std::fwide(3)

NAME
std::fwide - std::fwide

Synopsis
   Defined in header <cwchar>
   int fwide( std::FILE* stream, int mode );
   If mode > 0, attempts to make stream wide-oriented. If mode < 0, attempts to make
   stream byte-oriented. If mode == 0, only queries the current orientation of the
   stream.
   If the orientation of the stream has already been decided (by executing output or by
   an earlier call to fwide), this function does nothing.

Parameters
   stream - pointer to the C I/O stream to modify or query
   mode - integer value greater than zero to set the stream wide, less than zero to
            set the stream narrow, or zero to query only

Return value
   An integer greater than zero if the stream is wide-oriented after this call, less
   than zero if the stream is byte-oriented after this call, and zero if the stream has
   no orientation.

Example
   The following code sets and resets the stream orientation. // Run this code
 #include <cstdio>
 #include <cstdlib>
 #include <cwchar>
 #include <iostream>
 void show_orientation(int n)
 {
     n < 0 ? std::wcout << "\tnarrow orientation\n" :
     n > 0 ? std::wcout << "\twide orientation\n" :
             std::wcout << "\tno orientation\n";
 }
 void try_read(FILE* fp)
 {
     if (const int c = std::fgetc(fp); c == EOF)
         std::wcout << "\tnarrow character read failed\n";
     else
         std::wcout << "\tnarrow character read '" << static_cast<char>(c) << "'\n";
     if (const wint_t wc = std::fgetwc(fp); wc == WEOF)
         std::wcout << "\twide character read failed\n";
     else
         std::wcout << "\twide character read '" << static_cast<wchar_t>(wc) << "'\n";
 }
 int main()
 {
     enum fwide_orientation : int { narrow = -1, query, wide };
     FILE* fp = std::fopen("main.cpp", "r");
     if (!fp)
     {
         std::wcerr << "fopen() failed\n";
         return EXIT_FAILURE;
     }
     std::wcout << "1) A newly opened stream has no orientation.\n";
     show_orientation(std::fwide(fp, fwide_orientation::query));
     std::wcout << "2) Establish byte orientation.\n";
     show_orientation(std::fwide(fp, fwide_orientation::narrow));
     try_read(fp);
     std::wcout << "3) Only freopen() can reset stream orientation.\n";
     if (std::freopen("main.cpp", "r", fp) == NULL)
     {
         std::wcerr << "freopen() failed\n";
         return EXIT_FAILURE;
     }
     std::wcout << "4) A reopened stream has no orientation.\n";
     show_orientation(std::fwide(fp, fwide_orientation::query));
     std::wcout << "5) Establish wide orientation.\n";
     show_orientation(std::fwide(fp, fwide_orientation::wide));
     try_read(fp);
     std::fclose(fp);
 }

Possible output:
 1) A newly opened stream has no orientation.
         no orientation
 2) Establish byte orientation.
         narrow orientation
         narrow character read '#'
         wide character read failed
 3) Only freopen() can reset stream orientation.
 4) A reopened stream has no orientation.
         no orientation
 5) Establish wide orientation.
         wide orientation
         narrow character read failed
         wide character read '#'

See also
   fopen opens a file
         (function)
   C documentation for
   fwide