
boost::gregorian::date dt = Rcpp::as<boost::gregorian::date>(date);
return Rcpp::wrap( fdaf.get_date(dt) );
}
We can use this to, say, find the first Monday after New Year in 2020:
getFirstDayOfWeekAfter(1, as.Date("2020-01-01"))
[1] "2020-01-06"
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