The Rust Programming Language

Structs are similar to tuples, which were discussed in Chapter 3. Like tuples, the pieces of a struct can be different types. Unlike tuples, we name each piece of data so it’s clear what the values mean. As a result of these names, structs are more flexible than tuples: we don’t have to rely on the order of the data to specify or access the values of an instance.

To define a struct, we enter the keyword struct and name the entire struct. A struct’s name should describe the significance of the pieces of data being grouped together. Then, inside curly braces, we define the names and types of the pieces of data, which we call fields. For example, Listing 5-1 shows a struct to store information about a user account:

# #![allow(unused_variables)]
#fn main() {
struct User {
    username: String,
    email: String,
    sign_in_count: u64,
    active: bool,
}
#}

Listing 5-1: A User struct definition

To use a struct after we’ve defined it, we create an instance of that struct by specifying concrete values for each of the fields. We create an instance by stating the name of the struct, and then add curly braces containing key: value pairs where the keys are the names of the fields and the values are the data we want to store in those fields. We don’t have to specify the fields in the same order in which we declared them in the struct. In other words, the struct definition is like a general template for the type, and instances fill in that template with particular data to create values of the type. For example, we can declare a particular user as shown in Listing 5-2:

# #![allow(unused_variables)]
#fn main() {
# struct User {
#     username: String,
#     email: String,
#     sign_in_count: u64,
#     active: bool,
# }
#
let user1 = User {
    email: String::from("someone@example.com"),
    username: String::from("someusername123"),
    active: true,
    sign_in_count: 1,
};
#}

Listing 5-2: Creating an instance of the User struct

To get a specific value from a struct, we can use dot notation. If we wanted just this user’s email address, we can use user1.email wherever we want to use this value. If the instance is mutable, we can change a value by using the dot notation and assigning into a particular field. Listing 5-3 shows how to change the value in the email field of a mutable User instance:

# #![allow(unused_variables)]
#fn main() {
# struct User {
#     username: String,
#     email: String,
#     sign_in_count: u64,
#     active: bool,
# }
#
let mut user1 = User {
    email: String::from("someone@example.com"),
    username: String::from("someusername123"),
    active: true,
    sign_in_count: 1,
};

user1.email = String::from("anotheremail@example.com");
#}

Listing 5-3: Changing the value in the email field of a User instance

Note that the entire instance must be mutable; Rust doesn’t allow us to mark only certain fields as mutable. Also note that as with any expression, we can construct a new instance of the struct as the last expression in the function body to implicitly return that new instance.

Listing 5-4 shows a build_user function that returns a User instance with the given email and username. The active field gets the value of true, and the sign_in_count gets a value of 1.

# #![allow(unused_variables)]
#fn main() {
# struct User {
#     username: String,
#     email: String,
#     sign_in_count: u64,
#     active: bool,
# }
#
fn build_user(email: String, username: String) -> User {
    User {
        email: email,
        username: username,
        active: true,
        sign_in_count: 1,
    }
}
#}

Listing 5-4: A build_user function that takes an email and username and returns a User instance

It makes sense to name the function arguments with the same name as the struct fields, but having to repeat the email and username field names and variables is a bit tedious. If the struct had more fields, repeating each name would get even more annoying. Luckily, there's a convenient shorthand!

Because the parameter names and the struct field names are exactly the same in Listing 5-4, we can use the field init shorthand syntax to rewrite build_user so that it behaves exactly the same but doesn’t have the repetition of email and username in the way shown in Listing 5-5.

# #![allow(unused_variables)]
#fn main() {
# struct User {
#     username: String,
#     email: String,
#     sign_in_count: u64,
#     active: bool,
# }
#
fn build_user(email: String, username: String) -> User {
    User {
        email,
        username,
        active: true,
        sign_in_count: 1,
    }
}
#}

Listing 5-5: A build_user function that uses field init shorthand since the email and username parameters have the same name as struct fields

Here, we’re creating a new instance of the User struct, which has a field named email. We want to set the email field’s value to the value in the email parameter of the build_user function. Because the email field and the email parameter have the same name, we only need to write email rather than email: email.

It’s often useful to create a new instance of a struct that uses most of an old instance’s values, but changes some. We do this using struct update syntax.

