问C++11 luabind集成，功能失败

EN

struct app_t
{
    //...
    void exit();
    void reset();

    resource_mgr_t resources;
    //...
};

struct resource_mgr_t
{
    //...
    void prune();
    void purge();
    //...
};

extern app_t app;

luabind::module(state)
    [
        luabind::class_<resource_mgr_t>("resource_mgr")
        .def("prune", &resource_mgr_t::prune)
        .def("purge", &resource_mgr_t::purge)
    ];

luabind::module(state)
    [
        luabind::class_<app_t>("app")
        .def("exit", &app_t::exit)
        .def("reset", &app_t::reset)
        .def_readonly("resources", &app_t::resources)
    ];

luabind::globals(state)["app"] = &app;

app:exit()
app:reset()

app.resources:purge()

[string "app.resources:purge()"]:1: attempt to index field 'resources' (a function value)