Shadowing uses let to create a new variable with the same name as an existing variable. The new binding hides the earlier one, but does not destroy its value. Shadowing resembles mutability, yet it creates a different variable rather than changing the original. section 1.11 section 1.12
fn main() {
let my_number = 8; #1
println!("{}", my_number);
let my_number = 9.2; #2
println!("{}", my_number);
}
This minimal example binds my_number first to an integer and then shadows it with a floating-point value. section 1.11
Bindings follow scopes, the regions enclosed by braces. Within one scope, the newest binding is visible. A binding created inside a nested scope hides the outer binding only until that inner scope ends; afterward, the outer binding is visible again. section 1.11 section 1.12
fn main() {
let my_number = 8;
println!("{}", my_number);
{
let my_number = 9.2;
println!("{}", my_number); #1
}
println!("{}", my_number); #2
}
The inner my_number is visible inside its block, while the original binding becomes visible again after the block closes. section 1.11
Shadowing is useful when:
- A sequence of transformations makes intermediate names unnecessary, allowing one meaningful name to represent each successive value. section 1.11
- A value needs a new type, such as converting an integer binding into a floating-point binding. section 1.11
- A final expression returns the latest binding from a block, such as a value built through multiplication and addition. section 1.11
fn main() {
let country = String::from("Austria");
let country_ref = &country;
let country = 8;
println!("{country_ref} {country}");
}
A reference can continue pointing to the original String after country is shadowed by the integer 8, demonstrating that shadowing does not destroy the earlier value. section 2.6
Remember the distinction: mutability changes a binding's value, while shadowing creates a new binding that blocks the old name. The earlier value can remain available through another reference or after an inner scope ends. section 1.11 section 2.6