First, Listing 5-6 shows how we create a new User instance in user2 without the update syntax. We set new values for email and username, but otherwise use the same values from user1 that we created in Listing 5-2:

# #![allow(unused_variables)]
#fn main() {
# struct User {
#     username: String,
#     email: String,
#     sign_in_count: u64,
#     active: bool,
# }
#
# let user1 = User {
#     email: String::from("someone@example.com"),
#     username: String::from("someusername123"),
#     active: true,
#     sign_in_count: 1,
# };
#
let user2 = User {
    email: String::from("another@example.com"),
    username: String::from("anotherusername567"),
    active: user1.active,
    sign_in_count: user1.sign_in_count,
};
#}

Listing 5-6: Creating a new User instance using some of the values from user1

Using struct update syntax, we can achieve the same effect with less code, shown in Listing 5-7. The syntax .. specifies that the remaining fields not explicitly set should have the same value as the fields in the given instance.

# #![allow(unused_variables)]
#fn main() {
# struct User {
#     username: String,
#     email: String,
#     sign_in_count: u64,
#     active: bool,
# }
#
# let user1 = User {
#     email: String::from("someone@example.com"),
#     username: String::from("someusername123"),
#     active: true,
#     sign_in_count: 1,
# };
#
let user2 = User {
    email: String::from("another@example.com"),
    username: String::from("anotherusername567"),
    ..user1
};
#}

Listing 5-7: Using struct update syntax to set a new email and username values for a User instance but use the rest of the values from the fields of the instance in the user1 variable

The code in Listing 5-7 also creates an instance in user2 that has a different value for email and username but has the same values for the active and sign_in_count fields from user1.

We can also define structs that look similar to tuples, called tuple structs, that have the added meaning the struct name provides, but don’t have names associated with their fields, just the types of the fields. Tuple structs are useful when you want to give the whole tuple a name and make the tuple be a different type than other tuples, but naming each field as in a regular struct would be verbose or redundant.

To define a tuple struct you start with the struct keyword and the struct name followed by the types in the tuple. For example, here are definitions and usages of two tuple structs named Color and Point:

# #![allow(unused_variables)]
#fn main() {
struct Color(i32, i32, i32);
struct Point(i32, i32, i32);

let black = Color(0, 0, 0);
let origin = Point(0, 0, 0);
#}

Note that the black and origin values are different types, since they’re instances of different tuple structs. Each struct we define is its own type, even though the fields within the struct have the same types. For example, a function that takes a parameter of type Color cannot take a Point as an argument, even though both types are made up of three i32 values. Otherwise, tuple struct instances behave like tuples, which we covered in Chapter 3: you can destructure them into their individual pieces, you can use a . followed by the index to access an individual value, and so on.

We can also define structs that don’t have any fields! These are called unit-like structs since they behave similarly to (), the unit type. Unit-like structs can be useful in situations such as when you need to implement a trait on some type, but you don’t have any data that you want to store in the type itself. We’ll be discussing traits in Chapter 10.

Ownership of Struct Data

In the User struct definition in Listing 5-1, we used the owned String type rather than the &str string slice type. This is a deliberate choice because we want instances of this struct to own all of its data and for that data to be valid for as long as the entire struct is valid.

It’s possible for structs to store references to data owned by something else, but to do so requires the use of lifetimes, a Rust feature that is discussed in Chapter 10. Lifetimes ensure that the data referenced by a struct is valid for as long as the struct is. Let’s say you try to store a reference in a struct without specifying lifetimes, like this:

Filename: src/main.rs

struct User {
    username: &str,
    email: &str,
    sign_in_count: u64,
    active: bool,
}

fn main() {
    let user1 = User {
        email: "someone@example.com",
        username: "someusername123",
        active: true,
        sign_in_count: 1,
    };
}

The compiler will complain that it needs lifetime specifiers:

error[E0106]: missing lifetime specifier
 -->
  |
2 |     username: &str,
  |               ^ expected lifetime parameter

error[E0106]: missing lifetime specifier
 -->
  |
3 |     email: &str,
  |            ^ expected lifetime parameter

We’ll discuss how to fix these errors so you can store references in structs in Chapter 10, but for now, we’ll fix errors like these using owned types like String instead of references like &str.