const fn
- Rust course
- Advanced
- Lesson 67/81
- 4 min
Write compile-time functions with const fn.
const fn
A const fn is a function that can be run at compile time. It can be used to compute the values of constants, statics and array lengths; in normal calls it works like an ordinary function:
const fn kare(x: u32) -> u32 {
x * x
}
const BOYUT: u32 = kare(4); // at compile time: 16
static TABLO: [u8; BOYUT as usize] = [0; BOYUT as usize];
let n = kare(5); // works at runtime tooA computation done at compile time adds nothing to the program's running time.
Compile-time tables
Loops and conditions can be used inside a const fn. This makes it possible to generate lookup tables at compile time:
const fn kareler<const N: usize>() -> [u32; N] {
let mut t = [0; N];
let mut i = 0;
while i < N {
t[i] = (i * i) as u32;
i += 1;
}
t
}
const TABLO: [u32; 5] = kareler::<5>(); // [0, 1, 4, 9, 16]A for loop can't be used in const context yet, because it uses the iterator trait; while is used.
Practice Compile-time size
Write const fn kare(x: u32) -> u32 and define const BOYUT: u32 = kare(4);. Create the array [0u8; BOYUT as usize]; print BOYUT and the array's length separated by a space.
Starter code
fn main() {
// buraya yaz
}Sample solution
const fn kare(x: u32) -> u32 {
x * x
}
const BOYUT: u32 = kare(4);
fn main() {
let dizi = [0u8; BOYUT as usize];
println!("{} {}", BOYUT, dizi.len());
}Expected output
16 16
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which can't be used inside a const fn?
- Arithmetic
- Heap allocation (Vec::push, Box::new) and I/O Correct answer
- if and match
- while loops
Explanation: At compile time only deterministic, side-effect-free work can be done; what's allowed grows with each release.
2 Compile-time function
____ fn kare(x: u32) -> u32 {
x * x
}
const BOYUT: u32 = kare(4);const fn kare(x: u32) -> u32 {
x * x
}
const BOYUT: u32 = kare(4);Explanation: To compute a constant value, the function must be a const fn.
3 Expression and evaluation time
| Term | Meaning |
|---|---|
const B: u32 = kare(4); |
Compile time |
let n = kare(5); |
Runtime (usually) |
[0u8; kare(2) as usize] |
Compile time (array length) |
kareler::<5>() |
[0, 1, 4, 9, 16] |