NAME¶

std::isalnum - std::isalnum

Synopsis¶

Defined in header <cctype>
int isalnum( int ch );

Checks if the given character is an alphanumeric character as classified by the
current C locale. In the default locale, the following characters are alphanumeric:

* digits (0123456789)
* uppercase letters (ABCDEFGHIJKLMNOPQRSTUVWXYZ)
* lowercase letters (abcdefghijklmnopqrstuvwxyz)

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 an alphanumeric character, 0 otherwise.

Notes¶

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

Example¶

Demonstrates the use of std::isalnum with different locales (OS-specific).

// Run this code

#include <cctype>
#include <clocale>
#include <iostream>

int main()
{
unsigned char c = '\xdf'; // German letter ß in ISO-8859-1

std::cout << "isalnum(\'\\xdf\', default C locale) returned "
<< std::boolalpha << static_cast<bool>(std::isalnum(c)) << '\n';

if (std::setlocale(LC_ALL, "de_DE.iso88591"))
std::cout << "isalnum(\'\\xdf\', ISO-8859-1 locale) returned "
<< static_cast<bool>(std::isalnum(c)) << '\n';

}

Possible output:¶

isalnum('\xdf', default C locale) returned false
isalnum('\xdf', ISO-8859-1 locale) returned true

See also¶

isalnum(std::locale) checks if a character is classified as alphanumeric by a locale
(function template)
iswalnum checks if a wide character is alphanumeric
(function)
C documentation for
isalnum

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)