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Constants in Rust Programming
Rust provides us with two types of constants. These are −
- const − an unchangeable value
- static − possible mutable value with static lifetime.
If we try to assign another value to an already declared const value, the compiler will throw an error.
Example
Consider the example shown below −
static LANGUAGE: &str = "TutorialsPoint-Rust";
const THRESHOLD: i32 = 10;
fn is_small(n: i32) -> bool {
n < THRESHOLD
}
fn main() {
// Error! Cannot modify a `const`.
THRESHOLD = 5;
println!("Everything worked fine!");
}In the above code, we are trying to modify the value of a variable that is already declared with a const keyword, which is not allowed.
Output
error[E0070]: invalid left-hand side of assignment --> src/main.rs:12:15 | 12| THRESHOLD = 5; | --------- ^ | | | cannot assign to this expression
We can also access the constants that we declared in other functions.
Example
Consider the example shown below −
static LANGUAGE: &str = "TutorialsPoint-Rust";
const THRESHOLD: i32 = 11;
fn is_small(n: i32) -> bool {
n < THRESHOLD
}
fn main() {
let n = 19;
println!("This is {}", LANGUAGE);
println!("The threshold is {}", THRESHOLD);
println!("{} is {}", n, if is_small(n) { "small" } else { "big" });
}Output
This is TutorialsPoint-Rust The threshold is 11 19 is big
Updated on: 03-Apr-2021
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