Formatting a Date/Time as a String

Formatting a Date Time as a String

Problem

You want to convert a date and/or time to a formatted string.

Solution

You can use the time_put template class from the header, as shown in Example 5-4.

Example 5-4. Formatting a datetime string

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

using namespace std;

ostream& formatDateTime(ostream& out, const tm& t, const char* fmt) {
 const time_put& dateWriter = use_facet >(out.getloc( ));
 int n = strlen(fmt);
 if (dateWriter.put(out, out, ' ', &t, fmt, fmt + n).failed( )) {
 throw runtime_error("failure to format date time");
 }
 return out;
}

string dateTimeToString(const tm& t, const char* format) {
 stringstream s;
 formatDateTime(s, t, format);
 return s.str( );
}

tm now( ) {
 time_t now = time(0);
 return *localtime(&now);
}

int main( )
{
 try {
 string s = dateTimeToString(now( ), "%A %B, %d %Y %I:%M%p");
 cout << s << endl;
 s = dateTimeToString(now( ), "%Y-%m-%d %H:%M:%S");
 cout << s << endl;
 }
 catch(...) {
 cerr << "failed to format date time" << endl;
 return EXIT_FAILURE;
 }
 return EXIT_SUCCESS;
}

Output of the program in Example 5-4 will resemble the following, depending on your local settings:

Sunday July, 24 2005 05:48PM
2005-07-24 17:48:11

 

Discussion

The time_put member function put uses a formatting string specifier like the C printf function format string. Characters are output to the buffer as they appear in the format string unless they are preceded by a % sign. A character preceded by a % sign is a format specifier and has the special meaning shown in Table 5-1. Format specifiers may also support modifiers, such as an integer to specify the field width, as in %4B.

Table 5-1. Date/time format specifiers

Specifier	Description
a	Abbreviated weekday name (e.g., Mon)
A	Full weekday name (e.g., Monday)
b	Abbreviated month name (e.g., Dec)
B	Full month name (e.g., May)
c	Complete date and time
d	Day of the month (01-31)
H	Hour (00-23)
I	Hour (01-12)
j	Day of the year (001-366)
m	Month (01-12)
M	Minutes (00-59)
p	AM/PM designation
S	Second, including up to two leap seconds
U	Week number (00-53), with week 1 starting on the first Sunday
w	Weekday (0-6), where Sunday is 0
W	Week number (00-53), with week 1 starting on the first Monday
x	Date in form MM/DD/YY
X	Time in form HH/MM/SS with 24-hour clock
y	Year within the current century (00-99)
Y	Year
Z	Time zone abbreviation, or empty if the system doesn't know the time zone

The Boost date_time library discussed in later recipes does not have the formatting capabilities offered by time_put. For convenience Example 5-5 contains several routines to convert from the Boost date/time classes to a tm struct, so that you can use time_put routines.

Example 5-5. Converting from Boost date/time classes to a tm struct

using boost::gregorian;
using boost::posix_time;

void dateToTmAux(const date& src, tm& dest) {
 dest.tm_mday = src.day( );
 dest.tm_year = src.year( ) - 1900;
 dest.tm_mon = src.month( ) - 1;
}

void ptimeToTmAux(const ptime& src, tm& dest) {
 dest.tm_sec = src.seconds( );
 dest.tm_min = src.minutes( );
 dest.tm_hour = src.hours( );
 dateToTmAux(src.date( ), dest);
}

tm ptimeToTm(const ptime& t) {
 tm ret = tm( ); 
 ptimeToTmAux(t, ret);




 return ret;
}

 

See Also

Recipe 13.3