std::has_facet(3)

NAME
std::has_facet - std::has_facet

Synopsis
   Defined in header <locale>
   template< class Facet > (until C++11)
   bool has_facet( const locale& loc ) throw();
   template< class Facet > (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.

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 it can be const-qualified, but not
                      can be cv-qualified volatile-qualified

See also
   locale set of polymorphic facets that encapsulate cultural differences
             (class)
   use_facet obtains a facet from a locale
             (function template)