Rust by Example: Constants

Constants are immutable values that are bound to a name and are not allowed to change throughout the execution of the program. Constants can be declared in any scope, including the global scope, which makes them useful for values that many parts of code need to know about.

Constants are declared using the const keyword. Naming convention for constant's is to all upper case characters and underscores for spaces.

 Run code Copy code 
const GLOBAL_CONSTANT: u32 = 100_000;

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

integer

 
    const ONE: u32 = 1;
    println!("{}", ONE);

floating-point number

 
    const PI: f32 = 3.14159;
    println!("{}", PI);

boolean

 
    const TRUE: bool = true;
    println!("{}", TRUE);

character

 
    const CRAB: char = '🦀';
    println!("{}", CRAB);

Tuples, if all members are types that are valid for constants

 
    const TUPLE: (u32, f32, bool, char) = (ONE, PI, TRUE, CRAB);
    println!("{:?}", TUPLE);

Arrays, if all members are types that are valid for constants

 
    const ARRAY: [u32; 3] = [ONE, ONE, ONE];
    println!("{:?}", ARRAY);

The compiler is able to evaluate (limited) constant expressions at compile time and insert the resulting value into the code where the constant is used. This can be helpful for numeric values that you don't want to type out completely.

 
    const SECONDS_IN_A_DAY: u32 = 60 * 60 * 24;
    println!("{}", SECONDS_IN_A_DAY);
}

$ rustc constants.rs
$ ./constants
100000
5
3.14159
true
🦀
(5, 3.14159, true, '🦀')
[5, 5, 5]
86400
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