Associated constant

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 kenar

Exercise 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.

Show example 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>());
}

Test yourself

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
- It's a static variable
Show answer

The value belongs to the type, not to an instance — T::KENAR is substituted at compile time; there's no runtime cost.

Constant declaration

trait Cokgen {
    ____ KENAR: u32;
}

Show answer

trait Cokgen {
    const KENAR: u32;
}

Constant and value

Ucgen::KENAR

3
Kare::KENAR

4
u8::MAX

255
i8::MIN

-128

Next lesson: GAT

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