std::locale

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::numpunct <char> std::numpunct <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 .

#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