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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../ranges/../locale/use_facet.html below:

std::use_facet - cppreference.com

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.

[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.

#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 [edit] See also set of polymorphic facets that encapsulate cultural differences
(class) [edit] checks if a locale implements a specific facet
(function template) [edit]

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