I'm having trouble understand how to achieve following: I would like a reference to vector that i cannot change, containing data that i can change(c++ T* const equivalent) Is that achievable in rust syntax?
let mut a = Vec::new()
a.push(1) // should be allowed
a = Vec::new() // should be disallowed.
mut seems to be allowing mutability on both levels
This:
let mut a = Vec::new()
does not create a reference to a vector; rather it creates a variable bound to the vector itself. It is the equivalent to this in C++:
std::vector<int> a;
If you want an immutable reference to a mutable vector, you would have something like this:
let mut a = vec![1,2,3];
let r = &mut a;
r.push(4);
In the above snippet, r is an immutable variable bound to a reference to the mutable vector a. If you try to re-assign r to be a reference to another vector:
let mut b = vec![4,5,6];
r=&mut b;
r.push(7);
you will get this error:
9 | r=&mut b;
| ^^^^^^^^ cannot assign twice to immutable variable
Note however that due to the fact that Rust allows shadowing, you can use 'let' to create a new binding that shadows the old one in the same scope, so you could do this:
let mut a = vec![1, 2, 3];
let r = &mut a;
r.push(4);
let mut b = vec![4, 5, 6];
let r = &mut b;
r.push(7);
fn f(x: &RefCell<Vec<u8>>) { *x.borrow_mut() = Vec::new(); }Vec::pushcan copy the content of theVecto a new location and replace the oldVecwith the new one. This happens when there is not enough remaining capacity.