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std::use_facet(3)	      C++ Standard Libary	     std::use_facet(3)

NAME
       std::use_facet -	std::use_facet

Synopsis
	  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::lo-
       cale{"no_NO.UTF-8"});
	foo(f.curr_symbol()); // Error:	f internally uses a dangling reference
			      // to a std::locale object that  no  longer  ex-
       ists.
	// 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 lo-
       cale.

       // 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 retroac-
       tively to
	  previously published C++ standards.

	    DR	  Applied to	Behavior  as  published		       Correct
       behavior
			     the  returned reference	   the returned	refer-
       ence remains
	  LWG 31  C++98	     remained usable		  usable as
			     as	long as	the locale value  long as some	locale
       object refers to
			     itself exists		  that facet
	  LWG 38  C++98	     Facet was not required to	  required
			     have a direct member id
	  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  dif-
       ferences
		    (class)
	  has_facet checks if a	locale implements a specific facet
		    (function template)

http://cppreference.com		  2024.06.10		     std::use_facet(3)

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