std::islower(3)

NAME
std::islower - std::islower

Synopsis
   Defined in header <cctype>
   int islower( int ch );
   Checks if the given character is classified as a lowercase character according to
   the current C locale. In the default "C" locale, std::islower returns a nonzero
   value only for the lowercase letters (abcdefghijklmnopqrstuvwxyz).
   If islower returns a nonzero value, it is guaranteed that std::iscntrl,
   std::isdigit, std::ispunct, and std::isspace return zero for the same character in
   the same C locale.
   The behavior is undefined if the value of ch is not representable as unsigned char
   and is not equal to EOF.

Parameters
   ch - character to classify

Return value
   Non-zero value if the character is a lowercase letter, zero otherwise.

Notes
   Like all other functions from <cctype>, the behavior of std::islower 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:
 bool my_islower(char ch)
 {
     return std::islower(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:
 int count_lowers(const std::string& s)
 {
     return std::count_if(s.begin(), s.end(),
                       // static_cast<int(*)(int)>(std::islower) // wrong
                       // [](int c){ return std::islower(c); } // wrong
                       // [](char c){ return std::islower(c); } // wrong
                        [](unsigned char c){ return std::islower(c); } // correct
                        );
 }

Example
// Run this code
 #include <cctype>
 #include <clocale>
 #include <iostream>
 int main()
 {
     unsigned char c = '\xe5'; // letter å in ISO-8859-1
     std::cout << "islower(\'\\xe5\', default C locale) returned "
               << std::boolalpha << (bool)std::islower(c) << '\n';
     std::setlocale(LC_ALL, "en_GB.iso88591");
     std::cout << "islower(\'\\xe5\', ISO-8859-1 locale) returned "
               << std::boolalpha << (bool)std::islower(c) << '\n';
 }

Possible output:
 islower('\xe5', default C locale) returned false
 islower('\xe5', ISO-8859-1 locale) returned true

See also
   islower(std::locale) checks if a character is classified as lowercase by a locale
                        (function template)
   iswlower checks if a wide character is lowercase
                        (function)
   C documentation for
   islower
        ASCII values characters iscntrl isprint isspace isblank isgraph ispunct isalnum isalpha isupper islower isdigit isxdigit decimal hexadecimal octal iswcntrl iswprint iswspace iswblank iswgraph iswpunct iswalnum iswalpha iswupper iswlower iswdigit iswxdigit 0–8 \x0–\x8 \0–\10 control codes ≠0 0 0 0 0 0 0 0 0 0 0 0
                        (NUL, etc.) 9 \x9 \11 tab (\t) ≠0 0 ≠0 ≠0 0 0 0 0 0 0 0 0
                        whitespaces 10–13 \xA–\xD \12–\15 (\n, \v, \f, ≠0 0 ≠0 0 0 0 0 0 0 0 0 0
                        \r) 14–31 \xE–\x1F \16–\37 control codes ≠0 0 0 0 0 0 0 0 0 0 0 0 32 \x20 \40 space 0 ≠0 ≠0 ≠0 0 0 0 0 0 0 0 0 33–47 \x21–\x2F \41–\57 !"#$%&'()*+,-./ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0 48–57 \x30–\x39 \60–\71 0123456789 0 ≠0 0 0 ≠0 0 ≠0 0 0 0 ≠0 ≠0 58–64 \x3A–\x40 \72–\100 :;<=>?@ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0 65–70 \x41–\x46 \101–\106 ABCDEF 0 ≠0 0 0 ≠0 0 ≠0 ≠0 ≠0 0 0 ≠0 71–90 \x47–\x5A \107–\132 GHIJKLMNOP 0 ≠0 0 0 ≠0 0 ≠0 ≠0 ≠0 0 0 0
                        QRSTUVWXYZ 91–96 \x5B–\x60 \133–\140 [\]^_` 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0 97–102 \x61–\x66 \141–\146 abcdef 0 ≠0 0 0 ≠0 0 ≠0 ≠0 0 ≠0 0 ≠0 103–122 \x67–\x7A \147–\172 ghijklmnop 0 ≠0 0 0 ≠0 0 ≠0 ≠0 0 ≠0 0 0
                        qrstuvwxyz 123–126 \x7B–\x7E \172–\176 {|}~ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0 127 \x7F \177 backspace ≠0 0 0 0 0 0 0 0 0 0 0 0
                        character (DEL)