std::regex_traits::isctype(3)

🌸 NAME 🌸

okay so this is the manual page for std::regex_traits::isctype — literally the std::regex_traits::isctype member, from the C++ Standard Libary manual (yes, that's the header's spelling, not me 😭), section 3.

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🎀 Synopsis 🎀

bool isctype( CharT c, char_class_type f ) const;

So isctype determines whether the character c belongs to the character class identified by f, which, in turn, is a value returned by lookup_classname() or a bitwise OR of several such values. periodt.

The version of this function provided in the standard library specializations of std::regex_traits does the following, bestie:

1. First converts f to a value m of type std::ctype_base::mask. For each std::ctype category listed in the table in the page lookup_classname(), if the bits in f corresponding to the category are set, the corresponding bits in m will also be set.
2. Then attempts to classify the character in the imbued locale by calling std::use_facet<std::ctype<CharT>>(getloc()).is(m, c).
   • If that returns true, isctype() will also return true.
   • Otherwise, if c equals '_', and f includes the result of calling lookup_classname() for the character class [:w:], true is returned, otherwise false is returned.

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🫶 Parameters 🫶

• c — the character to classify
• f — the bitmask obtained from one or several calls to lookup_classname()

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✨ Return value ✨

true if c is classified by f, false otherwise.

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🍓 Example 🍓

okay, run this code:

#include <iostream>
#include <regex>
#include <string>

int main()
{
    std::regex_traits<char> t;
    std::string str_alnum = "alnum";
    auto a = t.lookup_classname(str_alnum.begin(), str_alnum.end());
    std::string str_w = "w"; // [:w:] is [:alnum:] plus '_'
    auto w = t.lookup_classname(str_w.begin(), str_w.end());
    std::cout << std::boolalpha
              << t.isctype('A', w) << ' ' << t.isctype('A', a) << '\n'
              << t.isctype('_', w) << ' ' << t.isctype('_', a) << '\n'
              << t.isctype(' ', w) << ' ' << t.isctype(' ', a) << '\n';
}

Output:

true true
true false
false false

this next one demonstrates a custom regex traits implementation of lookup_classname() / isctype():

#include <cwctype>
#include <iostream>
#include <locale>
#include <regex>

// This custom regex traits uses wctype/iswctype to implement lookup_classname/isctype.
struct wctype_traits : std::regex_traits<wchar_t>
{
    using char_class_type = std::wctype_t;

    template<class It>
    char_class_type lookup_classname(It first, It last, bool = false) const
    {
        return std::wctype(std::string(first, last).c_str());
    }

    bool isctype(wchar_t c, char_class_type f) const
    {
        return std::iswctype(c, f);
    }
};

int main()
{
    std::locale::global(std::locale("ja_JP.utf8"));
    std::wcout.sync_with_stdio(false);
    std::wcout.imbue(std::locale());

    std::wsmatch m;
    std::wstring in = L"風の谷のナウシカ";
    // matches all characters (they are classified as alnum)
    std::regex_search(in, m, std::wregex(L"([[:alnum:]]+)"));
    std::wcout << "alnums: " << m[1] << '\n'; // prints "風の谷のナウシカ"
    // matches only the katakana
    std::regex_search(in, m,
                      std::basic_regex<wchar_t,
                                       wctype_traits>(L"([[:jkata:]]+)"));
    std::wcout << "katakana: " << m[1] << '\n'; // prints "ナウシカ"
}

Output:

alnums: 風の谷のナウシカ
katakana: ナウシカ

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🦋 Defect reports 🦋

so these behavior-changing defect reports got applied retroactively to previously published C++ standards — not new behavior, retroactive tea ☕:

• DR LWG 2018, applied to C++11 — behavior as published: the value of m was unspecified; correct behavior: matches lookup_classname()'s minimal support.

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👀 See also 👀

• lookup_classname — gets a character class by name (public member function)
• do_is — classifies a character or a character sequence [virtual] (virtual protected member function of std::ctype<CharT>)
• iswctype — classifies a wide character according to the specified LC_CTYPE category (function)

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mwah 💖 — source: http://cppreference.com, version/date: 2024.06.10. bestie, you ate this page fr fr 🫶✨