std::towlower (3) - Linux Manuals
Defined in header <cwctype>
std::wint_t towlower( std::wint_t ch );
Converts the given wide character to lowercase, if possible.
If the value of ch is neither representable as a wchar_t nor equal to the value of the macro WEOF, the behavior is undefined.
ch - wide character to be converted
Lowercase version of ch or unmodified ch if no lowercase version is listed in the current C locale.
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::towlower cannot be used to obtain the correct lowercase form in this case.
ISO_30112 specifies which pairs of Unicode characters are included in this mapping.
// Run this code
tolower(std::locale) (function template)