Rust by Example

Ownership and moves

Because variables are in charge of freeing their own resources, resources can only have one owner. This also prevents resources from being freed more than once. Note that not all variables own resources (e.g. references).

When doing assignments (let x = y) or passing function arguments by value (foo(x)), the ownership of the resources is transferred. In Rust-speak, this is known as a move.

After moving resources, the previous owner can no longer be used. This avoids creating dangling pointers.

// This function takes ownership of the heap allocated memory

fn destroy_box(c: Box) {

println!("Destroying a box that contains {}", c);

// `c` is destroyed and the memory freed

}

fn main() {

// _Stack_ allocated integer

let x = 5u32;

// *Copy* `x` into `y` - no resources are moved

let y = x;

// Both values can be independently used

println!("x is {}, and y is {}", x, y);

// `a` is a pointer to a _heap_ allocated integer

let a = Box::new(5i32);

println!("a contains: {}", a);

// *Move* `a` into `b`

let b = a;

// The pointer address of `a` is copied (not the data) into `b`.

// Both are now pointers to the same heap allocated data, but

// `b` now owns it.

// Error! `a` can no longer access the data, because it no longer owns the

// heap memory

//println!("a contains: {}", a);

// TODO ^ Try uncommenting this line

// This function takes ownership of the heap allocated memory from `b`

destroy_box(b);

// Since the heap memory has been freed at this point, this action would

// result in dereferencing freed memory, but it's forbidden by the compiler

// Error! Same reason as the previous Error

//println!("b contains: {}", b);

// TODO ^ Try uncommenting this line

}

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