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std::ctype<CharT>::widen, do_widen

Defined in header <locale> 
public:
CharT widen( char c ) const;
 (1) 
public:
const char* widen( const char* beg, const char* end, CharT* dst ) const;
 (2) 
protected:
virtual CharT do_widen( char c ) const;
 (3) 
protected:
virtual const char* do_widen( const char* beg, const char* end, CharT* dst ) const;
 (4)
1,2) Public member function, calls the corresponding protected virtual member function do_widen overload of the most derived class. Overload (1) calls do_widen(c), overload (2) calls do_widen(beg, end, dst).
3) Converts the single-byte character c to the corresponding wide character representation using the simplest reasonable transformation. Typically, this applies only to the characters whose multibyte encoding is a single byte (e.g. U+0000-U+007F in UTF-8).
4) For every character in the character array [beg, end), writes the corresponding widened character to the successive locations in the character array pointed to by dst.

Widening always returns a wide character, but only the characters from the basic source character set (until C++23)basic character set (since C++23) are guaranteed to have a unique, well-defined, widening transformation, which is also guaranteed to be reversible (by narrow()). In practice, all characters whose multibyte representation is a single byte are usually widened to their wide character counterparts, and the rest of the possible single-byte values are usually mapped into the same placeholder value, typically CharT(-1).

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

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) widened character
2,4) end

Example

#include <locale>
#include <iostream>
 
void try_widen(const std::ctype<wchar_t>& f, char c)
{
    wchar_t w = f.widen(c);
    std::cout << "The single-byte character " << +(unsigned char)c
              << " widens to " << +w << '\n';
}
 
int main()
{
    std::locale::global(std::locale("cs_CZ.iso88592"));
    auto& f = std::use_facet<std::ctype<wchar_t>>(std::locale());
    std::cout << std::hex << std::showbase << "In Czech ISO-8859-2 locale:\n";
    try_widen(f, 'a');
    try_widen(f, '\xdf'); // German letter ß (U+00df) in ISO-8859-2
    try_widen(f, '\xec'); // Czech letter ě (U+011b) in ISO-8859-2
 
    std::locale::global(std::locale("cs_CZ.utf8"));
    auto& f2 = std::use_facet<std::ctype<wchar_t>>(std::locale());
    std::cout << "In Czech UTF-8 locale:\n";
    try_widen(f2, 'a');
    try_widen(f2, '\xdf'); 
    try_widen(f2, '\xec'); 
}

Possible output:

In Czech ISO-8859-2 locale:
The single-byte character 0x61 widens to 0x61
The single-byte character 0xdf widens to 0xdf
The single-byte character 0xec widens to 0x11b
In Czech UTF-8 locale:
The single-byte character 0x61 widens to 0x61
The single-byte character 0xdf widens to 0xffffffff
The single-byte character 0xec widens to 0xffffffff

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 153 C++98 widen always called overload (4) calls the corresponding overload

See also

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

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