template<class T>
class proxy
{
public:
    proxy(T &obj)
        m_obj(obj)
    {
    }

    template<typename fn, typename rt, typename ...arg_types>
    rt do_sync(arg_types ...args)
    {
        rt return_var;
        obj->*fn(std::forward<arg_types>(args)..., [this, return_var]
            (cb_arg)
            {
                return_var = cb_arg;
                m_cv.notify();
            });

        m_cv.wait();
        return return_var;
    }

private:
    T &m_obj;
    std::condition_variable m_cv;
};

// Usage example

class gameserver
{
public:
    game::map_list get_map_list(int offset, int count)
    {
        proxy<gameserver> gs_proxy(*this);
        return gs_proxy.do_sync<gameserver::get_map_list_async>(offset, count);
    }

    void get_map_list_async(int offset, int count, void(cb*)(const game::map_list &map_list))
    {
        // Impl
    }

private:
};