Rust Essentials - Second Edition

Moving closure

In the previous chapter we introduced the concept of a moving closure, which takes ownership of its variables. The following code illustrates clearly the difference between a normal closure and a moving closure:

// see code in Chapter 7/code/moving_closure.rs 
struct Block { 
    number: i32 
} 
 
fn main() { 
    let block = Block{ number:1 }; 
    // ordinary closure: 
    let closure = || { println!("n: {:?}", block.number); }; 
    closure(); 
    println!("n: {:?}", block.number); 
 
    let block = Block{ number:1 }; 
    // moving closure: 
    // closure takes ownership of the block value 
    let closure = move || {println!("n: {:?}", block.number); }; 
    closure(); 
    // error: use of moved value: `block.number` 
    // println!("n: {:?}", block.number); 
} 

This prints out the following ...