std::use_facet
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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 >
const Facet& use_facet( const std::locale & loc );
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.
[edit] Parameters
loc
-
the locale object to query
[edit] 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.
[edit] Exceptions
std::bad_cast if std::has_facet <Facet>(loc) == false.
[edit] 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.
[edit] 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
[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 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 |