gistfile1.cpp

struct FileChooserDialog {
  private:
   struct FileCompare {
      bool operator() (const Glib::RefPtr<Gio::File>& a, const Glib::RefPtr<Gio::File>& b) const {
         std::clog << (a->equal(b) ? "equal" : "not equal") << std::endl;
         return a->equal(b);
      }
   };

  public:
   std::set<Glib::RefPtr<Gio::File>, FileCompare> chosen_files;
};

void add_files_button_clicked_handler() {
   auto& dialog = app_data.file_chooser_dialog;
   for (auto file : dialog.dialog->get_files()) {
      auto& chosen_files = dialog.chosen_files;
      if (chosen_files.count(file) == 0) {
         chosen_files.insert(file);
      }
   }
}