01_variables.md

Variables

Every variable and reference is immutable by default.

Declaration

• let declares a variable
• let mut declares a mutable variable
• const declares a constant that gets inlined during compilation
• static declares a variable with a static location in memory
• static mut declares a mutable variable with a static location in memory

Variables

Variables declared using let or let mut:

fn main() {
    let x: i32 = 5;

    let mut y: i32 = 10;
    y = 12;
}

Type can be inferred:

fn main() {
    let x = -4; // i32
}

Initialization can be deferred, type annotation is optional:

fn main() {
    let z;
    // ...
    z = 6;
}

Initializing the same variable multiple times shadows the previous declaration, types can differ:

fn main() {
    let x = 5;
    // ...
    let x = "kek";
}

Statics

Static variables are declared using static in the global scope. They can be mutable, although mutable static variables is a bad practice because they're not thread safe. They must have a type annotation.

Static variables have a static place in memory and can be passed around as references.

static NICKNAME: &str = "henchbruv";
static mut PLAYERS: u32 = 0; // :(

Constants

Constants are declared using const and are values that are inlined during compilation. They can be declared in any scope. They must have a type annotation.

const MAX_VALUE: u32 = 1337;

Unless interior mutability or a static place in memory is required, constants are preferred over statics.