std::setlocale

char* setlocale( int category, const char* locale );

The setlocale function installs the specified system locale or its portion as the new C locale. The modifications remain in effect and influences the execution of all locale-sensitive C library functions until the next call to setlocale. If locale is a null pointer, setlocale queries the current C locale without modifying it.

Parameters

Return value

Pointer to a narrow null-terminated string identifying the C locale after applying the changes, if any, or null pointer on failure.

A copy of the returned string along with the category used in this call to std::setlocale may be used later in the program to restore the locale back to the state at the end of this call.

Notes

During program startup, the equivalent of std::setlocale(LC_ALL, "C"); is executed before any user code is run.

Although the return type is char*, modifying the pointed-to characters is undefined behavior.

Because setlocale modifies global state which affects execution of locale-dependent functions, it is undefined behavior to call it from one thread, while another thread is executing any of the following functions: std::fprintf, std::isprint, std::iswdigit, std::localeconv, std::tolower, std::fscanf, std::ispunct, std::iswgraph, std::mblen, std::toupper, std::isalnum, std::isspace, std::iswlower, std::mbstowcs, std::towlower, std::isalpha, std::isupper, std::iswprint, std::mbtowc, std::towupper, std::isblank, std::iswalnum, std::iswpunct, std::setlocale, std::wcscoll, std::iscntrl, std::iswalpha, std::iswspace, std::strcoll, std::wcstod, std::isdigit, std::iswblank, std::iswupper, std::strerror, std::wcstombs, std::isgraph, std::iswcntrl, std::iswxdigit, std::strtod, std::wcsxfrm, std::islower, std::iswctype, std::isxdigit.

POSIX also defines a locale named "POSIX", which is always accessible and is exactly equivalent to the default minimal "C" locale.

POSIX also specifies that the returned pointer, not just the contents of the pointed-to string, may be invalidated by subsequent calls to setlocale.

Example

#include <clocale>
#include <cstdio>
#include <ctime>
#include <cwchar>
#include <iterator>
#include <string>
 
int main()
{
    // Make a "deep copy" of current locale name.
    std::string prev_loc = std::setlocale(LC_ALL, nullptr);
 
    // The C locale will be UTF-8 enabled English,
    // decimal dot will be German,
    // date and time formatting will be Japanese.
    if (const char* loc = std::setlocale(LC_ALL, "en_US.UTF-8"))
        std::wprintf(L"New LC_ALL locale: %s\n", loc);
    if (const char* loc = std::setlocale(LC_NUMERIC, "de_DE.UTF-8"))
        std::wprintf(L"New LC_NUMERIC locale: %s\n", loc);
    if (const char* loc = std::setlocale(LC_TIME, "ja_JP.UTF-8"))
        std::wprintf(L"New LC_TIME locale: %s\n", loc);
 
    wchar_t buf[100];
    std::time_t t = std::time(nullptr);
    std::wcsftime(buf, std::size(buf), L"%A %c", std::localtime(&t));
    std::wprintf(L"Number: %.2f\nDate: %Ls\n", 3.14, buf);
 
    // Restore the previous locale.
    if (const char* loc = std::setlocale(LC_ALL, prev_loc.c_str()))
        std::wprintf(L"Restorred LC_ALL locale: %s\n", loc);
}

New LC_ALL locale: en_US.UTF-8
New LC_NUMERIC locale: de_DE.UTF-8
New LC_TIME locale: ja_JP.UTF-8
Number: 3,14
Date: 日曜日 2022年11月06日 20時40分59秒
Restorred LC_ALL locale: C

See also