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std::isdigit(std::locale) - cppreference.com

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template< class CharT > bool isdigit( CharT ch, const locale& loc );

Checks if the given character is classified as a digit by the given locale's std::ctype facet.

Parameters

ch - character
loc - locale

Return value

Returns true if the character is classified as a digit, false otherwise.

Possible implementation

template<class CharT>
bool isdigit(CharT ch, const std::locale& loc)
{
    return std::use_facet<std::ctype<CharT>>(loc).is(std::ctype_base::digit, ch);
}

Example

#include <iostream>
#include <locale>
#include <string>
#include <unordered_set>

struct jdigit_ctype : std::ctype<wchar_t>
{
    std::unordered_set<wchar_t> jdigits{
        L'一', L'二', L'三', L'四', L'五', L'六', L'七', L'八', L'九', L'十'
    };

    bool do_is(mask m, char_type c) const override
    {
        return (m & digit) && jdigits.contains(c)
            ? true // Japanese digits will be classified as digits
            : ctype::do_is(m, c); // leave the rest to the parent class
    }
};

int main()
{
    std::wstring text = L"123一二三１２３";
    std::locale loc(std::locale(""), new jdigit_ctype);

    std::locale::global(std::locale("en_US.utf8"));
    std::wcout.imbue(std::locale());

    for (const wchar_t c : text)
        if (std::isdigit(c, loc))
            std::wcout << c << " is a digit\n";
        else
            std::wcout << c << " is NOT a digit\n";
}

Possible output: 

1 is a digit
2 is a digit
3 is a digit
一 is a digit
二 is a digit
三 is a digit
１ is NOT a digit
２ is NOT a digit
３ is NOT a digit

See also