Rust: Ref Usage

2024-07-07

When using the match expression, the ref keyword allows the match arm to work with a reference instead of moving the variable.

The match Expression

match y {
    Some(ref p) => println!("Co-ordinates are {},{} ", p.x, p.y),
    _ => panic!("no match!"),
}

• Here, we use a match expression to handle the different possibilities of the Option value stored in y.

• Some(ref p): This arm matches when y contains a Some value.

  • ref p creates a reference (&Point) to the Point inside the Some variant. This avoids consuming (moving) the value out of the Option.
  • The code then prints the coordinates of the point using p.x and p.y.

• _: This is a wildcard pattern that matches any other case (i.e., None).

  • If y is None, this arm executes and panics (abruptly ends the program) with the message “no match!“.

When to Use ref

The primary purpose of ref is to create a reference (&) within a pattern. This is particularly useful in two main scenarios:

1. Pattern Matching with match, if let, and while let: When you want to match against a value and simultaneously create a reference to that value for further use within the match arm.

   let my_option: Option<i32> = Some(42);
   
   match my_option {
       Some(ref x) => println!("The value is: {}", x), // x is a reference to the i32 inside Some
       None => println!("No value found."),
   }

   In this example, ref x creates a reference to the i32 value inside the Some variant, allowing you to use x directly without consuming (moving) the value out of the Option.

2. Destructuring with let: When you want to create references to parts of a structure or tuple.

   struct Person {
       name: String,
       age: u8,
   }
   
   let person = Person { name: "Alice".to_string(), age: 30 };
   
   let Person { name: ref name_ref, age: ref age_ref } = person;
   
   println!("Name: {}, Age: {}", name_ref, age_ref); // name_ref and age_ref are references

   Here, ref name_ref and ref age_ref create references to the name and age fields of the person struct.