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Boost.Locale
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The iostream manipulators are very useful, but when we create a messages for the user, sometimes we need something like good old printf or boost::format.
Unfortunately boost::format has several limitations in context of localization:
ostream locale. For example: std::locale::global(std::locale("en_US.UTF-8")); output.imbue(std::locale("de_DE.UTF-8")) output << boost::format("%1%") % 1234.345;
printf-like syntax is very limited for formatting complex localized data, not allowing the formatting of dates, times, or currenciesThus a new class, boost::locale::format, was introduced. For example:
Each format specifier is enclosed within {} brackets, is separated with a comma "," and may have an additional option after an equals symbol '='. This option may be simple ASCII text or single-quoted localized text. If a single-quote should be inserted within the text, it may be represented with a pair of single-quote characters.
Here is an example of a format string:
"Ms. {1} had arrived at {2,ftime='%I o''clock'} at home. The exact time is {2,time=full}"
The syntax is described by following grammar:
format : '{' parameters '}'
parameters: parameter | parameter ',' parameters;
parameter : key ["=" value] ;
key : [0-9a-zA-Z<>]+ ;
value : ascii-string-excluding-"}"-and="," | local-string ;
local-string : quoted-text | quoted-text local-string;
quoted-text : '[^']*' ;
You can include literal '{' and '}' by inserting double "{{" or "}}" to the text.
Would display something like
Unexpected `{' in line 5 in file source.cpp
The following format key-value pairs are supported:
[0-9]+ -- digits, the index of the formatted parameter -- required.num or number -- format a number. Options are: hex -- display in hexadecimal formatoct -- display in octal formatsci or scientific -- display in scientific formatfix or fixed -- display in fixed format number=sci cur or currency -- format currency. Options are: iso -- display using ISO currency symbol.nat or national -- display using national currency symbol. per or percent -- format a percentage value.date, time, datetime or dt -- format a date, a time, or a date and time. Options are: s or short -- display in short format.m or medium -- display in medium format.l or long -- display in long format.f or full -- display in full format.ftime with string (quoted) parameter -- display as with strftime. See as::ftime manipulator.spell or spellout -- spell the number.ord or ordinal -- format an ordinal number (1st, 2nd... etc)left or < -- align-left.right or > -- align-right.width or w -- set field width (requires parameter).precision or p -- set precision (requires parameter).locale -- with parameter -- switch locales for the current operation. This command generates a locale with formatting facets, giving more fine grained control of formatting. For example: timezone or tz -- the name of the timezone to display the time in. For example:local - display the time in local timegmt - display the time in UTC time scale The constructor for the format class can take an object of type message, simplifying integration with message translation code.
For example:
A formatted string can be fetched directly by using the str(std::locale const &loc=std::locale()) member function. For example:
std::wstring de = (wformat(translate("Adding {1} to {2}, we get {3}")) % a % b % (a+b)).str(de_locale); std::wstring fr = (wformat(translate("Adding {1} to {2}, we get {3}")) % a % b % (a+b)).str(fr_locale);
boost::format and boost::locale::format: Boost.Locale's format converts its parameters only when written to an ostream or when the `str()` member function is called. It only saves references to the objects that can be written to a stream.This is generally not a problem when all operations are done in one statement, such as:
cout << format("Adding {1} to {2}, we get {3}") % a % b % (a+b);
Because the temporary value of (a+b) exists until the formatted data is actually written to the stream. But following code is wrong:
format fmt("Adding {1} to {2}, we get {3}"); fmt % a; fmt % b; fmt % (a+b); cout << fmt;
Because the temporary value of (a+b) no longer exists when fmt is written to the stream. A correct solution would be:
format fmt("Adding {1} to {2}, we get {3}"); fmt % a; fmt % b; int a_plus_b = a+b; fmt % a_plus_b; cout << fmt;