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std::numpunct

Defined in header `<locale>`
template< class CharT > class numpunct;

The facet std::numpunct encapsulates numeric punctuation preferences. Stream I/O operations use std::numpunct through std::num_get and std::num_put for parsing numeric input and formatting numeric output.

The numbers that are supported by std::numpunct have the format described below. Here digit represents the radix set specified by the fmtflags argument value, thousands-sep and decimal-point are the results of thousands_sep() and decimal_point() functions respectively. The format of integer values is as follows:

integer     ::= [sign] units
sign        ::= plusminus
plusminus   ::= '+' | '-'
units       ::= digits [thousands-sep units]
digits      ::= digit [digits]

The number of digits between the thousand-seps (maximum size of digits) is specified by the result of grouping().

The format of floating-point values is as follows:

floatval    ::= [sign] units [decimal-point [digits]] [e [sign] digits] |
                [sign]        decimal-point  digits   [e [sign] digits]
e           ::= 'e' | 'E'

Inheritance diagram.

Specializations

The standard library is guaranteed to provide the following specializations (they are required to be implemented by any locale object):

Defined in header `<locale>`
std::numpunct<char>	provides equivalents of the "C" locale preferences
std::numpunct<wchar_t>	provides wide character equivalents of the "C" locale preferences

Member types

Member type	Definition
`char_type`	`CharT`
`string_type`	std::basic_string<CharT>

Member functions

(constructor)	constructs a new `numpunct` facet (public member function)
(destructor)	destructs a `numpunct` facet (protected member function)

decimal_point	invokes `do_decimal_point` (public member function)
thousands_sep	invokes `do_thousands_sep` (public member function)
grouping	invokes `do_grouping` (public member function)
truenamefalsename	invokes `do_truename` or `do_falsename` (public member function)

Protected member functions

do_decimal_point [virtual]	provides the character to use as decimal point (virtual protected member function)
do_thousands_sep [virtual]	provides the character to use as thousands separator (virtual protected member function)
do_grouping [virtual]	provides the numbers of digits between each pair of thousands separators (virtual protected member function)
do_truenamedo_falsename [virtual]	provides the string to use as the name of the boolean `true` and `false` (virtual protected member function)

Member objects

static std::locale::id id

id of the locale
(public member object)

Example

The following example changes the string representations of true and false:

#include <iostream>
#include <locale>
 
struct french_bool : std::numpunct<char>
{
    string_type do_truename() const override { return "vrai"; }
    string_type do_falsename() const override { return "faux"; }
};
 
int main()
{
    std::cout << "default locale: "
              << std::boolalpha << true << ", " << false << '\n';
    std::cout.imbue(std::locale(std::cout.getloc(), new french_bool));
    std::cout << "locale with modified numpunct: "
              << std::boolalpha << true << ", " << false << '\n';
}

Output:

default locale: true, false
locale with modified numpunct: vrai, faux

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 338	C++98	the `sign` token allowed an optional whitespace following `+` or `-`	removed the whitespace