std::tolower(3)

NAME
std::tolower - std::tolower

Synopsis
   Defined in header <cctype>
   int tolower( int ch );
   Converts the given character to lowercase according to the character conversion
   rules defined by the currently installed C locale.
   In the default "C" locale, the following uppercase letters
   ABCDEFGHIJKLMNOPQRSTUVWXYZ are replaced with respective lowercase letters
   abcdefghijklmnopqrstuvwxyz.

Parameters
   ch - character to be converted. If the value of ch is not representable as unsigned
        char and does not equal EOF, the behavior is undefined

Return value
   Lowercase version of ch or unmodified ch if no lowercase version is listed in the
   current C locale.

Notes
   Like all other functions from <cctype>, the behavior of std::tolower is undefined if
   the argument's value is neither representable as unsigned char nor equal to EOF. To
   use these functions safely with plain chars (or signed chars), the argument should
   first be converted to unsigned char:
 char my_tolower(char ch)
 {
     return static_cast<char>(std::tolower(static_cast<unsigned char>(ch)));
 }
   Similarly, they should not be directly used with standard algorithms when the
   iterator's value type is char or signed char. Instead, convert the value to unsigned
   char first:
 std::string str_tolower(std::string s)
 {
     std::transform(s.begin(), s.end(), s.begin(),
                 // static_cast<int(*)(int)>(std::tolower) // wrong
                 // [](int c){ return std::tolower(c); } // wrong
                 // [](char c){ return std::tolower(c); } // wrong
                    [](unsigned char c){ return std::tolower(c); } // correct
                   );
     return s;
 }

Example
// Run this code
 #include <cctype>
 #include <clocale>
 #include <iostream>
 int main()
 {
     unsigned char c = '\xb4'; // the character Ž in ISO-8859-15
                        // but ´ (acute accent) in ISO-8859-1
     std::setlocale(LC_ALL, "en_US.iso88591");
     std::cout << std::hex << std::showbase;
     std::cout << "in iso8859-1, tolower('0xb4') gives "
               << std::tolower(c) << '\n';
     std::setlocale(LC_ALL, "en_US.iso885915");
     std::cout << "in iso8859-15, tolower('0xb4') gives "
               << std::tolower(c) << '\n';
 }

Possible output:
 in iso8859-1, tolower('0xb4') gives 0xb4
 in iso8859-15, tolower('0xb4') gives 0xb8

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