std::ctype::is,std::ctype::do_is(3)

NAME
std::ctype::is,std::ctype::do_is - std::ctype::is,std::ctype::do_is

Synopsis
   Defined in header <locale>
   public: (1)
   bool is( mask m, CharT c ) const;
   public: (2)
   const CharT* is( const CharT* low, const CharT* high, mask* vec ) const;
   protected: (3)
   virtual bool do_is( mask m, CharT c ) const;
   protected:
   virtual const CharT* do_is( const CharT* low, const CharT* high, mask* vec ) (4)
   const;
   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 <cstddef>
 #include <iostream>
 #include <locale>
 #include <utility>
 #include <vector>
 // 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+" << static_cast<wint_t>(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
   is table
            (public member function of std::ctype<char>)
   iswctype classifies a wide character according to the specified LC_CTYPE category
            (function)