rust - Newtype pattern of the combinations of Rc, RefCell and Box -

because don't want type code rc::new(refcell::new(box::new(mytype::new(args...)))) again , again, decided create newtype rcrefbox below:

use std::rc::rc; use std::cell::refcell;  struct rcrefbox<t>(rc<refcell<box<t>>>);  impl<t> rcrefbox<t> {     fn new(value: box<t>) -> rcrefbox<t> {         rcrefbox(rc::new(refcell::new(value)))     } }  trait interface {}  struct a; impl interface {}  fn main() {     let iface: rcrefbox<interface> = rcrefbox::new(box::new(a)); } 

the code doesn't compile errors below: (playpen: http://is.gd/itir8q)

<anon>:19:16: 19:35 error: trait `core::marker::sized` not implemented type `interface` [e0277] <anon>:19     let iface: rcrefbox<interface> = rcrefbox::new(box::new(a));                          ^~~~~~~~~~~~~~~~~~~ <anon>:19:16: 19:35 note: `interface` not have constant size known @ compile-time <anon>:19     let iface: rcrefbox<interface> = rcrefbox::new(box::new(a));                      ^~~~~~~~~~~~~~~~~~~ <anon>:19:38: 19:51 error: trait `core::marker::sized` not implemented type `interface` [e0277] <anon>:19     let iface: rcrefbox<interface> = rcrefbox::new(box::new(a));                                                ^~~~~~~~~~~~~ <anon>:19:38: 19:51 note: `interface` not have constant size known @ compile-time <anon>:19     let iface: rcrefbox<interface> = rcrefbox::new(box::new(a));                                                ^~~~~~~~~~~~~ error: aborting due 2 previous errors 

how can fix errors?

the problem not struct itself, trait pass.

trait objects dynamically sized objects (do not implement sized).

normally, when specify bounds on generic type (t here), constrain it, when sized introduced decided applied as default bound because generic code deals sized types , result t alone means t: sized.

there special "widening" bound "t may not sized": ?sized, have apply if wish able take trait objects. adding code:

use std::rc::rc; use std::cell::refcell;  struct rcrefbox<t: ?sized>(rc<refcell<box<t>>>);  // ?sized  impl<t: ?sized> rcrefbox<t> {                     // ?sized     fn new(value: box<t>) -> rcrefbox<t> {         rcrefbox(rc::new(refcell::new(value)))     } }  trait interface {}  struct a; impl interface {}  fn main() {     let iface: rcrefbox<interface> = rcrefbox::new(box::new(a)); } 

makes work (http://is.gd/pszkk2).