Synchronization of locales between streams.

gistfile1.cpp

#include <iostream>
#include <sstream>

static int tied_stream()
{
    static int i = std::ios_base::xalloc(); return i;
}

static int first_check()
{
    static int i = std::ios_base::xalloc(); return i;
}

void sync_locales(std::ios_base::event evt, std::ios_base& str, int index)
{
    if (evt & std::ios_base::imbue_event)
    {
        void*& p = str.pword(index);
        if (p)
        {
            std::ios_base& other = *static_cast<std::ios_base*>(p);
            other.imbue(str.getloc());
        }
    }
}

void tie_locales(std::ios_base& src, std::ios_base& dest)
{
    static long&  done   = src.iword(first_check());
    static void*& stream = dest.pword(tied_stream());

    if (!done)
    {
        dest.imbue(src.getloc());
        done = true;
    }

    if (!stream)
    {
        stream = &src;
        dest.register_callback(sync_locales, tied_stream());
    }
}

int main()
{
    std::ostringstream o1, o2;
    o1.imbue(std::locale("en_US.UTF8"));

    std::cout << std::boolalpha;
    std::cout << "First check: " << (o1.getloc() == o2.getloc()) << std::endl;

    tie_locales(o1, o2);

    std::cout << "Second check: " << (o1.getloc() == o2.getloc()) << std::endl;
}