1. Rc <T> pointer reference count
Rc <T> is the count of pointer reference, may be used clone
so that the pointer points to the data having a plurality of owners.
enum List {
Cons(i32, Rc<List>),
Nil,
}
use List::{Cons, Nil};
use std::rc::Rc;
fn main() {
let a = Rc::new(Cons(5, Rc::new(Cons(10, Rc::new(Nil)))));
let b = Cons(3, Rc::clone(&a));
let c = Cons(4, Rc::clone(&a));
}
Note that, Rc <T> pointer data is immutable
2. RefCell <T> internal pointer variable
RefCell<T>
Borrowing compile pointer can bypass check, and an ordinary pointer references the same time only one or a plurality of variable reference immutable reference. And RefCell<T>
this examination be postponed until runtime. If there are references in violation of the principle of borrowing, the program will panic!
use std::cell::RefCell;
struct S{
data:i32
}
fn main() {
let s = RefCell::new(S{data:3});
s.borrow_mut().data=5;
println!("{}",s.borrow().data);
}
By &self.borrow()
obtaining a borrow immutable, &self.borrrow_mut()
to obtain a variable borrowing.
use std::cell::RefCell;
struct S{
data:i32
}
fn main() {
let s = RefCell::new(S{data:3});
let s1=s.borrow_mut();
let s2 = s.borrow_mut();
}
s two variable references, which is a clear violation of the principle borrowed reference. So after running you will get a panic.
thread 'main' panicked at 'already borrowed: BorrowMutError', src\libcore\result.rs:997:5
stack backtrace:
0: std::sys_common::backtrace::_print
at src\libstd\sys\windows\backtrace/mod.rs:95
at src\libstd\sys\windows\backtrace/mod.rs:82
at src\libstd\sys_common/backtrace.rs:71
1: std::panicking::default_hook::{{closure}}
at src\libstd\sys_common/backtrace.rs:59
at src\libstd/panicking.rs:197
2: std::panicking::default_hook
at src\libstd/panicking.rs:211
3: std::panicking::rust_panic_with_hook
at src\libstd/panicking.rs:474
4: std::panicking::continue_panic_fmt
at src\libstd/panicking.rs:381
5: rust_begin_unwind
at src\libstd/panicking.rs:308
6: core::panicking::panic_fmt
at src\libcore/panicking.rs:85
7: core::result::unwrap_failed
at /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e\src\libcore/macros.rs:18
8: core::result::Result<T,E>::expect
at /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e\src\libcore/result.rs:825
9: core::cell::RefCell<T>::borrow_mut
at /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e\src\libcore/cell.rs:873
10: untitled::main
at src/main.rs:9
11: std::rt::lang_start::{{closure}}
at /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e\src\libstd/rt.rs:64
12: std::panicking::try::do_call
at src\libstd/rt.rs:49
at src\libstd/panicking.rs:293
13: _rust_maybe_catch_panic
at src\libpanic_unwind/lib.rs:87
14: std::rt::lang_start_internal
at src\libstd/panicking.rs:272
at src\libstd/panic.rs:388
at src\libstd/rt.rs:48
15: std::rt::lang_start
at /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e\src\libstd/rt.rs:64
16: main
17: _tmainCRTStartup
18: mainCRTStartup
19: unit_addrs_search
20: unit_addrs_search
3. Rc <T> and RefCell <T> construct having a plurality of variable reference owners
use std::cell::RefCell;
use std::rc::Rc;
struct S{
data:i32
}
fn main() {
let s = Rc::new( RefCell::new(S{data:3}));
let s1 = Rc::clone(&s);
s1.borrow_mut().data=43;
println!("{}",RefCell::borrow(&s).data);
}
Output 43