21.1 可变性
Mutability of data can be changed when ownership is transferred.
fn main() {
let immutable_box = box 5u;
println!("immutable_box contains {}", immutable_box);
// Mutability error
//*immutable_box = 4;
// Hand over the box, changing the mutability
let mut mutable_box = immutable_box;
println!("mutable_box contained {}", mutable_box);
// Modify the contents of the box
*mutable_box = 4;
println!("mutable_box now contains {}", mutable_box);
}