Defining and Instantiating Structs
Structs are similar to tuples, discussed in “The Tuple Type” section, in that both hold multiple related values. Like tuples, the pieces of a struct can be different types. Unlike with tuples, in a struct you’ll name each piece of data so it’s clear what the values mean. Adding these names means that 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.
struct User { active: bool, username: String, email: String, sign_in_count: u64, } fn main() {}
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.
struct User { active: bool, username: String, email: String, sign_in_count: u64, } fn main() { let user1 = User { email: String::from("someone@example.com"), username: String::from("someusername123"), active: true, sign_in_count: 1, }; }
To get a specific value from a struct, we use dot notation. For example, to
access this user’s email address, we use user1.email
. 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.
struct User { active: bool, username: String, email: String, sign_in_count: u64, } fn main() { 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"); }
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
.
struct User { active: bool, username: String, email: String, sign_in_count: u64, } fn build_user(email: String, username: String) -> User { User { email: email, username: username, active: true, sign_in_count: 1, } } fn main() { let user1 = build_user( String::from("someone@example.com"), String::from("someusername123"), ); }
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
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.
struct User { active: bool, username: String, email: String, sign_in_count: u64, } fn build_user(email: String, username: String) -> User { User { email, username, active: true, sign_in_count: 1, } } fn main() { let user1 = build_user( String::from("someone@example.com"), String::from("someusername123"), ); }
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 includes most of the values from another instance, but changes some. You can do this using struct update syntax.
First, in Listing 5-6 we show how to create a new User
instance in user2
regularly, without the update syntax. We set a new value for email
but
otherwise use the same values from user1
that we created in Listing 5-2.
struct User { active: bool, username: String, email: String, sign_in_count: u64, } fn main() { // --snip-- let user1 = User { email: String::from("someone@example.com"), username: String::from("someusername123"), active: true, sign_in_count: 1, }; let user2 = User { active: user1.active, username: user1.username, email: String::from("another@example.com"), sign_in_count: user1.sign_in_count, }; }
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.
struct User { active: bool, username: String, email: String, sign_in_count: u64, } fn main() { // --snip-- 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"), ..user1 }; }
The code in Listing 5-7 also creates an instance in user2
that has a
different value for email
but has the same values for the username
,
active
, and sign_in_count
fields from user1
. The ..user1
must come last
to specify that any remaining fields should get their values from the
corresponding fields in user1
, but we can choose to specify values for as
many fields as we want in any order, regardless of the order of the fields in
the struct’s definition.
Note that the struct update syntax uses =
like an assignment; this is
because it moves the data, just as we saw in the “Ways Variables and Data
Interact: Move” section. In this example, we can no
longer use user1
after creating user2
because the String
in the
username
field of user1
was moved into user2
. If we had given user2
new
String
values for both email
and username
, and thus only used the
active
and sign_in_count
values from user1
, then user1
would still be
valid after creating user2
. The types of active
and sign_in_count
are
types that implement the Copy
trait, so the behavior we discussed in the
“Stack-Only Data: Copy” section would apply.
Using Tuple Structs without Named Fields to Create Different Types
Rust also supports 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 a different type from other tuples, and when 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 we define and use
two tuple structs named Color
and Point
:
struct Color(i32, i32, i32); struct Point(i32, i32, i32); fn main() { 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 might 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 are similar to tuples in that you can
destructure them into their individual pieces, and you can use a .
followed
by the index to access an individual value.
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 that
we mentioned in “The Tuple Type” section. Unit-like
structs can be useful when 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. Here’s an example of declaring and instantiating a unit struct
named AlwaysEqual
:
struct AlwaysEqual; fn main() { let subject = AlwaysEqual; }
To define AlwaysEqual
, we use the struct
keyword, the name we want, then a
semicolon. No need for curly brackets or parentheses! Then we can get an
instance of AlwaysEqual
in the subject
variable in a similar way: using the
name we defined, without any curly brackets or parentheses. Imagine that later
we’ll implement behavior for this type such that every instance of
AlwaysEqual
is always equal to every instance of any other type, perhaps to
have a known result for testing purposes. We wouldn’t need any data to
implement that behavior! You’ll see in Chapter 10 how to define traits and
implement them on any type, including unit-like structs.
Ownership of Struct Data
In the
User
struct definition in Listing 5-1, we used the ownedString
type rather than the&str
string slice type. This is a deliberate choice because we want each instance 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 also 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 the following; this won’t work:
Filename: src/main.rs
struct User { active: bool, username: &str, email: &str, sign_in_count: u64, } 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:
$ cargo run Compiling structs v0.1.0 (file:///projects/structs) error[E0106]: missing lifetime specifier --> src/main.rs:3:15 | 3 | username: &str, | ^ expected named lifetime parameter | help: consider introducing a named lifetime parameter | 1 ~ struct User<'a> { 2 | active: bool, 3 ~ username: &'a str, | error[E0106]: missing lifetime specifier --> src/main.rs:4:12 | 4 | email: &str, | ^ expected named lifetime parameter | help: consider introducing a named lifetime parameter | 1 ~ struct User<'a> { 2 | active: bool, 3 | username: &str, 4 ~ email: &'a str, | For more information about this error, try `rustc --explain E0106`. error: could not compile `structs` due to 2 previous errors
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
.