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Rust has two main types of strings: String and &str. What is the difference?

  • &str is a simple string. When you write let my_variable = "Hello, world!", you create a &str. A &str is very fast.
  • String is a more complicated string. It is a bit slower, but it has more functions. A String is a pointer, with data on the heap.

Also note that &str has the & in front of it because you need a reference to use a str. That's because of the reason we saw above: the stack needs to know the size. So we give it a & that it knows the size of, and then it is happy. Also, because you use a & to interact with a str, you don't own it. But a String is an owned type. We will soon learn why that is important to know.

Both &str and String are UTF-8. For example, you can write:

fn main() {
    let name = "서태지"; // This is a Korean name. No problem, because a &str is UTF-8.
    let other_name = String::from("Adrian Fahrenheit Țepeș"); // Ț and ș are no problem in UTF-8.

You can see in String::from("Adrian Fahrenheit Țepeș") that it is easy to make a String from a &str. The two types are very closely linked together, even though they are different.

You can even write emojis, thanks to UTF-8.

fn main() {
    let name = "😂";
    println!("My name is actually {}", name);

On your computer that will print My name is actually 😂 unless your command line can't print it. Then it will show My name is actually �. But Rust has no problem with emojis or any other Unicode.

Let's look at the reason for using a & for strs again to make sure we understand.

  • str is a dynamically sized type (dynamically sized = the size can be different). For example, the names "서태지" and "Adrian Fahrenheit Țepeș" are not the same size:
fn main() {

    println!("A String is always {:?} bytes. It is Sized.", std::mem::size_of::<String>()); // std::mem::size_of::<Type>() gives you the size in bytes of a type
    println!("And an i8 is always {:?} bytes. It is Sized.", std::mem::size_of::<i8>());
    println!("And an f64 is always {:?} bytes. It is Sized.", std::mem::size_of::<f64>());
    println!("But a &str? It can be anything. '서태지' is {:?} bytes. It is not Sized.", std::mem::size_of_val("서태지")); // std::mem::size_of_val() gives you the size in bytes of a variable
    println!("And 'Adrian Fahrenheit Țepeș' is {:?} bytes. It is not Sized.", std::mem::size_of_val("Adrian Fahrenheit Țepeș"));

This prints:

A String is always 24 bytes. It is Sized.
And an i8 is always 1 bytes. It is Sized.
And an f64 is always 8 bytes. It is Sized.
But a &str? It can be anything. '서태지' is 9 bytes. It is not Sized.
And 'Adrian Fahrenheit Țepeș' is 25 bytes. It is not Sized.

That is why we need a &, because & makes a pointer, and Rust knows the size of the pointer. So the pointer goes on the stack. If we wrote str, Rust wouldn't know what to do because it doesn't know the size.

There are many ways to make a String. Here are some:

  • String::from("This is the string text"); This is a method for String that takes text and creates a String.
  • "This is the string text".to_string(). This is a method for &str that makes it a String.
  • The format! macro. This is like println! except it creates a String instead of printing. So you can do this:
fn main() {
    let my_name = "Billybrobby";
    let my_country = "USA";
    let my_home = "Korea";

    let together = format!(
        "I am {} and I come from {} but I live in {}.",
        my_name, my_country, my_home

Now we have a String named together, but did not print it yet.

One other way to make a String is called .into() but it is a bit different because .into() isn't just for making a String. Some types can easily convert to and from another type using From and .into(). And if you have From, then you also have .into(). From is clearer because you already know the types: you know that String::from("Some str") is a String from a &str. But with .into(), sometimes the compiler doesn't know:

fn main() {
    let my_string = "Try to make this a String".into(); // ⚠️

Rust doesn't know what type you want, because many types can be made from a &str. It says, "I can make a &str into a lot of things. Which one do you want?"

error[E0282]: type annotations needed
 --> src\
2 |     let my_string = "Try to make this a String".into();
  |         ^^^^^^^^^ consider giving `my_string` a type

So you can do this:

fn main() {
    let my_string: String = "Try to make this a String".into();

And now you get a String.