Archive for the ‘STL’ Category

STL Design Problem?

Thursday, 24 September 2009

Consider the following functions:

#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/date_time/date_facet.hpp>
#include <clocale>

std::string format_date_crash (::boost::gregorian::date dt)
{
  std::locale locale_local ("");

  ::boost::gregorian::date_facet date_output;
  std::locale locale_adjusted (locale_local, &date_output);

  std::ostringstream date_ss;
  date_ss.imbue (locale_adjusted);

  p_date_output.format("%x");
  date_ss << dt;

  return date_ss.str();
}

std::string format_date_leakquery (::boost::gregorian::date dt)
{
  std::locale locale_local ("");

  ::boost::gregorian::date_facet * p_date_output
      = new ::boost::gregorian::date_facet;
  std::locale locale_adjusted (locale_local, p_date_output);

  std::ostringstream date_ss;
  date_ss.imbue (locale_adjusted);

  (*p_date_output).format("%x");
  date_ss << dt;

  // *** don't delete p_date_output ***
  return date_ss.str();
}

Here, I have used Boost too, but I don’t believe that the problem is in Boost.

Calls to format_date_crash() crash in the ~std::locale() destructor [oddly, I found that it crashes only on the second call].

On the other hand format_date_leakquery() looks as though it might leak: it has a new without a corresponding delete. However a test showed that it does not leak.

It seems that the std::locale constructor takes ownership of the facet, so that when the locale object is destroyed, so is the facet (ie. the facet is deleted).

Is this an instance of poor design in the STL, or is there some compelling reason that things are arranged this way?