std::iscntrl

 

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

Checks if the given character is a control character as classified by the currently installed C locale. In the default, "C" locale, the control characters are the characters with the codes 0x00-0x1F and 0x7F.

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 control character, zero otherwise.

Notes

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

Example

Run this code

#include <iostream>
#include <cctype>
#include <clocale>
 
int main()
{
    unsigned char c = '\x94'; // the control code CCH in ISO-8859-1
 
    std::cout << "iscntrl(\'\\x94\', default C locale) returned "
               << std::boolalpha << (bool)std::iscntrl(c) << '\n';
 
    std::setlocale(LC_ALL, "en_GB.iso88591");
    std::cout << "iscntrl(\'\\x94\', ISO-8859-1 locale) returned "
              << std::boolalpha << (bool)std::iscntrl(c) << '\n';
 
}

Output:

iscntrl('\x94', default C locale) returned false
iscntrl('\x94', ISO-8859-1 locale) returned true

See also

iscntrl(std::locale)	checks if a character is classified as a control character by a locale (function template)
iswcntrl	checks if a wide character is a control character (function)
C documentation for iscntrl

ASCII values (hex)		characters	iscntrl iswcntrl	isprint iswprint	isspace iswspace	isblank iswblank	isgraph iswgraph	ispunct iswpunct	isalnum iswalnum	isalpha iswalpha	isupper iswupper	islower iswlower	isdigit iswdigit	isxdigit iswxdigit
0 - 8	0x00-0x08	control codes (NUL, etc.)	≠0	0	0	0	0	0	0	0	0	0	0	0
9	0x09	tab (\t)	≠0	0	≠0	≠0	0	0	0	0	0	0	0	0
10 - 13	0x0A-0x0D	whitespaces (\n,\v,\f,\r)	≠0	0	≠0	0	0	0	0	0	0	0	0	0
14 - 31	0x0E-0x1F	control codes	≠0	0	0	0	0	0	0	0	0	0	0	0
32	0x20	space	0	≠0	≠0	≠0	0	0	0	0	0	0	0	0
33 - 47	0x21-0x2F	!"#$%&'()*+,-./	0	≠0	0	0	≠0	≠0	0	0	0	0	0	0
48 - 57	0x30-0x39	0123456789	0	≠0	0	0	≠0	0	≠0	0	0	0	≠0	≠0
58 - 64	0x3a-0x40	:;<=>?@	0	≠0	0	0	≠0	≠0	0	0	0	0	0	0
65 - 70	0x41-0x46	ABCDEF	0	≠0	0	0	≠0	0	≠0	≠0	≠0	0	0	≠0
71 - 90	0x47-0x5A	GHIJKLMNOPQRSTUVWXYZ	0	≠0	0	0	≠0	0	≠0	≠0	≠0	0	0	0
91 - 96	0x5B-0x60	[\]^_`	0	≠0	0	0	≠0	≠0	0	0	0	0	0	0
97 -102	0x61-0x66	abcdef	0	≠0	0	0	≠0	0	≠0	≠0	0	≠0	0	≠0
103-122	0x67-0x7A	ghijklmnopqrstuvwxyz	0	≠0	0	0	≠0	0	≠0	≠0	0	≠0	0	0
123-126	0x7B-0x7E	{|}~	0	≠0	0	0	≠0	≠0	0	0	0	0	0	0
127	0x7F	backspace character (DEL)	≠0	0	0	0	0	0	0	0	0	0	0	0