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

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'

• i.e. for any digit character c, the expression (do_narrow(c, dflt) - '0')

Parameters

Return value

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');
}

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.

See also