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

From cppreference.com
< cpp‎ | locale‎ | ctype

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`

### [edit] 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

### [edit] Return value

1,3) true if `c` is classified by `m`
2,4) `high`

### [edit] 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

### [edit] See also

iswctype | classifies a wide character according to the specified LC_CTYPE category (function) [edit]

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