Question

#include <locale>
#include <fstream>
#include <iostream>

using namespace std;

int main()
{
    ofstream fout("test.txt");

    fout.imbue(locale("chs"));
    cout.imbue(locale("C"));

    cout.rdbuf(fout.rdbuf());
    cout << "中文"; // Which locale will apply to here? "C" or "chs"?
}

The question is commented in the code.

Answer 1

From http://www.cplusplus.com/reference/ios/ios/imbue/

std::ios::imbue <ios>

locale imbue ( const locale& loc );

Imbue locale

Associates loc to both the stream and its associated stream buffer (if
any) as the new locale object to be used with locale-sensitive
operations.

All callback functions registered with register_callback with
imbue_event as its first parameter are called.

In fact, this member function calls its inherited homonym
ios_base::imbue(loc), and if the stream is associated with a stream
buffer, also calls rdbuf()->pubimbue(loc).

Also, see http://stdcxx.apache.org/doc/stdlibug/27-4.html

27.4.4 Collaboration of Locales and Iostreams

The base class ios_base contains a locale object. The formatting and
parsing functions defined by the derived stream classes use the
numeric facets of that locale.

The class template basic_ios holds a pointer to the stream buffer.
This stream buffer has a locale object, too, usually a copy of the
same locale object used by the functions of the stream classes. The
stream buffer’s input and output functions use the code conversion
facet of the attached locale.

In your case, it would use the “C” numeric locale and the “chs” character locale.