NAME¶

std::ctype::narrow,do_narrow - std::ctype::narrow,do_narrow

Synopsis¶

Defined in header <locale>
public: (1)
char narrow( CharT c, char dflt ) const;
public:

const CharT* narrow( const CharT* beg, const CharT* end, (2)

char dflt, char* dst ) const;
protected: (3)
virtual char do_narrow( CharT c, char dflt ) const;
protected:

virtual const CharT* do_narrow( const CharT* beg, const CharT* end, (4)

char dflt, char* dst ) const;

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
1. the code representing reversibility
was
do_widen(do_narrow(c), 0) == c
LWG 126 C++98 2. the code representing category corrected both
preservation was
is(m, c) || !ctc.is(m, do_narrow(c),
dflt)
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)