Rust-Usage-of-Cow

Signed-off-by: Vineel Kovvuri <[email protected]>

content/blog/Rust-Usage-of-Cow/index.md

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+---
+title: "Rust: Usage of Cow(Clone on Write)"
+date: 2025-01-17 14:03:43
+tags: ['Rust']
+---
+
+## Rust: Usage of Cow(Clone on Write)
+
+Today, After reading following blog
+post(https://hermanradtke.com/2015/05/29/creating-a-rust-function-that-returns-string-or-str.html/),
+I finally understood the need for Cow in Rust. Below is my code sample with
+relevant comments.
+
+```rust
+use std::borrow::Cow;
+
+#[derive(Clone, Debug)]
+struct Book {
+    name: String,
+    price: u32,
+}
+
+fn update_book_price<'a>(book: &'a Book, price: u32) -> Cow<'a, Book> {
+    // If the price matches, there's no need to create a new book.
+    if book.price == price {
+        return Cow::Borrowed(book);
+    }
+
+    // If the price does not match, create a new book (updating the title to
+    // make it easy to distinguish the objects).
+    Cow::Owned(Book {
+        name: format!("{}-updated", book.name.clone()),
+        price,
+    })
+}
+
+fn main() {
+    let book = Book {
+        name: "Rust programming".to_string(),
+        price: 100,
+    };
+
+    // CASE 1: Since there is no change in price, we get a `Cow` wrapped
+    // borrowed version of the existing book.
+    let updated_book: Cow<'_, Book> = update_book_price(&book, 100);
+    // The returned `Cow` object is always immutable. The following line will
+    // not work even though we can access the object fields via the `Deref`
+    // trait:
+    // updated_book.price = 200;
+    println!("{:?}", updated_book); // Book { name: "Rust programming", price: 100 }
+
+    // If we ever need to own the above object (e.g., for modifying it), we can
+    // call `into_owned()` on `Cow` to get a cloned version of the book.
+    // This cloned object will be mutable.
+    let mut updated_book: Book = updated_book.into_owned();
+    updated_book.price = 200;
+    println!("{:?}", updated_book); // Book { name: "Rust programming", price: 200 }
+
+    // ========================================
+
+    // CASE 2: Since the price has changed, we get a `Cow` wrapped owned version
+    // of the book. This owned version is the object we created in the
+    // `update_book_price()` method.
+    let updated_book: Cow<'_, Book> = update_book_price(&book, 300);
+    // Again, the returned object is immutable, so the following line will not
+    // work:
+    // updated_book.price = 400;
+    println!("{:?}", updated_book); // Book { name: "Rust programming-updated", price: 300 }
+
+    // If we ever need to own this object (e.g., for modifying it), we can call
+    // `into_owned()` on `Cow` to get the owned object. Importantly, in this
+    // case, `Cow` simply returns the already owned object instead of cloning
+    // it. The returned object will be mutable.
+    let mut updated_book: Book = updated_book.into_owned();
+    updated_book.price = 400;
+    println!("{:?}", updated_book); // Book { name: "Rust programming-updated", price: 400 }
+
+    // Takeaway 1: `Cow` allows us to defer cloning a borrowed object until it
+    // is required for modification (ownership). When ownership is needed, `Cow`
+    // provides the following behavior via `into_owned()`:
+    // 1. `Cow` either calls `clone()` on the borrowed object.
+    // 2. Or, `Cow` directly returns the underlying owned object.
+
+    // Takeaway 2: With the help of `Cow` (Clone-On-Write), we can design
+    // methods optimized to return the borrowed object wrapped inside `Cow`.
+    // This approach helps delay the cloning process. Specifically, the
+    // `update_book_price()` method returns either a borrowed `Cow` or an owned
+    // `Cow`, catering to both cases. However, this flexibility is not always
+    // necessary. Since every type `T` that `Cow` wraps requires
+    // `#[derive(Clone)]`, `Cow` can decide when to clone the borrowed object
+    // and when to return the underlying owned object directly, depending on the
+    // need when `into_owned()` is called.
+
+    // Reference: https://hermanradtke.com/2015/05/29/creating-a-rust-function-that-returns-string-or-str.html/
+}
+```

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@@ -35,6 +35,11 @@ <h1>Blog</h1>
 
 
 
+    <li>
+      <span class="date">2025/01/17</span>
+      <a href="/blog/rust-usage-of-cow/">Rust: Usage of Cow(Clone on Write) - blog</a>
+    </li>
+
     <li>
       <span class="date">2025/01/14</span>
       <a href="/blog/rust-from-vs-into-traits-part-2/">Rust: From and Into trait - Part 2 - blog</a>

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