2013-01-20-custom-as-and-wrap-example.md

title	author	license	tags	summary	layout	src
Custom as and wrap converters example	Dirk Eddelbuettel	GPL (>= 2)	basics boost	This post shows how to use custom as and wrap converters	post	2013-01-20-custom-as-and-wrap-example.cpp

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:

{% highlight cpp %} #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);

} {% endhighlight %}

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.

{% highlight cpp %} #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 ));
}

} {% endhighlight %}

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.

{% highlight cpp %} // [[Rcpp::export]] Rcpp::Date getFirstDayOfWeekAfter(int weekday, SEXP date) { boost::gregorian::first_day_of_the_week_after fdaf(weekday); boost::gregorian::date dt = Rcpp::asboost::gregorian::date(date); return Rcpp::wrap( fdaf.get_date(dt) ); } {% endhighlight %}

We can use this to, say, find the first Monday after New Year in 2020:

{% highlight r %} getFirstDayOfWeekAfter(1, as.Date("2020-01-01")) {% endhighlight %}

[1] "2020-01-06"

We can also write a more concise form, knowing that compileAttributes() will insert the call to as<>() as needed.

{% highlight cpp %} // [[Rcpp::export]] Rcpp::Date getFirstDayOfWeekAfter2(int weekday, boost::gregorian::date dt) { boost::gregorian::first_day_of_the_week_after fdaf(weekday); return Rcpp::wrap(fdaf.get_date(dt)); } {% endhighlight %}

We still need a call to wrap() as the return-type of the function is Rcpp::Date() whereas we defined our wrap() converter with a return-type of SEXP.