CODEWITHSUNDEEP
rust
Kai Sellgren
Kai Sellgren

Reputation: 30312

Implementing a trait does not work when splitting code into files

I have a simple Rust problem that arises when modularizing the code.

The following works:

pub trait B {
    fn bar(&self) -> int;
}

pub struct A {
    foo: int
}

impl B for A {
    fn bar(&self) -> int { 5 }
}

// Later...
let a = A { foo: 5 };
println!("{}", a.bar());

It prints 5, but as soon as I modularize the code:

// lib.rs
mod a;
mod b;

// b.rs
pub trait B {
    fn bar(&self) -> int;
}

// a.rs
use b::B;

pub struct A {
    foo: int
}

impl B for A {
    fn bar(&self) -> int { 5 }
}

// Anywhere:
let test = a::A { foo: 5 };
println!("{}", test.bar());

I get a compilation error:

error: type a::A does not implement any method in scope named bar

I'm slightly puzzled.

I'm using: rustc 0.12.0-pre-nightly (0bdac78da 2014-09-01 21:31:00 +0000)

Upvotes: 7

Views: 3026

Answers (1)

amgaera
amgaera

Reputation: 1838

Trait B must be in scope whenever you want to call its methods on an object implementing it. You likely forgot to import B into the file where you use A:

// At the top:
use b::B;

// Anywhere:
let test = a::A { foo: 5 };
println!("{}", test.bar());

This answer explains why that's needed.

Upvotes: 6

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