Boost.Locale
|
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: 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:
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:
Because the temporary value of (a+b) exists until the formatted data is actually written to the stream. But following code is wrong:
Because the temporary value of (a+b) no longer exists when
fmt
is written to the stream. A correct solution would be: