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

std::locale - cppreference.com

An object of class std::locale is an immutable indexed set of immutable facets. Each stream object of the C++ input/output library is associated with an std::locale object and uses its facets for parsing and formatting of all data. In addition, a locale object is associated with each std::basic_regex object.(since C++11) Locale objects can also be used as predicates that perform string collation with the standard containers and algorithms and can be accessed directly to obtain or modify the facets they hold.

Each locale constructed in a C++ program holds at least the following standard facets (i.e. std::has_facet returns true for all these facet types), but a program may define additional specializations or completely new facets and add them to any existing locale object.

Supported facets std::ctype<char>
std::ctype<wchar_t> std::codecvt<char, char, std::mbstate_t>
std::codecvt<wchar_t, char, std::mbstate_t> std::num_get<char>
std::num_get<wchar_t> std::numpunct<char>
std::numpunct<wchar_t> std::num_put<char>
std::num_put<wchar_t> std::money_get<char>
std::money_get<wchar_t> std::moneypunct<char>
std::moneypunct<char, true>
std::moneypunct<wchar_t>
std::moneypunct<wchar_t, true> std::money_put<char>
std::money_put<wchar_t> std::time_get<char>
std::time_get<wchar_t> std::collate<char>
std::collate<wchar_t> std::time_put<char>
std::time_put<wchar_t> std::messages<char>
std::messages<wchar_t> Deprecated facets std::codecvt<char16_t, char, std::mbstate_t> (since C++11)(deprecated in C++20)
std::codecvt<char32_t, char, std::mbstate_t> (since C++11)(deprecated in C++20)
std::codecvt<char16_t, char8_t, std::mbstate_t> (since C++20)(deprecated)
std::codecvt<char32_t, char8_t, std::mbstate_t> (since C++20)(deprecated)

Internally, a locale object is implemented as if it is a reference-counted pointer to an array (indexed by std::locale::id) of reference-counted pointers to facets: copying a locale only copies one pointer and increments several reference counts. To maintain the standard C++ library thread safety guarantees (operations on different objects are always thread-safe), both the locale reference count and each facet reference count are updated in a thread-safe manner, similar to std::shared_ptr.

[edit] Member types Type Description the facet index type: each facet class must declare or inherit a public static member of this type
(class) [edit] the base class for all facet categories: each facet of any category is derived from this type
(class) [edit] int
(typedef) [edit] Member constants Name Explanation

const category none

[static]

a zero value indicating no facet category
(public static member constant)

const category collate

[static]

a bitmask value indicating the collate facet category
(public static member constant)

const category ctype

[static]

a bitmask value indicating the ctype facet category
(public static member constant)

const category monetary

[static]

a bitmask value indicating the monetary facet category
(public static member constant)

const category numeric

[static]

a bitmask value indicating the numeric facet category
(public static member constant)

const category time

[static]

a bitmask value indicating the time facet category
(public static member constant)

const category messages

[static]

a bitmask value indicating the messages facet category
(public static member constant)

const category all

[static]

std::locale member functions expecting a category argument require one of the category values defined above, or the union of two or more such values. The LC constants are not accepted.

[edit] Member functions constructs a new locale
(public member function) [edit] destructs the locale and the facets whose reference count becomes zero
(public member function) [edit] replaces a locale
(public member function) [edit] constructs a locale with compile-time identified facet copied from another locale
(public member function) [edit] returns the name of the locale or "*" if unnamed
(public member function) [edit] returns the character encoding scheme associated with the locale
(public member function) [edit] equality comparison between locale objects
(public member function) [edit] lexicographically compares two strings using this locale's collate facet
(public member function) [edit] changes the global locale
(public static member function) [edit] obtains a reference to the "C" locale
(public static member function) [edit] [edit] Example

Demonstrates the typical prologue of a locale-sensitive program (cross-platform).

#include <iostream>
#include <locale>
 
int main()
{
    std::wcout << L"User-preferred locale setting is "
               << std::locale("").name().c_str() << L'\n';
    // on startup, the global locale is the "C" locale
    std::wcout << 1000.01 << L'\n';
 
    // replace the C++ global locale and the "C" locale with the user-preferred locale
    std::locale::global(std::locale(""));
    // use the new global locale for future wide character output
    std::wcout.imbue(std::locale());
 
    // output the same number again
    std::wcout << 1000.01 << L'\n';
}

Possible output:

User-preferred locale setting is en_US.UTF8
1000.01
1,000.01

[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 340 C++98 the set of standard facets that all locales need to hold was unclear made clear LWG 347 C++98 parameters of type category could accept LC constants not accepted anymore [edit] See also

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