Trait objects in Rust

A Vec of Box<dyn Trait> holds different concrete types behind one interface.

trait Check {
    fn name(&self) -> &str;
    fn passes(&self, text: &str) -> bool;
}

struct MinLength(usize);
struct NoSpaces;

impl Check for MinLength {
    fn name(&self) -> &str {
        "min length"
    }
    fn passes(&self, text: &str) -> bool {
        text.len() >= self.0
    }
}

impl Check for NoSpaces {
    fn name(&self) -> &str {
        "no spaces"
    }
    fn passes(&self, text: &str) -> bool {
        !text.contains(' ')
    }
}

fn main() {
    let checks: Vec<Box<dyn Check>> =
        vec![Box::new(MinLength(3)), Box::new(NoSpaces)];

    for candidate in ["ada", "ada lovelace", "x"] {
        let failed: Vec<&str> = checks
            .iter()
            .filter(|c| !c.passes(candidate))
            .map(|c| c.name())
            .collect();
        println!("{candidate:?} failed {:?}", failed);
    }
}

How it works

1. dyn Trait dispatches at run time through a vtable.
2. Boxing is what makes the sizes uniform.
3. Generics would force every element to be the same type.

Keywords and builtins used here

• Box
• MinLength
• NoSpaces
• Vec
• bool
• dyn
• fn
• for
• impl
• in
• let
• main
• name
• passes
• self
• str
• struct
• trait
• usize

The run, in numbers

Lines

39

Characters to type

682

Tokens

234

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 128 seconds.

Type this snippet

Step 1 of 3 in Dynamic dispatch, step 16 of 19 in Traits & generics.

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