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.
There is a special manipulator
as::posix that "unsets" locale-specific settings and returns them to the default
iostream formatting and parsing methods. Please note, such formats may still be localized by the default
Here are the manipulators for number formatting:
as::number– format number according to local specifications, it takes into account various
std::ios_baseflags like scientific format and precision.
as::percent– format number as "percent" format. For example:
as::spellout– spell the number. For example, under the English locale, 103 may be displayed as "one hundred three". Note: not all locales provide rules for spelling numbers. In such a case the number would be displayed in decimal format.
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.
These are the manipulators for currency formatting:
as::currency– set the format to currency mode.
as::currency_iso– change the currency format to international, like "USD" instead of "$". This flag is supported when using ICU 4.2 and above.
as::currency_national– change currency format to national, like "$".
as::currency_default– return to the default (national) currency format.
as::currency_XYZmanipulators 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.
Dates and times are represented as POSIX time. When date-time formatting is turned on in the
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:
as::date– date only
as::time– time only
as::datetime– both date and time
as::ftime– parameterized manipulator that allows specification of time in the format that is used in the
strftimefunction. Note: not all formatting flags of
More fine-grained control of date-time formatting is also available:
as::time_full– change time formatting.
as::date_full– change date formatting.
These manipulators, when used together with the
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:
as::gmt– display date and time in GMT.
as::local_time– display in local time zone (default).
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.
There is a list of supported
strftime flags by ICU backend:
%a– Abbreviated weekday (Sun.)
%A– Full weekday (Sunday)
%b– Abbreviated month (Jan.)
%B– Full month (January)
%c– Locale date-time format. Note: prefer using
%d– Day of Month [01,31]
%e– Day of Month [1,31]
%h– Same as
%H– 24 clock hour [00,23]
%I– 12 clock hour [01,12]
%j– Day of year [1,366]
%m– Month [01,12]
%M– Minute [00,59]
%n– New Line
%p– AM/PM in locale representation
%r– Time with AM/PM, same as
%R– Same as
%S– Second [00,61]
%t– Tab character
%T– Same as
%x– Local date representation. Note: prefer using
%X– Local time representation. Note: prefer using
%y– Year [00,99]
%Y– 4 digits year. (2009)
%Z– Time Zone
%%– Percent symbol
strftime flags are:
%W . Also, the
E modifiers are not supported.
Formatting information is stored in a stream class by using the
register_callback member functions of
std::ios_base . All the information is stored and managed using a special object bound to
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
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.