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Currently I use this code

string now() {
    time_t t = time(0);
    char buffer[9] = {0};

    strftime(buffer, 9, "%H:%M:%S", localtime(&t));
    return string(buffer);
}

to format time. I need to add milliseconds, so the output has the format: 16:56:12.321

Answers

You can use Boost's Posix Time.

You can use boost::posix_time::microsec_clock::local_time() to get current time from microseconds-resolution clock:

boost::posix_time::ptime now = boost::posix_time::microsec_clock::local_time();

Then you can compute time offset in current day (since your duration output is in the form <hours>:<minutes>:<seconds>.<milliseconds>, I'm assuming they are calculated as current day offset; if they are not, feel free to use another starting point for duration/time interval):

boost::posix_time::time_duration td = now.time_of_day();

Then you can use .hours(), .minutes(), .seconds() accessors to get the corresponding values.
Unfortunately, there doesn't seem to be a .milliseconds() accessor, but there is a .total_milliseconds() one; so you can do a little subtraction math to get the remaining milliseconds to be formatted in the string.

Then you can use sprintf() (or sprintf()_s if you are interested in non-portable VC++-only code) to format those fields into a raw char buffer, and safely wrap this raw C string buffer into a robust convenient std::string instance.

See the commented code below for further details.

Output in console is something like:

11:43:52.276

Sample code:

///////////////////////////////////////////////////////////////////////////////

#include <stdio.h>      // for sprintf()

#include <iostream>     // for console output
#include <string>       // for std::string

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


//-----------------------------------------------------------------------------
// Format current time (calculated as an offset in current day) in this form:
//
//     "hh:mm:ss.SSS" (where "SSS" are milliseconds)
//-----------------------------------------------------------------------------
std::string now_str()
{
    // Get current time from the clock, using microseconds resolution
    const boost::posix_time::ptime now = 
        boost::posix_time::microsec_clock::local_time();

    // Get the time offset in current day
    const boost::posix_time::time_duration td = now.time_of_day();

    //
    // Extract hours, minutes, seconds and milliseconds.
    //
    // Since there is no direct accessor ".milliseconds()",
    // milliseconds are computed _by difference_ between total milliseconds
    // (for which there is an accessor), and the hours/minutes/seconds
    // values previously fetched.
    //
    const long hours        = td.hours();
    const long minutes      = td.minutes();
    const long seconds      = td.seconds();
    const long milliseconds = td.total_milliseconds() -
                              ((hours * 3600 + minutes * 60 + seconds) * 1000);

    //
    // Format like this:
    //
    //      hh:mm:ss.SSS
    //
    // e.g. 02:15:40:321
    //
    //      ^          ^
    //      |          |
    //      123456789*12
    //      ---------10-     --> 12 chars + \0 --> 13 chars should suffice
    //  
    // 
    char buf[40];
    sprintf(buf, "%02ld:%02ld:%02ld.%03ld", 
        hours, minutes, seconds, milliseconds);

    return buf;
}

int main()
{
    std::cout << now_str() << '\n';    
}

///////////////////////////////////////////////////////////////////////////////