2.6 Constants and Statics

Rust provides constants for values known at compile time and statics for immutable global data. Constants remain unchanged throughout execution and must have an explicitly declared type. Unlike variables, they do not occupy a specific memory location and can be freely copied or optimized by the compiler.

Statics, by contrast, represent values stored at a fixed memory address. They are initialized once at runtime and remain available throughout the program’s execution.

2.6.1 Constants

By convention, constant names use SCREAMING_SNAKE_CASE:

const MAX_POINTS: u32 = 100_000;

fn main() {
    println!("The maximum points are: {}", MAX_POINTS);
}

2.6.2 Statics

Statics differ from constants by having a fixed memory location:

static GREETING: &str = "Hello, world!";

fn main() {
    println!("{}", GREETING);
}

Mutable statics are generally discouraged in Rust because modifying global data can introduce race conditions. If mutability is required, it must be wrapped in synchronization primitives (such as Mutex or Atomic* types), and access to it requires unsafe code.

2.6.3 Comparison with C

#define MAX_POINTS 100000
const int max_points = 100000;
  • #define is a preprocessor directive and does not enforce type safety.
  • const in C applies type safety but does not necessarily guarantee immutability at the machine level.
  • In Rust, const ensures compile-time evaluation and optimization, while static provides a persistent memory location.