# std::ctype<CharT>::narrow, do_narrow

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< cpp‎ | locale‎ | ctype

Defined in header `<locale>` |  |
public: char narrow( CharT c, char dflt ) const; | (1) |
public: const CharT* narrow( const CharT* beg, const CharT* end, char dflt, char* dst ) const; | (2) |
protected: virtual char do_narrow( CharT c, char dflt ) const; | (3) |
protected: virtual const CharT* do_narrow( const CharT* beg, const CharT* end, char dflt, char* dst ) const; | (4) |

1,2) Public member function, calls the corresponding protected virtual member function `do_narrow` overload of the most derived class. Overload (1) calls do_narrow(c, dflt), overload (2) calls do_narrow(beg, end, dflt, dst).
3) Converts the (possibly wide) character c to multibyte representation if the character can be represented with a single byte (for example, ASCII characters in UTF-8 encoding are single bytes). Returns dflt if such conversion does not exist.
4) For every character in the character array `[`beg`,` end`)`, writes narrowed characters (or dflt whenever narrowing fails) to the successive locations in the character array pointed to by dst.

Narrowing is always successful and is always reversible (by calling widen()) for all characters from the basic source character set (until C++23) basic character set (since C++23).

- i.e. do_widen(do_narrow(c, 0)) == c always holds for any character c in the basic source character set (until C++23) basic character set (since C++23).

Narrowing, if successful, preserves all character classification categories known to is().

- i.e. is(m, c) || !ctc.is(m, do_narrow(c, dflt)) is always true for any named `ctype` category with a `ctype<char>` facet ctc and `ctype_base::mask` value m (unless `do_narrow` returns dflt).

Narrowing of any digit character guarantees that if the result is subtracted from the character literal '0', the difference equals the digit value of the original character.

- i.e. for any digit character c, the expression (do_narrow(c, dflt) - '0') evaluates to the digit value of the character.

### Parameters

c | - | character to convert
dflt | - | default value to produce if the conversion fails
beg | - | pointer to the first character in an array of characters to convert
end | - | one past the end pointer for the array of characters to convert
dst | - | pointer to the first element of the array of characters to fill

### Return value

1,3) Narrowed character or dflt if narrowing fails.
2,4) end

### Example

Run this code

    #include <iostream>
    #include <locale>

    void try_narrow(const std::ctype<wchar_t>& f, wchar_t c)
    {
        char n = f.narrow(c, 0);
        if (n)
            std::wcout << '\'' << c << "' narrowed to " << +(unsigned char)n << '\n';
        else
            std::wcout << '\'' << c << "' could not be narrowed\n";
    }

    int main()
    {
        std::locale::global(std::locale("en_US.utf8"));
        std::wcout.imbue(std::locale());
        std::wcout << std::hex << std::showbase << "In US English UTF-8 locale:\n";
        auto& f = std::use_facet<std::ctype<wchar_t>>(std::locale());
        try_narrow(f, L'A');
        try_narrow(f, L'Ａ');
        try_narrow(f, L'ě');

        std::locale::global(std::locale("cs_CZ.iso88592"));
        auto& f2 = std::use_facet<std::ctype<wchar_t>>(std::locale());
        std::wcout << "In Czech ISO-8859-2 locale:\n";
        try_narrow(f2, L'A');
        try_narrow(f2, L'Ａ');
        try_narrow(f2, L'ě');
    }

Possible output:

    In US English UTF-8 locale:
    'A' narrowed to 0x41
    'Ａ' could not be narrowed
    'ě' could not be narrowed
    In Czech ISO-8859-2 locale:
    'A' narrowed to 0x41
    'Ａ' could not be narrowed
    'ě' narrowed to 0xec

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 126 | C++98 | 1. the code representing reversibility was do_widen(do_narrow(c), 0) == c 2. the code representing category preservation was is(m, c) || !ctc.is(m, do_narrow(c), dflt) | corrected both
LWG 153 | C++98 | `narrow` always called overload (4) | calls the corresponding overload

### See also

widen | invokes `do_widen` (public member function)
narrow | narrows characters (public member function of `std::basic_ios<CharT,Traits>`)
wctob | narrows a wide character to a single-byte narrow character, if possible (function)

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