std::btowc(3)

NAME
std::btowc - std::btowc

Synopsis
   Defined in header <cwchar>
   std::wint_t btowc( int c );
   Widens a single-byte character c to its wide character equivalent.
   Most multibyte character encodings use single-byte codes to represent the characters
   from the ASCII character set. This function may be used to convert such characters
   to wchar_t.

Parameters
   c - single-byte character to widen

Return value
   WEOF if c is EOF.
   Wide character representation of c if (unsigned char)c is a valid single-byte
   character in the initial shift state, WEOF otherwise.

Example
// Run this code
 #include <clocale>
 #include <cwchar>
 #include <iostream>
 void try_widen(char c)
 {
     std::wint_t w = std::btowc(c);
     if (w != WEOF)
         std::cout << "The single-byte character " << +(unsigned char)c
                   << " widens to " << +w << '\n';
     else
         std::cout << "The single-byte character " << +(unsigned char)c
                   << " failed to widen\n";
 }
 int main()
 {
     std::setlocale(LC_ALL, "lt_LT.iso88594");
     std::cout << std::hex << std::showbase << "In Lithuanian ISO-8859-4 locale:\n";
     try_widen('A');
     try_widen('\xdf'); // German letter ß (U+00df) in ISO-8859-4
     try_widen('\xf9'); // Lithuanian letter ų (U+0173) in ISO-8859-4
     std::setlocale(LC_ALL, "lt_LT.utf8");
     std::cout << "In Lithuanian UTF-8 locale:\n";
     try_widen('A');
     try_widen('\xdf');
     try_widen('\xf9');
 }

Possible output:
 In Lithuanian ISO-8859-4 locale:
 The single-byte character 0x41 widens to 0x41
 The single-byte character 0xdf widens to 0xdf
 The single-byte character 0xf9 widens to 0x173
 In Lithuanian UTF-8 locale:
 The single-byte character 0x41 widens to 0x41
 The single-byte character 0xdf failed to widen
 The single-byte character 0xf9 failed to widen

See also
   wctob narrows a wide character to a single-byte narrow character, if possible
             (function)
   do_widen converts a character or characters from char to CharT
   [virtual] (virtual protected member function of std::ctype<CharT>)
   C documentation for
   btowc