A 'static lifetime is longest possible lifetime, and lasts for
the lifetime of the running program. A 'static lifetime may also be
coerced to a shorter lifetimes. There are two ways to make a variable
with 'static lifetime, and both are stored in the read-only memory
of the binary:
Make a constant with the static declaration.
Make a string literal which has type: &'static str.
See the following example for a display of each method:
// Make a constant with `'static` lifetime.
static NUM: i32 = 18;
// Returns a reference to `NUM` where its `'static`
// lifetime is coerced to that of the input argument.
fn coerce_static<'a>(_: &'a i32) -> &'a i32 {
&NUM
}
fn main() {
{
// Make a `string` literal and print it:
let static_string = "I'm in read-only memory";
println!("static_string: {}", static_string);
// When `static_string` goes out of scope, the reference
// can no longer be used, but the data remains in the binary.
}
{
// Make an integer to use for `coerce_static`:
let lifetime_num = 9;
// Coerce `NUM` to lifetime of `lifetime_num`:
let coerced_static = coerce_static(&lifetime_num);
println!("coerced_static: {}", coerced_static);
}
println!("NUM: {} stays accessible!", NUM);
}