Rust - Borrowing


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It is very inconvenient to pass the ownership of a variable to another function and then return the ownership. Rust supports a concept, borrowing, where the ownership of a value is transferred temporarily to an entity and then returned to the original owner entity.

Consider the following −

fn main(){
   // a list of nos
   let v = vec![10,20,30];
   print_vector(v);
   println!("{}",v[0]); // this line gives error
}
fn print_vector(x:Vec<i32>){
   println!("Inside print_vector function {:?}",x);
}

The main function invokes a function print_vector(). A vector is passed as parameter to this function. The ownership of the vector is also passed to the print_vector() function from the main(). The above code will result in an error as shown below when the main() function tries to access the vector v.

|  print_vector(v);
|     - value moved here
|  println!("{}",v[0]);
|     ^ value used here after move

This is because a variable or value can no longer be used by the function that originally owned it once the ownership is transferred to another function.

What is Borrowing?

When a function transfers its control over a variable/value to another function temporarily, for a while, it is called borrowing. This is achieved by passing a reference to the variable (& var_name) rather than passing the variable/value itself to the function. The ownership of the variable/ value is transferred to the original owner of the variable after the function to which the control was passed completes execution.

fn main(){
   // a list of nos
   let v = vec![10,20,30];
   print_vector(&v); // passing reference
   println!("Printing the value from main() v[0]={}",v[0]);
}
fn print_vector(x:&Vec<i32>){
   println!("Inside print_vector function {:?}",x);
}

Output

Inside print_vector function [10, 20, 30]
Printing the value from main() v[0] = 10

Mutable References

A function can modify a borrowed resource by using a mutable reference to such resource. A mutable reference is prefixed with &mut. Mutable references can operate only on mutable variables.

Illustration: Mutating an integer reference

fn add_one(e: &mut i32) {
   *e+= 1;
}
fn main() {
   let mut i = 3;
   add_one(&mut i);
   println!("{}", i);
}

The main() function declares a mutable integer variable i and passes a mutable reference of i to the add_one(). The add_one() increments the value of the variable i by one.

Illustration: Mutating a string reference

fn main() {
   let mut name:String = String::from("TutorialsPoint");
   display(&mut name); 
   //pass a mutable reference of name
   println!("The value of name after modification is:{}",name);
}
fn display(param_name:&mut String){
   println!("param_name value is :{}",param_name);
   param_name.push_str(" Rocks"); 
   //Modify the actual string,name
}

The main() function passes a mutable reference of the variable name to the display() function. The display function appends an additional string to the original name variable.

Output

param_name value is :TutorialsPoint
The value of name after modification is:TutorialsPoint Rocks
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