How to create a vector of RwLock in rust? - rust

Question

I need to make a vector of struct. Each element of the vector have to be protected by a RwLock. Threads need to read and write in this vector ( thanks to RwLock ). How can I do that in Rust.
I've tried using a vector of Arc.
#[derive(Default, Debug, Clone, Copy)]
pub struct shared_values {
x: usize,
y: usize,
}
fn main() {
let mut shared = vec![Arc::new(RwLock::new(shared_values { x: 0, y: 0 })); 10];
//changing value of the element [0]
{
let mut sh = shared[0].write().unwrap();
*sh = shared_values { x: 10, y: 10 };
}
//Printing
{
println!("Print RwLock");
for i in 0..g.ns {
{
let val = shared[i].read().unwrap();
println!("{:?}", val);
}
}
}
}
The result is like that :
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
RwLockReadGuard { lock: RwLock { data: shared_values { suitor: 10, ws: 10 } } }
I expected the element 0 to be set with { x : 10, y : 10 }
I think Arc increase the counting reference of shared_values { x : 0 , y : 0 } but doesn't create an independant element for each index in the vector.

Answer 1

This vector initialization clones the parameter. Use std::iter::repeat_with:
use std::sync::{Arc, RwLock};
#[derive(Default, Debug, Clone, Copy)]
pub struct SharedValues {
x: usize,
y: usize,
}
fn main() {
let shared: Vec<_> =
std::iter::repeat_with(|| Arc::new(RwLock::new(SharedValues { x: 0, y: 0 })))
.take(10)
.collect();
//changing value of the element [0]
{
let mut sh = shared[0].write().unwrap();
*sh = SharedValues { x: 10, y: 10 };
}
//Printing
{
println!("Print RwLock");
for x in shared {
println!("{:?}", x.read().unwrap());
}
}
}
If you miss the simplicity of a macro, you can write your own:
macro_rules! vec_no_clone {
( $val:expr; $n:expr ) => {{
let result: Vec<_> = std::iter::repeat_with(|| $val).take($n).collect();
result
}};
}
fn main() {
let shared = vec_no_clone![Arc::new(RwLock::new(SharedValues { x: 0, y: 0 })); 10];
}