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

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).

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

#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

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

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