std::ctype<CharT>::is, std::ctype<CharT>::do_is

 

C++

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Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Localizations library

Locales and facets
locale
use_facet	has_facet
Character classification
isspace isblank (C++11) iscntrl isupper islower isalpha	isdigit ispunct isxdigit isalnum isprint isgraph
Conversions
toupper tolower	wstring_convert (C++11) wbuffer_convert (C++11)
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
Facet categories
ctype ctype<char> codecvt collate messages time_get time_put	num_get num_put numpunct money_get money_put moneypunct
Locale-specific facets
ctype_byname codecvt_byname messages_byname collate_byname	time_get_byname time_put_byname numpunct_byname moneypunct_byname
Code conversion facets
codecvt_utf8 (C++11) codecvt_utf16 (C++11)	codecvt_utf8_utf16 (C++11) codecvt_mode (C++11)
C locale
setlocale localeconv lconv	LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME

 

std::ctype

Member functions

ctype::ctype

ctype::~ctype

ctype::isctype::do_is

ctype::scan_isctype::do_scan_is

ctype::scan_notctype::do_scan_not

ctype::toupperctype::do_toupper

ctype::tolowerctype::do_tolower

ctype::widenctype::do_widen

ctype::narrowctype::do_narrow

Member functions of ctype<char>

ctype<char>::ctype

ctype<char>::~ctype

ctype<char>::table

ctype<char>::classic_table

ctype<char>::is

ctype<char>::scan_is

ctype<char>::scan_not

 

Defined in header <locale>
public: bool is(mask m, CharT c) const;	(1)
public: const CharT* is(const CharT* low, const CharT* high, mask* vec) const;	(2)
protected: virtual bool do_is(mask m, CharT c) const;	(3)
protected: virtual const CharT* do_is(const CharT* low, const CharT* high, mask* vec) const;	(4)

1,2) public member function, calls the protected virtual member function do_is of the most derived class.

3) Checks if the character c is classified by the mask m

4) For every character in the character array [low, high), identifies the complete classification mask (e.g. digit|xdigit|alnum|print|graph for the digit '0' in the default locale), and stores the masks in the corresponding elements of the array pointed to by vec

Parameters

c	-	character to classify
m	-	mask to use for classifying a single character
low	-	pointer to the first character in an array of characters to classify
high	-	one past the end pointer for the array of characters to classify
vec	-	pointer to the first element of the array of masks to fill

Return value

1,3) true if c is classified by m

2,4) high

Example

Run this code

#include <locale>
#include <utility>
#include <vector>
#include <iostream>
 
// utility wrapper to make locale-bound facets destructible
template<class Facet>
struct deletable_facet : Facet
{
    template<class ...Args>
    deletable_facet(Args&& ...args) : Facet(std::forward<Args>(args)...) {}
    ~deletable_facet() {}
};
 
int main()
{
    // classify a single character using the default locale
    auto& f = std::use_facet<std::ctype<char>>(std::locale());
    char c = '0';
    if (f.is(std::ctype_base::digit, c)) // or isdigit(c, locale());
    {
        std::cout << "'" << c << "' is a digit\n";
    }
 
    // classify every character in a string using a named locale
    deletable_facet<std::ctype_byname<wchar_t>> f2("en_US.utf8");
    std::wstring str = L"z\u00df\u6c34\U0001d10b";
    std::vector<std::ctype_base::mask> vec(str.size());
    f2.is(&str[0], &str[0] + str.size(), &vec[0]);
 
    for (std::size_t n = 0; n < str.size(); ++n) {
       std::cout << std::hex << "U+" << str[n] << " is: ";
       if (vec[n] & std::ctype_base::alnum) 
          std::cout << " alnum ";
       if (vec[n] & std::ctype_base::punct) 
          std::cout << " punct ";
       std::cout << '\n';
    }
}

Output:

'0' is a digit
U+7a is:  alnum 
U+df is:  alnum 
U+6c34 is:  alnum 
U+1d10b is:  punct

See also

is	classifies a character or a character sequence, using the classification table (public member function of std::ctype<char>)
iswctype	classifies a wide character according to the specified LC_CTYPE category (function)