Associated constant
- Rust course
- Advanced
- Lesson 61/81
- 4 min
Define an associated constant.
Associated constants
Traits can declare constants too; each implementation gives its own value. The values are known at compile time and accessed through the type:
trait Cokgen {
const KENAR: u32;
const AD: &'static str;
}
struct Ucgen;
impl Cokgen for Ucgen {
const KENAR: u32 = 3;
const AD: &'static str = "üçgen";
}
fn ozet<T: Cokgen>() -> String {
format!("{}: {} kenar", T::AD, T::KENAR)
}A constant can have a default value too: const AZAMI: usize = 64;.
Usage examples
In the standard library: u32::MAX, f64::EPSILON, char::MAX. In your own code: unit conversions, protocol versions, buffer sizes. Associated constants can also be used as array sizes together with const generics:
struct Kare;
impl Cokgen for Kare {
const KENAR: u32 = 4;
const AD: &'static str = "kare";
}
println!("{}", ozet::<Kare>()); // kare: 4 kenarPractice Polygon constants
Implement the Cokgen trait above for Ucgen and Kare and write the ozet function. Print the results of ozet::<Ucgen>() and ozet::<Kare>() on separate lines.
Starter code
fn main() {
// buraya yaz
}Sample solution
trait Cokgen {
const KENAR: u32;
const AD: &'static str;
}
struct Ucgen;
struct Kare;
impl Cokgen for Ucgen {
const KENAR: u32 = 3;
const AD: &'static str = "üçgen";
}
impl Cokgen for Kare {
const KENAR: u32 = 4;
const AD: &'static str = "kare";
}
fn ozet<T: Cokgen>() -> String {
format!("{}: {} kenar", T::AD, T::KENAR)
}
fn main() {
println!("{}", ozet::<Ucgen>());
println!("{}", ozet::<Kare>());
}Expected output
üçgen: 3 kenar kare: 4 kenar
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why can associated constants be used without <code>self</code>?
- It can't be
- Because it's global
- The value belongs to the type, not to an instance Correct answer
- It's a static variable
Explanation: T::KENAR is substituted at compile time; there's no runtime cost.
2 Constant declaration
trait Cokgen {
____ KENAR: u32;
}trait Cokgen {
const KENAR: u32;
}Explanation: Constants in a trait are declared with const.
3 Constant and value
| Term | Meaning |
|---|---|
Ucgen::KENAR |
3 |
Kare::KENAR |
4 |
u8::MAX |
255 |
i8::MIN |
-128 |