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std::ctype::widen,do_widen(3) C++ Standard Libary std::ctype::widen,do_widen(3)


std::ctype::widen,do_widen - std::ctype::widen,do_widen


Defined in header <locale>
public: (1)
CharT widen( char c ) const;
public: (2)
const char* widen( const char* beg, const char* end, CharT* dst ) const;
protected: (3)
virtual CharT do_widen( char c ) const;
virtual const char* do_widen( const char* beg, const char* end, CharT* dst ) (4)

1,2) public member function, calls the protected virtual member function do_widen of
the most derived class.
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

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

Widening, if successful, preserves all character classification categories known to


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 char to fill

Return value

1,3) widened character
2,4) end


// Run this code

#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()
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

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');


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

See also

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