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 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. To change a value in a struct, if the instance is mutable, we
can use the dot notation and assign 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
Field Init Shorthand when Variables Have the Same Name as Fields
If you have variables with the same names as struct fields, you can use field
init shorthand. This can make functions that create new instances of structs
more concise. The function named build_user shown here in Listing 5-4 has
parameters named email and username. The function creates and returns a
User instance:
# #![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
Because the parameter names email and username are the same as the User
struct's field names email and username, we can write build_user without
the repetition of email and username as shown in Listing 5-5. This version
of build_user behaves the same way as the one in Listing 5-4. The field init
syntax can make cases like this shorter to write, especially when structs have
many fields.
# #![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
syntax since the email and username parameters have the same name as struct
fields
Creating Instances From Other Instances With Struct Update Syntax
It's often useful to create a new instance from an old instance, using most of
the old instance's values but changing some. Listing 5-6 shows an example of
creating a new User instance in user2 by setting the values of email and
username but using the same values for the rest of the fields from the
user1 instance 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, user2, and
setting some fields to the values of the same fields from user1
The struct update syntax achieves the same effect as the code in Listing 5-6
using less code. The struct update syntax uses .. to specify that the
remaining fields not set explicitly should have the same value as the fields in
the given instance. 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 that user1 has:
# #![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
Tuple Structs without Named Fields to Create Different Types
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. The definition of a
tuple struct still starts with the struct keyword and the struct name, which
are followed by the types in the tuple. For example, here are definitions and
usages of 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. Otherwise, tuple
struct instances behave like tuples, which we covered in Chapter 3.
Unit-Like Structs without Any Fields
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
Userstruct definition in Listing 5-1, we used the ownedStringtype rather than the&strstring 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 parameterWe’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
Stringinstead of references like&str.