boost/libs/locale/doc/formatting_and_parsing.txt

//
//  Copyright (c) 2009-2011 Artyom Beilis (Tonkikh)
//
//  Distributed under the Boost Software License, Version 1.0. (See
//  accompanying file LICENSE_1_0.txt or copy at
//  http://www.boost.org/LICENSE_1_0.txt)
//

// vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4 filetype=cpp.doxygen
/*!

\page formatting_and_parsing Numbers, Time and Currency formatting and parsing

All formatting and parsing is performed via the standard I/O streams. Each of the above information types is represented as a number.
The formatting information is set using iostream manipulators. All manipulators are placed in the boost::locale::as namespace.

For example:

\code
    cout << as::currency << 123.45 << endl;
    // display 123.45 in local currency representation.
    cin >> as::currency >> x ;
    // Parse currency representation and store it in x
\endcode

There is a special manipulator \c as::posix that "unsets" locale-specific settings and returns them to the default \c iostream formatting
and parsing methods. Please note, such formats may still be localized by the default \c std::num_put and \c std::num_get facets.

\section numbers_formatting Numbers and number manipulators

Here are the manipulators for number formatting:

-   \c as::number -- format number according to local specifications, it takes into account various \c std::ios_base flags like scientific
    format and precision.
    \n
-   \c as::percent -- format number as "percent" format. For example:
    \code
        cout << as::percent << 0.25 <<endl;
    \endcode
    Would create an output that may look like this:
    \verbatim
    25%
    \endverbatim
    \n
-   \c as::spellout -- spell the number. For example, under the English locale, 103 may be displayed as "one hundred three".
    \b Note: not all locales provide rules for spelling numbers. In such a case the number would be displayed in decimal format.
    \n
-   \c as::ordinal -- display an order-of element. For example "2" would be displayed as "2nd" under the English locale. As in
    the above case, not all locales provide ordinal rules.

\section currency_formatting Currency formatting

These are the manipulators for currency formatting:

-   \c as::currency -- set the format to currency mode.
-   \c as::currency_iso -- change the currency format to international, like "USD" instead of "$". This flag is supported
    when using ICU 4.2 and above.
-   \c as::currency_national -- change currency format to national, like "$".
-   \c as::currency_default -- return to the default (national) currency format.

\note \c as::currency_XYZ  manipulators have no effect on general formatting, only on the currency format. You must use both currency
and number manipulators to use a non-default format.

\section date_and_time_formatting Date and Time formatting

Dates and times are represented as POSIX time. When date-time formatting is turned on in the \c iostream, each number is treated as a
POSIX time. The number may be an integer or a double.

There are four major manipulators for Date and Time formatting:

-   \c as::date -- date only
-   \c as::time -- time only
-   \c as::datetime -- both date and time
-   \c as::ftime -- parameterized manipulator that allows specification of time in the format that is used in the \c strftime function.
    \b Note: not all formatting flags of \c strftime are supported.

For example:

\code
    time_t now=time(0);
    cout << "Today is "<< as::date << now << " and tomorrow is " << now+24*3600 << endl;
    cout << "Current time is "<< as::time << now << endl;
    cout << "The current weekday is "<< as::ftime("%A") << now << endl;
\endcode

More fine-grained control of date-time formatting is also available:

-   \c as::time_default , \c as::time_short , \c as::time_medium , \c as::time_long , \c as::time_full  -- change time formatting.
-   \c as::date_default , \c as::date_short , \c as::date_medium , \c as::date_long , \c as::date_full  -- change date formatting.

These manipulators, when used together with the \c as::date, \c as::time, or \c as::datetime manipulators, change the date-time representation.
The default format is medium.


By default, the date and time are shown in the local time zone. This behavior may be changed with the following manipulators:

-   \c as::gmt -- display date and time in GMT.
-   \c as::local_time  -- display in local time zone (default).
-   \c as::time_zone  -- parameterized manipulator that sets the time-zone ID for date-time formatting and parsing. It
    takes a string parameter that represents the time zone ID.

For example:

\code
    double now=time(0);
    cout << as::datetime << as::local_time << "Local time is: "<< now << endl;
    cout << as::gmt << "GMT Time is: "<< now <<endl;
    cout << as::time_zone("EST") << "Eastern Standard Time is: "<< now <<endl;
\endcode

There is a list of supported \c strftime flags by ICU backend:

-   \c \%a  -- Abbreviated  weekday (Sun.)
-   \c \%A  -- Full weekday (Sunday)
-   \c \%b  -- Abbreviated month (Jan.)
-   \c \%B  -- Full month (January)
-   \c \%c  -- Locale date-time format. \b Note: prefer using \c as::datetime
-   \c \%d  -- Day of Month [01,31]
-   \c \%e  -- Day of Month [1,31]
-   \c \%h  -- Same as \c \%b 
-   \c \%H  -- 24 clock hour [00,23]
-   \c \%I  -- 12 clock hour [01,12]
-   \c \%j  -- Day of year [1,366]
-   \c \%m  -- Month [01,12]
-   \c \%M  -- Minute [00,59]
-   \c \%n  -- New Line
-   \c \%p  -- AM/PM in locale representation
-   \c \%r  -- Time with AM/PM, same as \c \%I:\%M:\%S \%p 
-   \c \%R  -- Same as \c \%H:\%M 
-   \c \%S  -- Second [00,61]
-   \c \%t  -- Tab character
-   \c \%T  -- Same as \c \%H:\%M:\%S 
-   \c \%x  -- Local date representation. **Note:** prefer using \c as::date
-   \c \%X  -- Local time representation. **Note:** prefer using \c as::time
-   \c \%y  -- Year [00,99]
-   \c \%Y  -- 4 digits year. (2009)
-   \c \%Z  -- Time Zone
-   \c \%\%  -- Percent symbol

Unsupported \c strftime flags are: \c \%C , \c \%u , \c \%U , \c \%V , \c \%w , \c \%W . Also, the \c O and \c E modifiers are not supported.


\b General \b recommendations

- Prefer using generic date-time manipulators rather than specifying the full format using \c as::ftime.
- Remember that current calendars may be not Gregorian.


\section formatting_internals Internals

Formatting information is stored in a stream class by using the \c xalloc, \c pword, and \c register_callback  member functions
of \c std::ios_base . All the information is stored and managed using a special object bound to \c iostream, and the manipulators just
change its state.

When a number is written to or read from the stream, a custom Boost.Locale facet accesses the object and checks the required formatting
information. Then it creates a special object that actually formats the number and caches it in the \c iostream. The
next time a number is written to the stream, the same formatter would be used unless some flags had changed and formatter object is
invalid.

*/