How to format date time object with format dd/mm/yyyy?

If you're using Boost.Date_Time, this is done using IO facets.

You need to include boost/date_time/posix_time/posix_time_io.hpp to get the correct facet typedefs (wtime_facet, time_facet, etc.) for boost::posix_time::ptime. Once this is done, the code is pretty simple. You call imbue on the ostream you want to output to, then just output your ptime:

#include <iostream>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/date_time/posix_time/posix_time_io.hpp>

using namespace boost::posix_time;
using namespace std;

int main(int argc, char **argv) {
  time_facet *facet = new time_facet("%d-%b-%Y %H:%M:%S");
  cout.imbue(locale(cout.getloc(), facet));
  cout << second_clock::local_time() << endl;
}

Output:

14-Dec-2009 16:13:14

See also the list of format flags in the boost docs, in case you want to output something fancier.

With the {fmt} library you can print the date in the required format as follows:

#include <boost/date_time/posix_time/posix_time.hpp>
#include <fmt/time.h>

int main() {
  auto now = boost::posix_time::second_clock::local_time();
  fmt::print("{:%d-%b-%Y %H:%M:%S}\n", to_tm(now));
}

This formatting facility is being proposed for standardization in C++20: P0645.

Alternatively you can use std::put_time which was introduced in C++11:

#include <boost/date_time/posix_time/posix_time.hpp>
#include <iomanip>
#include <iostream>

int main() {
  boost::posix_time::ptime now = boost::posix_time::second_clock::local_time();
  auto tm = to_tm(now);
  std::cout << std::put_time(&tm, "%d-%b-%Y %H:%M:%S");
}

Disclaimer: I'm the author of {fmt}.