std::use_facet

 

C++

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Freestanding and hosted

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Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

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Diagnostics library

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Ranges library (C++20)

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Technical specifications

Symbols index

External libraries

 

Localization library

Locales and facets
Locales
has_facet use_facet	locale
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
ctype facets
ctype ctype<char> ctype_byname	codecvt codecvt_byname
numeric facets
num_get num_put	numpunct numpunct_byname
collate facets
collate	collate_byname
time facets
time_get time_put	time_get_byname time_put_byname
monetary facets
money_get money_put	moneypunct moneypunct_byname
messages facets
messages	messages_byname

Character classification and conversion
Character classification
isspace isblank (C++11) iscntrl	isupper islower isalpha	ispunct isdigit isxdigit	isalnum isprint isgraph

Character conversions

toupper

tolower

String and stream conversions

wstring_convert (C++11/17/26*)

wbuffer_convert (C++11/17/26*)

Text encoding identifications

text_encoding (C++26)

Unicode conversion facets

codecvt_utf8 (C++11/17/26*)

codecvt_utf16 (C++11/17/26*)

codecvt_utf8_utf16 (C++11/17/26*)

codecvt_mode (C++11/17/26*)

C library locales
LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME	setlocale localeconv lconv

 

Defined in header <locale>
template< class Facet > const Facet& use_facet( const std::locale& loc );

Obtains a reference to a facet implemented by loc.

The program is ill-formed if Facet is not a facet whose definition contains the public static member id or it is a volatile-qualified facet.

Parameters

loc

-

the locale object to query

Return value

Returns a reference to the facet. The reference returned by this function is valid as long as any std::locale object refers to that facet.

Exceptions

std::bad_cast if std::has_facet<Facet>(loc) == false.

Notes

A std::locale object should not be a temporary if a reference to the Facet object obtained from use_facet is used after the end of statement:

// BAD:
auto& f = std::use_facet<std::moneypunct<char, true>>(std::locale{"no_NO.UTF-8"});
foo(f.curr_symbol()); // Error: f internally uses a dangling reference
                      // to a std::locale object that no longer exists.
// GOOD:
auto loc = std::locale{"is_IS.UTF-8"}; // OK: a non-temporary object
auto& f = std::use_facet<std::moneypunct<char, true>>(loc);
foo(f.curr_symbol()); // OK: f internally uses a reference to existing locale object.

Example

Display the 3-letter currency name used by the user's preferred locale.

Run this code

#include <iostream>
#include <locale>
 
int main()
{
    for (const char* name: {"en_US.UTF-8", "de_DE.UTF-8", "en_GB.UTF-8"})
        std::cout << "Your currency string is "
                  << std::use_facet<std::moneypunct<char, true>>(std::locale{name}).
                     curr_symbol() << '\n';
}

Output:

Your currency string is USD
Your currency string is EUR
Your currency string is GBP

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 31	C++98	the returned reference remained usable as long as the locale value itself exists	the returned reference remains usable as long as some locale object refers to that facet
LWG 38	C++98	Facet was not required to have a direct member id	required
LWG 436	C++98	it was unclear whether Facet can be cv-qualified	it can be const-qualified, but not volatile-qualified

See also

locale	set of polymorphic facets that encapsulate cultural differences (class)
has_facet	checks if a locale implements a specific facet (function template)