std::tolower(std::locale)(3)

NAME
std::tolower(std::locale) - std::tolower(std::locale)

Synopsis
   Defined in header <locale>
   template< class CharT >
   CharT tolower( CharT ch, const locale& loc );
   Converts the character ch to lowercase if possible, using the conversion rules
   specified by the given locale's std::ctype facet.

Parameters
   ch - character
   loc - locale

Return value
   Returns the lowercase form of ch if one is listed in the locale, otherwise return ch
   unchanged.

Notes
   Only 1:1 character mapping can be performed by this function, e.g. the Greek
   uppercase letter 'Σ' has two lowercase forms, depending on the position in a word:
   'σ' and 'ς'. A call to std::tolower cannot be used to obtain the correct lowercase
   form in this case.

Possible implementation
   template<class CharT>
   CharT tolower(CharT ch, const std::locale& loc)
   {
       return std::use_facet<std::ctype<CharT>>(loc).tolower(ch);
   }

Example
// Run this code
 #include <cwctype>
 #include <iostream>
 #include <locale>
 int main()
 {
     wchar_t c = L'\u0190'; // Latin capital open E ('Ɛ')
     std::cout << std::hex << std::showbase;
     std::cout << "in the default locale, tolower(" << (std::wint_t)c << ") = "
               << (std::wint_t)std::tolower(c, std::locale()) << '\n';
     std::cout << "in Unicode locale, tolower(" << (std::wint_t)c << ") = "
               << (std::wint_t)std::tolower(c, std::locale("en_US.utf8")) << '\n';
 }

Possible output:
 in the default locale, tolower(0x190) = 0x190
 in Unicode locale, tolower(0x190) = 0x25b

See also
                        converts a character to uppercase using the ctype facet of a
   toupper(std::locale) locale
                        (function template)
   tolower converts a character to lowercase
                        (function)
   towlower converts a wide character to lowercase
                        (function)