int main()
{
  using namespace std;
  wstring ws = L"wide string";
  //setlocale(LC_ALL, "russian");
  ostream_iterator<char> osi(cout);
  transform(ws.begin(), ws.end(), osi, wctob);
}

переносимо сделать преобразование

ofstream& operator<<(ofstream&, char);

int main()
{
  using namespace std;
  wstring ws = L"wide строка.";
  //setlocale(LC_ALL, ".1251"); // хз как получить кодовую стариницу, потому альтернатива ниже
  locale::global(locale(".1251")); // кодировка для конверсии wctob
  cout << cout.getloc().name() << '\n';
  cout.imbue(locale()); // устанавливаем для стрима глобальную локаль, кодировка выбрана выше
  cout << cout.getloc().name() << '\n';
  cout.imbue(locale(".1252")); // не влияет на конверсию
  cout << cout.getloc().name() << '\n';
  ostream_iterator<char> osi(cout);
  transform(ws.begin(), ws.end(), osi, wctob);
}

C
Russian_Russia.1251
Russian_Russia.1252
wide строка.

setlocale(LC_ALL, "russian");

MC>А если локаль — utf8?

Say, for example, you have a code conversion facet called codecvt_utf8 that you want to use to output to cout a UTF-8 multibyte sequence corresponding to a wide string

Note that the Standard C++ library currently uses code conversion facets only within template class basic_filebuf, for converting from multibyte sequences when reading from a file and for converting to multibyte sequences when writing to a file

  wfstream wf("log", std::ios::out|std::ios::trunc);
  wf << L"ddd";

GN>int main()
GN>{
GN>  using namespace std;
GN>  wstring ws = L"wide string";
GN>  //setlocale(LC_ALL, "russian");
GN>  ostream_iterator<char> osi(cout);
GN>  transform(ws.begin(), ws.end(), osi, wctob);
GN>}
GN>

#include <stdlib.h>
#include <stdio.h>
#include <locale.h>
#include <wchar.h>

int main()
{
const wchar_t *str = L"Превед мир.";
char buf[256];
int i;
    setlocale(LC_ALL, "");

    i=wcstombs(buf, str, sizeof(buf));
    if (i<0) {
        printf("wcstombs: can't convert in current locale.\n");
    } else {
        printf("wcstombs: %s\n", buf);
    }

    fputs("wctomb: ", stdout);
    while (*str) {
        i=wctob(*str++);
        if (i>0) putchar(i);
    }
    putchar('\n');
    
    return 0;
}

$ gcc -Wall test.c
$ locale |grep LC_CTYPE
LC_CTYPE=ru_RU.KOI8-R
$ ./a.out 
wcstombs: Перевед мир.
wctomb: Перевед мир.
$ LC_ALL=ru_RU.UTF-8 ./a.out | iconv -f utf8
wcstombs: Перевед мир.
wctomb:  .

Description
The wctob function determines whether c corresponds to a member of the extended
character set whose multibyte character representation is a single byte when in the initial
shift state.

Returns
The wctob returns EOF if c does not correspond to a multibyte character with length
one in the initial shift state. Otherwise, it returns the single-byte representation of that
character as an unsigned char converted to an int.