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Boost.Locale
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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:
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 std::num_put and std::num_get facets.
Here are the manipulators for number formatting:
as::number – format number according to local specifications, it takes into account various std::ios_base flags like scientific format and precision. as::percent – format number as "percent" format. For example: 25%
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_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.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 onlyas::time – time onlyas::datetime – both date and timeas::ftime – parameterized manipulator that allows specification of time in the format that is used in the strftime function. Note: not all formatting flags of strftime are supported.For example:
More fine-grained control of date-time formatting is also available:
as::time_default , as::time_short , as::time_medium , as::time_long , as::time_full – change time formatting.as::date_default , as::date_short , as::date_medium , as::date_long , as::date_full – change date formatting.These manipulators, when used together with the as::date, as::time, or 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.For example:
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 as::datetime %d – Day of Month [01,31]%e – Day of Month [1,31]%h – Same as %b %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 %I:%M:%S %p%R – Same as %H:%M %S – Second [00,61]%t – Tab character%T – Same as %H:%M:%S %x – Local date representation. Note: prefer using as::date %X – Local time representation. Note: prefer using as::time %y – Year [00,99]%Y – 4 digits year. (2009)%Z – Time Zone%% – Percent symbolUnsupported strftime flags are: %C , %u , %U , %V , %w , %W . Also, the O and E modifiers are not supported.
General recommendations
as::ftime.Formatting information is stored in a stream class by using the xalloc, pword, and 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.