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std::ctype::is, std::ctype::do_is

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

#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

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

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