Defining and Instantiating Structs - The Rust Programming Language

## Defining and Instantiating Structs

Structs are similar to tuples, which were discussed in Chapter 3. Like tuples, the pieces of a struct can be different types. Unlike with tuples, you’ll 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: you 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 brackets, we define the names and types of the pieces of data, which we call fields. For example, Listing 5-1 shows a struct that stores 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 brackets 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 could use `user1.email` wherever we wanted 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. 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 parameters 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!

### Using the Field Init Shorthand when Variables and Fields Have the Same Name

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`, as 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 because 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`.

### Creating Instances From Other Instances With Struct Update Syntax

It’s often useful to create a new instance of a struct that uses most of an old instance’s values but changes some. You’ll 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, as 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 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`.

### Using Tuple Structs without Named Fields to Create Different Types

You can also define structs that look similar to tuples, called tuple structs. Tuple structs have the added meaning the struct name provides but don’t have names associated with their fields; rather, they just have 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 from other tuples, and naming each field as in a regular struct would be verbose or redundant.

To define a tuple struct, 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, because they’re instances of different tuple structs. Each struct you 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: you can destructure them into their individual pieces, you can use a `.` followed by the index to access an individual value, and so on.

### Unit-Like Structs Without Any Fields

You can also define structs that don’t have any fields! These are called unit-like structs because they behave similarly to `()`, the unit type. Unit-like structs can be useful in situations in which you need to implement a trait on some type but don’t have any data that you want to store in the type itself. We’ll discuss 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 we’ll discuss 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, which won’t work:

> 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

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