Understanding Rust Strings: A Comprehensive Guide

Theoretical Foundations

Before diving into the practical aspects, it’s essential to understand the theoretical foundations of Rust strings. Both String and str are guaranteed to be valid UTF-8, which means they can represent any Unicode character.

What is String?

String is an owned type that needs to be allocated on the heap. It has a dynamic size, which means its length can change at runtime. This flexibility comes at the cost of performance, as the allocation and deallocation of memory can be expensive.

let s = String::from("Hello, world!");

A String can be modified, and its contents can be changed after creation:

let mut s = String::from("Hello, ");
s.push_str("world!");
println!("{}", s); // Output: Hello, world!

What is str?

str is a string slice that references a sequence of characters. It’s an unowned type, meaning it doesn’t own the memory it points to. Instead, it borrows the memory from another string.

let s: &str = "Hello, world!";

A str cannot be modified, as it’s a read-only view into the original string:

// This will not compile
let mut s: &str = "Hello, ";
s.push_str("world!");

Key Differences

• String is owned, while str is unowned.
• String has a dynamic size, while str has a fixed size.
• String can be modified, while str is read-only.

When to Use Each

The choice between String and str depends on the specific use case:

• Use String when you need to own and modify the string.
• Use str when you need a read-only view into a string.

By understanding the differences between String and str, you can write more efficient and effective Rust code.