std::has_facet

 

C++

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Freestanding and hosted

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

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Strings library

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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 > bool has_facet( const locale& loc ) throw();		(until C++11)
template< class Facet > bool has_facet( const locale& loc ) noexcept;		(since C++11)

Checks if the locale loc implements the facet Facet.

The program is ill-formed if Facet is not a facet or it is a volatile-qualified facet.

Parameters

loc

-

the locale object to query

Return value

Returns true if the facet Facet was installed in the locale loc, false otherwise.

Notes

std::has_facet must return true for all locales loc if Facet is one of the standard facets given here.

Example

Run this code

#include <iostream>
#include <locale>
 
// minimal custom facet
struct myfacet : public std::locale::facet
{
    static std::locale::id id;
};
 
std::locale::id myfacet::id;
 
int main()
{
    // loc is a "C" locale with myfacet added
    std::locale loc(std::locale::classic(), new myfacet);
    std::cout << std::boolalpha
              << "Can loc classify chars? "
              << std::has_facet<std::ctype<char>>(loc) << '\n'
              << "Can loc classify char32_t? "
              << std::has_facet<std::ctype<char32_t>>(loc) << '\n'
              << "Does loc implement myfacet? "
              << std::has_facet<myfacet>(loc) << '\n';
}

Output:

Can loc classify chars? true
Can loc classify char32_t? false
Does loc implement myfacet? true

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 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)
use_facet	obtains a facet from a locale (function template)