std::has_facet
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C++
Feature test macros (C++20)
Concepts library (C++20)
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Localization library
String and stream conversions Unicode conversion facets C library locales
(C++11/17/26*)
(C++11/17/26*)
(C++11/17/26*)
(C++11/17/26*)
(C++11/17/26*)
(C++11/17/26*)
Defined in header
<locale>
template< class Facet >
bool has_facet( const locale& loc ) throw();
(until C++11)
bool has_facet( const locale& loc ) throw();
template< class Facet >
bool has_facet( const locale& loc ) noexcept;
(since C++11)
bool has_facet( const locale& loc ) noexcept;
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.
[edit] Parameters
loc
-
the locale object to query
[edit] Return value
Returns true if the facet Facet was installed in the locale loc, false otherwise.
[edit] Notes
std::has_facet must return true for all locales loc if Facet is one of the standard facets given here.
[edit] 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
[edit] 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 |