rust / intermediate
Snippet
Associated Types in Trait Definitions
Associated types let you declare a placeholder type within a trait. Implementors of the trait specify the concrete type. This is different from generics—a Container<T> would let callers decide the type, but with associated types, each implementation fixes one type.
snippet.rs
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trait Container {type Item;fn get(&self, index: usize) -> Option<&Self::Item>;fn len(&self) -> usize;}struct Stack<T> {items: Vec<T>,}impl<T> Container for Stack<T> {type Item = T;fn get(&self, index: usize) -> Option<&Self::Item> {self.items.get(index)}fn len(&self) -> usize {self.items.len()}}struct Matrix {data: Vec<Vec<f64>>,}impl Container for Matrix {type Item = Vec<f64>;fn get(&self, index: usize) -> Option<&Self::Item> {self.data.get(index)}fn len(&self) -> usize {self.data.len()}}fn main() {let stack: Stack<i32> = Stack { items: vec![1, 2, 3] };println!("Stack length: {}", stack.len());println!("Stack item at 1: {:?}", stack.get(1));let matrix = Matrix { data: vec![vec![1.0, 2.0], vec![3.0, 4.0]] };println!("Matrix rows: {}", matrix.len());println!("Matrix row at 0: {:?}", matrix.get(0));}
Breakdown
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type Item;
Placeholder type declared in trait; must be specified in impl
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Self::Item
References the associated type from the implementing type
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impl<T> Container for Stack<T>
Generic impl; Container::Item becomes concrete T
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type Item = Vec<f64>;
Matrix implementation uses Vec<f64> as its Item type
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stack.get(1)
Returns Option<&i32> for Stack, Option<&Vec<f64>> for Matrix