Rust for C-Programmers

12.7 Additional Topics

Below are a few advanced patterns and features related to closures.

12.7.1 Returning Closures

You can return closures from functions in two ways:

Using a Trait Object

fn returns_closure() -> Box<dyn Fn(i32) -> i32> {
    Box::new(|x| x + 1)
}

Trait objects allow returning different closure types but require dynamic dispatch and potentially a heap allocation.

Using impl Trait

fn returns_closure() -> impl Fn(i32) -> i32 {
    |x| x + 1
}

Here, the compiler monomorphizes the code, often optimizing as if it were a normal function.

12.7.2 Partial Captures

Modern Rust partially captures only the fields of a struct that the closure uses, reducing unnecessary moves. This helps when you only need to capture part of a larger data structure:

struct Container {
    data: Vec<i32>,
    label: String,
}

fn main() {
    let c = Container {
        data: vec![1, 2, 3],
        label: "Numbers".to_string(),
    };

    // Only moves c.data into the closure
    let consume_data = move || {
        println!("Consumed data: {:?}", c.data);
    };

    // c.label is still accessible
    println!("Label is still available: {}", c.label);
    consume_data();
}

12.7.3 Real-World Use Cases

• GUIs: Closures as event handlers, triggered by user actions.
• Async / Futures: Passing closures to asynchronous tasks.
• Configuration / Strategy: Using closures for custom logic in libraries or frameworks.