BOOST_LOG_TRIVIAL and boost::posix_time::ptime output formatting - c++

Question

Need to set custom facet for posix_time::ptime which are printed with BOOST_LOG_TRIVIAL. Say, change default facet to "%Y--%m--%d %H:%M:%S".
I've tried to imbue log sink with new locale but had no success.
The same imbueing trick works perfectly well when applied to std::cout.
What could be the problem and is there a way to deal with it?
#include <boost/log/trivial.hpp>
#include <boost/log/utility/setup.hpp>
#include <boost/log/support/date_time.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <locale>
namespace logging = boost::log;
namespace pt = boost::posix_time;
void main(void)
{
// console sink
auto sink = logging::add_console_log(std::cout);
// standart output
boost::posix_time::ptime t1(boost::posix_time::time_from_string("2014-11-23 23:59:59.117"));
BOOST_LOG_TRIVIAL(info) << "before " << t1;
// adding custom facet...
pt::time_facet *facet = new boost::posix_time::time_facet("%Y--%m--%d %H:%M:%S");
sink->imbue(std::locale(sink->getloc(), facet));
// ... but output doesn't change
BOOST_LOG_TRIVIAL(info) << "after " << t1;
}

Answer 1

Use sink->set_formatter like this
sink->set_formatter
(
expr::stream << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%Y--%m--%d %H:%M:%S")
);
Also, make sure you call logging::add_common_attributes();
Working example: Coliru
#include <boost/log/trivial.hpp>
#include <boost/log/utility/setup.hpp>
#include <boost/log/support/date_time.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/log/utility/setup/common_attributes.hpp>
#include <boost/log/expressions/formatters/date_time.hpp>
namespace logging = boost::log;
namespace pt = boost::posix_time;
namespace expr = boost::log::expressions;
int main(void)
{
// console sink
auto sink = logging::add_console_log(std::cout);
// standart output
boost::posix_time::ptime t1(boost::posix_time::time_from_string("2014-11-23 23:59:59.117"));
BOOST_LOG_TRIVIAL(info) << "before " << t1;
sink->set_formatter
(
expr::stream << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%Y--%m--%d %H:%M:%S")
);
sink->imbue(sink->getloc());
logging::add_common_attributes();
// ... but output doesn't change
BOOST_LOG_TRIVIAL(info) << "after " << t1;
return 0;
}
Output:
before 2014-Nov-23 23:59:59.117000
2015--09--25 10:37:42

Answer 2

The problem is that the locale in the sink is not used for log message string formatting. It can't be used because there can be multiple sinks and the choice is ambiguous. That locale is used for log record formatting, which is performed for every sink individually.
The log message string is composed with a stream that is initialized for every log message, so you can't imbue it with a custom locale in a way that keeps the locale used for multiple records.
What you can do is set your custom locale as the global one somewhere in the initialization code of your application, before you use Boost.Log.
void main(void)
{
pt::time_facet *facet = new boost::posix_time::time_facet("%Y--%m--%d %H:%M:%S");
std::locale::global(std::locale(std::locale(), facet));
// console sink
auto sink = logging::add_console_log(std::cout);
// ...
BOOST_LOG_TRIVIAL(info) << "after " << t1;
}
Note that this will affect all formatting in your application, for logging purposes and not.
If that is not acceptable you can try different approaches. First, you can write a stream manipulator that will imbue the stream with a locale before formatting date/time objects. To automate the use of this manipulator you can define your own logging macro.
struct locale_manip {};
std::ostream& operator<< (std::ostream& strm, locale_manip)
{
pt::time_facet *facet = new boost::posix_time::time_facet("%Y--%m--%d %H:%M:%S");
strm.imbue(std::locale(std::locale(), facet));
return strm;
}
#define MY_LOG(sev) BOOST_LOG_TRIVIAL(sev) << locale_manip()
void main(void)
{
// ...
MY_LOG(info) << "after " << t1;
}
Another solution is to transform your logging statements so that the date/time objects are added to log records as attributes. Then you can use the formatters and the sink-specific locale to customize the output.