A facet that holds a stream's filename

filename.cpp

#include <iostream>
#include <fstream>

struct filename_facet : std::locale::facet
{
private:
    std::string m_name;
public:
    filename_facet(std::string const& name, std::size_t refs = 0)
        : std::locale::facet(refs)
        , m_name(name)
    { }
    
    std::string name() const { return m_name; }
    static std::locale::id id;
};

std::locale::id filename_facet::id;

std::string fname(std::istream& ifs)
{
    return std::has_facet<filename_facet>(ifs.getloc())
        ? std::use_facet<filename_facet>(ifs.getloc()).name()
        : std::string();
}

int main()
{
    std::ifstream ifs("in.txt");
    ifs.imbue(std::locale(ifs.getloc(), new filename_facet("in.txt")));
    std::cout << fname(ifs);
}