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Is it possible / desirable to create non-copyable shared pointer analogue (to enable weak_ptr tracking / borrow-type sem

By : user6095774
Date : September 17 2020, 04:00 AM
it helps some times Unique ownership is unique, full stop. One place owns this resource and will release it when that code so chooses.
Shared ownership is shared. Multiple places can own this resource, and the resource will only be released when all of them have done so. This is a binary state: either one place owns the resource or multiple places do.
code :

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Can I borrow a pointer to a shared trait in Rust?

By : Michael Bateman
Date : March 29 2020, 07:55 AM
Hope that helps This is a known bug in the current version of Rust:
#3794: Casting to a trait doesn't auto-coerce to a &T type

Cannot borrow pointer even if previous borrow is "forgotten"?

By : user3060885
Date : March 29 2020, 07:55 AM
wish of those help Currently, borrow scopes are lexical, that is, they correspond to the braces in the code. There's a bug to change this to account for cases like your fn1, but like you see in fn2, there are workarounds for now.
For fn3, if you try to use y again, you will get this error:
code :
error: use of moved value: `*y`

Does it make sense to create a `Copyable` type interface instead of using `Cloneable`?

By : user2927754
Date : March 29 2020, 07:55 AM
Hope that helps To solve your problem you need to polymorphically make a copy of dataObject like this:
code :
T cloneOfDataObject = dataObject.clone();
interface Copyable<T> {
    public T clone();
public class DangerousCallWrapper<T extends Copyable<T>> 
    implements DangerousCaller<T> {

How to ensure end of immutable borrow after function call in order to enable mutable borrow?

By : user3058424
Date : March 29 2020, 07:55 AM
Hope that helps It seems that the problem was with the Sum trait's lifetime parameter, and here is a solution without removing this trait
code :
fn compute_mean_of_vec<'g, T>(input_vec: &'g Vec<T>) -> T
    for<'x> T: Copy
        + num::Zero
        + std::ops::Add<T, Output = T>
        + std::ops::Div<T, Output = T>
        + num::FromPrimitive
        + std::iter::Sum<&'x T>,
    let sum: T = input_vec.iter().sum();
    sum / num::FromPrimitive::from_usize(input_vec.len()).unwrap()

fn normalise_cost_vec<'a, T>(cost_vec: &'a mut Vec<T>)
    for<'x> T: std::ops::SubAssign
        + Copy
        + num::traits::identities::Zero
        + std::ops::Div<Output = T>
        + num::traits::cast::FromPrimitive
        + std::iter::Sum<&'x T>,
    let mean = compute_mean_of_vec(cost_vec);
    for c in cost_vec.iter_mut() {
        *c -= mean;

fn main() {
    let mut my_vec = vec![5.0f32; 5];
    normalise_cost_vec(&mut my_vec);
    for e in my_vec.iter() {
        println!("{}", e);

weak_ptr<User_Class> is none pointer type

By : Sanyika Blg Balogh
Date : March 29 2020, 07:55 AM
This might help you You don't want to use std::weak_ptr to maintain memory. In fact, it is used to release memory as soon as possible while being able to possible retrieve an object if it is used elsewhere. Depending on your ownership semantics, you want to use std::unique_ptr for a single owner of the object or std::shared_ptr for shared ownership of the object.
The idea of std::weak_ptr is that you might have the need to find objects if they do exist but you don't want to cling onto these objects to strongly. That is, if there is no other owner, i.e., std::shared_ptr, of an object, the object is released although there is a std::weak_ptr pointing to it. The std::weak_ptr gets informed about the other object being released, however. To use an object pointed to by a std::weak_ptr, you need to convert it into a std::shared_ptr after you made sure that it indeed points to an object which is still alive (if the object referenced from a std::weak_ptr doesn't exist anymore when converting it to a std::shared_ptr, an exception gets thrown).
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