Constants can be declared in the global scope using the static keyword, the
type annotation is obligatory in this case. These constants are placed in a
read-only section of the memory and can be accessed in any other part of the
program.
String literals like "string" can also be assigned to static variables. These
variables have type signature &'static str, and are references to strings
allocated in read-only memory. 'static is a special lifetime that outlives
all the other lifetimes, and indicates that the referenced data is available in
all the scopes.
static LANGUAGE: &'static str = "Rust";
static THRESHOLD: i32 = 10;
fn is_big(n: i32) -> bool {
// Access constant in some function
n > THRESHOLD
}
fn main() {
let n = 16;
// Access constant in the main thread
println!("This is {}", LANGUAGE);
println!("The threshold is {}", THRESHOLD);
println!("{} is {}", n, if is_big(n) { "big" } else { "small" });
// Error! Cannot modify a static item
THRESHOLD = 5;
// FIXME ^ Comment out this line
{
// String literals are references to read-only memory
let _static_string: &'static str = "In read-only memory";
// When `_static_string` goes out of scope, we can no longer refer to
// the underlying data, but the string remains in the read-only memory
}
}