Closures: Capturing Environment Variables

Snippet

Closures: Capturing Environment Variables

Closures can capture variables from their surrounding environment. Rust determines the capture mode automatically based on how the closure uses the variable: by reference, by mutable reference, or by value. The move keyword forces a closure to take ownership of captured variables, which is essential when passing closures to threads or when you need guaranteed ownership.

snippet.rs
rust
fn main() {
    let x = 4;
    
    let equal_to_x = |z| z == x;
    
    println!("Equal to 4: {}", equal_to_x(4));
    
    let y = 4;
    let owns_y = move || {
        println!("y taken: {}", y);
    };
    
    let v = vec![1, 2, 3];
    let uses_v = || println!("v.len(): {}", v.len());
    
    uses_v();
    drop(v);
}

Breakdown

let equal_to_x = |z| z == x;

Closure captures x by reference since it only reads x

let owns_y = move || { ... }

move forces ownership transfer; closure now owns y and can use it even after y goes out of scope elsewhere

let uses_v = || println!("v.len(): {}", v.len())

Captures v by reference since only reading; v can still be dropped after this closure is used

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Closures: Capturing Environment Variables

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