cow-type-for-clone-on-write

Cow type for clone-on-write

https://doc.rust-lang.org/std/borrow/enum.Cow.html

A Rust Cow type is a clone-on-write smart pointer. When a function receives a Cow type as an argument, the function can modify the data without actually modifying the original data structure. Instead, the Cow type makes a clone of the data when it is modified, and any other references to the original data continue to point to the original data.

The Cow type is implemented as an enum with two variants: Borrowed and Owned, which express "either a reference, or an owned type". You choose which variant you want depending on your goal.

use std::borrow::Cow;

fn main() {
    let a = ['a', 'b', 'c'];
    let mut b = Cow::Borrowed(&a);

    // The `b` Cow enum is borrowed, and points to `a`.
    match b {
        Cow::Borrowed(_) => println!("Borrowed"),
        Cow::Owned(_) => println!("Owned"),
    }

    // Convert `b` to mutable i.e. clone it, then change it.
    b.to_mut()[0] = 'x';

    // Now the `b` Cow enum is Owned i.e. has its own data.
    match b {
        Cow::Borrowed(_) => println!("Borrowed"),
        Cow::Owned(_) => println!("Owned"),
    }
}

A typical use case for Cow is optimization by not doing copies. For example, you write a function that returns a String, but there are cases when you already have a &'static str containing the data; you can return Cow::Borrowed so you don't need to allocate and copy a new String.