c++ - Cast a boost::log::expressions::attr< std::string > to std::string

Question

While using Boost.Log, I am trying to keep my TimeStamp formatter such as:

  logging::add_file_log
    (
     keywords::file_name = "my.log",
     keywords::format =
     (
      expr::stream
      << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%Y-%m-%d %H:%M:%S")
      << "," << expr::attr< int >("Line")
      << " " << expr::attr< std::string >("File")
      << " " << logging::trivial::severity
      << " - " << expr::smessage
     )
    );

It is said that I cannot use the other form of formatter since I'll have much difficulties turning "TimeStamp" into my custom format:

static void my_formatter(logging::record_view const& rec, logging::formatting_ostream& strm)
{
  strm << logging::extract< boost::posix_time::ptime >("TimeStamp", rec);

will output something like: 2015-Jul-01 16:06:31.514053, while I am only interested in: "%Y-%m-%d %H:%M:%S". However the first form is extremely hard to use, for instance I am not able to cast an expr::attr< std::string > to a simple std::string for instance:

  logging::add_file_log
    (
     keywords::file_name = "my.log",
     keywords::format =
     (
      expr::stream
      << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%Y-%m-%d %H:%M:%S")
      << "," << expr::attr< int >("Line")
      << " " << boost::filesystem::path(expr::attr< std::string >("File"))
        .filename().string()
      << " " << logging::trivial::severity
      << " - " << expr::smessage
     )
    );

The above code does not even compile.

Is there an easy way to both print TimeStamp using my custom format and at the same time use a custom cast to string to be able to use boost::filesystem::path::filename() ?

Answer 1

In custom formatter you can easily build the timestamp in "%Y-%m-%d %H:%M:%S" format:

void my_formatter(logging::record_view const& rec, logging::formatting_ostream& strm)
{
    const boost::posix_time::ptime &pt = *logging::extract< boost::posix_time::ptime >("TimeStamp", rec);
    strm << pt.date() << " " << pt.time_of_day().hours() << ":" << pt.time_of_day().minutes() << ":" << pt.time_of_day().seconds()
    ...
    << rec[expr::smessage];
}