Here's an example:
use std::rc::Rc;
#[derive(PartialEq, Eq)]
struct MyId;
pub fn main() {
let rc_a_0 = Rc::new(MyId);
let rc_a_1 = rc_a_0.clone();
let rc_b_0 = Rc::new(MyId);
let rc_b_1 = rc_b_0.clone();
println!("rc_a_0 == rc_a_1: {:?}", rc_a_0 == rc_a_1);
println!("rc_a_0 == rc_b_0: {:?}", rc_a_0 == rc_b_0);
}
Both println!s above print true. Is there a way distinguish between the rc_a_* and rc_b_* pointers?
2017 stabilization update (in 2020).
In Rust 1.17 and forward, you can use Rc::ptr_eq. It does the same as ptr::eq, without the need of converting the Rc to a reference or pointer.
As the other answers mention Rc::ptr_eq (and ptr::eq) checks for reference equality, i.e. whether the two references "point" to the same address.
let five = Rc::new(5);
let same_five = Rc::clone(&five);
let other_five = Rc::new(5);
// five and same_five reference the same value in memory
assert!(Rc::ptr_eq(&five, &same_five));
// five and other_five does not reference the same value in memory
assert!(!Rc::ptr_eq(&five, &other_five));
The example is from the Rust Rc::ptr_eq docs.
Rc implements PartialEq, so simply use == as always, to perform value equality, i.e. whether the values are equal, irrelevant of whether they reference the same address in memory.
use std::rc::Rc;
let five = Rc::new(5);
let other_five = Rc::new(5);
let ten = Rc::new(10);
assert!(five == other_five);
assert!(ten != five);
assert!(ten != other_five);
You can cast &*rc to *const T to get a pointer to the underlying data and compare the value of those pointers:
use std::rc::Rc;
#[derive(PartialEq, Eq)]
struct MyId;
pub fn main() {
let rc_a_0 = Rc::new(MyId);
let rc_a_1 = rc_a_0.clone();
let rc_b_0 = Rc::new(MyId);
let rc_b_1 = rc_b_0.clone();
println!(
"rc_a_0 == rc_a_1: {:?}",
&*rc_a_0 as *const MyId == &*rc_a_1 as *const MyId
);
println!(
"rc_a_0 == rc_b_0: {:?}",
&*rc_a_0 as *const MyId == &*rc_b_0 as *const MyId
);
}
prints
rc_a_0 == rc_a_1: true
rc_a_0 == rc_b_0: false
The same answer as Dogbert, but maybe a bit cleaner:
use std::ptr;
println!(
"rc_a_0 == rc_a_1: {:?}",
ptr::eq(rc_a_0.as_ref(), rc_a_1.as_ref())
);
println!(
"rc_a_0 == rc_b_0: {:?}",
ptr::eq(rc_a_0.as_ref(), rc_b_0.as_ref())
);
rc_a_0 == rc_a_1: true
rc_a_0 == rc_b_0: false
In short, you want reference equality, not value equality. A raw pointer's value is the memory address, so comparing the value of a raw pointer is equivalent to reference equality.
See also: