The Rust language has a number of types that are considered ‘primitive’. This means that they’re built-in to the language. Rust is structured in such a way that the standard library also provides a number of useful types built on top of these ones, as well, but these are the most primitive.
Rust has a built-in boolean type, named bool. It has two values, true and false:
let x = true; let y: bool = false;
A common use of booleans is in if conditionals.
You can find more documentation for bools in the standard library
documentation.
charThe char type represents a single Unicode scalar value. You can create chars
with a single tick: (')
let x = 'x'; let two_hearts = '💕';
Unlike some other languages, this means that Rust’s char is not a single byte,
but four.
You can find more documentation for chars in the standard library
documentation.
Rust has a variety of numeric types in a few categories: signed and unsigned, fixed and variable, floating-point and integer.
These types consist of two parts: the category, and the size. For example,
u16 is an unsigned type with sixteen bits of size. More bits lets you have
bigger numbers.
If a number literal has nothing to cause its type to be inferred, it defaults:
fn main() { let x = 42; // x has type i32 let y = 1.0; // y has type f64 }let x = 42; // x has type i32 let y = 1.0; // y has type f64
Here’s a list of the different numeric types, with links to their documentation in the standard library:
Let’s go over them by category:
Integer types come in two varieties: signed and unsigned. To understand the
difference, let’s consider a number with four bits of size. A signed, four-bit
number would let you store numbers from -8 to +7. Signed numbers use
“two’s complement representation”. An unsigned four bit number, since it does
not need to store negatives, can store values from 0 to +15.
Unsigned types use a u for their category, and signed types use i. The i
is for ‘integer’. So u8 is an eight-bit unsigned number, and i8 is an
eight-bit signed number.
Fixed-size types have a specific number of bits in their representation. Valid
bit sizes are 8, 16, 32, and 64. So, u32 is an unsigned, 32-bit integer,
and i64 is a signed, 64-bit integer.
Rust also provides types whose particular size depends on the underlying machine
architecture. Their range is sufficient to express the size of any collection, so
these types have ‘size’ as the category. They come in signed and unsigned varieties
which account for two types: isize and usize.
Rust also has two floating point types: f32 and f64. These correspond to
IEEE-754 single and double precision numbers.
Like many programming languages, Rust has list types to represent a sequence of things. The most basic is the array, a fixed-size list of elements of the same type. By default, arrays are immutable.
fn main() { let a = [1, 2, 3]; // a: [i32; 3] let mut m = [1, 2, 3]; // m: [i32; 3] }let a = [1, 2, 3]; // a: [i32; 3] let mut m = [1, 2, 3]; // m: [i32; 3]
Arrays have type [T; N]. We’ll talk about this T notation in the generics
section. The N is a compile-time constant, for the length of the
array.
There’s a shorthand for initializing each element of an array to the same
value. In this example, each element of a will be initialized to 0:
let a = [0; 20]; // a: [i32; 20]
You can get the number of elements in an array a with a.len():
let a = [1, 2, 3]; println!("a has {} elements", a.len());
You can access a particular element of an array with subscript notation:
fn main() { let names = ["Graydon", "Brian", "Niko"]; // names: [&str; 3] println!("The second name is: {}", names[1]); }let names = ["Graydon", "Brian", "Niko"]; // names: [&str; 3] println!("The second name is: {}", names[1]);
Subscripts start at zero, like in most programming languages, so the first name
is names[0] and the second name is names[1]. The above example prints
The second name is: Brian. If you try to use a subscript that is not in the
array, you will get an error: array access is bounds-checked at run-time. Such
errant access is the source of many bugs in other systems programming
languages.
You can find more documentation for arrays in the standard library
documentation.
A ‘slice’ is a reference to (or “view” into) another data structure. They are useful for allowing safe, efficient access to a portion of an array without copying. For example, you might want to reference only one line of a file read into memory. By nature, a slice is not created directly, but from an existing variable binding. Slices have a defined length, can be mutable or immutable.
Internally, slices are represented as a pointer to the beginning of the data and a length.
You can use a combo of & and [] to create a slice from various things. The
& indicates that slices are similar to references, which we will cover in
detail later in this section. The []s, with a range, let you define the
length of the slice:
let a = [0, 1, 2, 3, 4]; let complete = &a[..]; // A slice containing all of the elements in a let middle = &a[1..4]; // A slice of a: only the elements 1, 2, and 3
Slices have type &[T]. We’ll talk about that T when we cover
generics.
You can find more documentation for slices in the standard library documentation.
strRust’s str type is the most primitive string type. As an unsized type,
it’s not very useful by itself, but becomes useful when placed behind a
reference, like &str. We'll elaborate further when we cover
Strings and references.
You can find more documentation for str in the standard library
documentation.
A tuple is an ordered list of fixed size. Like this:
fn main() { let x = (1, "hello"); }let x = (1, "hello");
The parentheses and commas form this two-length tuple. Here’s the same code, but with the type annotated:
fn main() { let x: (i32, &str) = (1, "hello"); }let x: (i32, &str) = (1, "hello");
As you can see, the type of a tuple looks like the tuple, but with each
position having a type name rather than the value. Careful readers will also
note that tuples are heterogeneous: we have an i32 and a &str in this tuple.
In systems programming languages, strings are a bit more complex than in other
languages. For now, read &str as a string slice, and we’ll learn more
soon.
You can assign one tuple into another, if they have the same contained types and arity. Tuples have the same arity when they have the same length.
fn main() { let mut x = (1, 2); // x: (i32, i32) let y = (2, 3); // y: (i32, i32) x = y; }let mut x = (1, 2); // x: (i32, i32) let y = (2, 3); // y: (i32, i32) x = y;
You can access the fields in a tuple through a destructuring let. Here’s an example:
fn main() { let (x, y, z) = (1, 2, 3); println!("x is {}", x); }let (x, y, z) = (1, 2, 3); println!("x is {}", x);
Remember before when I said the left-hand side of a let statement was more
powerful than assigning a binding? Here we are. We can put a pattern on
the left-hand side of the let, and if it matches up to the right-hand side,
we can assign multiple bindings at once. In this case, let “destructures”
or “breaks up” the tuple, and assigns the bits to three bindings.
This pattern is very powerful, and we’ll see it repeated more later.
You can disambiguate a single-element tuple from a value in parentheses with a comma:
fn main() { (0,); // single-element tuple (0); // zero in parentheses }(0,); // single-element tuple (0); // zero in parentheses
You can also access fields of a tuple with indexing syntax:
fn main() { let tuple = (1, 2, 3); let x = tuple.0; let y = tuple.1; let z = tuple.2; println!("x is {}", x); }let tuple = (1, 2, 3); let x = tuple.0; let y = tuple.1; let z = tuple.2; println!("x is {}", x);
Like array indexing, it starts at zero, but unlike array indexing, it uses a
., rather than []s.
You can find more documentation for tuples in the standard library documentation.
Functions also have a type! They look like this:
fn main() { fn foo(x: i32) -> i32 { x } let x: fn(i32) -> i32 = foo; }fn foo(x: i32) -> i32 { x } let x: fn(i32) -> i32 = foo;
In this case, x is a ‘function pointer’ to a function that takes an i32 and
returns an i32.