Expand description
A static item is a value which is valid for the entire duration of your
program (a 'static
lifetime).
On the surface, static
items seem very similar to const
s: both contain
a value, both require type annotations and both can only be initialized with
constant functions and values. However, static
s are notably different in
that they represent a location in memory. That means that you can have
references to static
items and potentially even modify them, making them
essentially global variables.
Static items do not call drop
at the end of the program.
There are two types of static
items: those declared in association with
the mut
keyword and those without.
Static items cannot be moved:
static VEC: Vec<u32> = vec![];
fn move_vec(v: Vec<u32>) -> Vec<u32> {
v
}
// This line causes an error
move_vec(VEC);
RunSimple static
s
Accessing non-mut
static
items is considered safe, but some
restrictions apply. Most notably, the type of a static
value needs to
implement the Sync
trait, ruling out interior mutability containers
like RefCell
. See the Reference for more information.
static FOO: [i32; 5] = [1, 2, 3, 4, 5];
let r1 = &FOO as *const _;
let r2 = &FOO as *const _;
// With a strictly read-only static, references will have the same address
assert_eq!(r1, r2);
// A static item can be used just like a variable in many cases
println!("{FOO:?}");
RunMutable static
s
If a static
item is declared with the mut
keyword, then it is allowed
to be modified by the program. However, accessing mutable static
s can
cause undefined behavior in a number of ways, for example due to data races
in a multithreaded context. As such, all accesses to mutable static
s
require an unsafe
block.
Despite their unsafety, mutable static
s are necessary in many contexts:
they can be used to represent global state shared by the whole program or in
extern
blocks to bind to variables from C libraries.
In an extern
block:
extern "C" {
static mut ERROR_MESSAGE: *mut std::os::raw::c_char;
}
RunMutable static
s, just like simple static
s, have some restrictions that
apply to them. See the Reference for more information.