pub const unsafe fn read<T>(src: *const T) -> T
Expand description
Reads the value from src
without moving it. This leaves the
memory in src
unchanged.
§Safety
Behavior is undefined if any of the following conditions are violated:
-
src
must be valid for reads. -
src
must be properly aligned. Useread_unaligned
if this is not the case. -
src
must point to a properly initialized value of typeT
.
Note that even if T
has size 0
, the pointer must be non-null and properly aligned.
§Examples
Basic usage:
let x = 12;
let y = &x as *const i32;
unsafe {
assert_eq!(std::ptr::read(y), 12);
}
RunManually implement mem::swap
:
use std::ptr;
fn swap<T>(a: &mut T, b: &mut T) {
unsafe {
// Create a bitwise copy of the value at `a` in `tmp`.
let tmp = ptr::read(a);
// Exiting at this point (either by explicitly returning or by
// calling a function which panics) would cause the value in `tmp` to
// be dropped while the same value is still referenced by `a`. This
// could trigger undefined behavior if `T` is not `Copy`.
// Create a bitwise copy of the value at `b` in `a`.
// This is safe because mutable references cannot alias.
ptr::copy_nonoverlapping(b, a, 1);
// As above, exiting here could trigger undefined behavior because
// the same value is referenced by `a` and `b`.
// Move `tmp` into `b`.
ptr::write(b, tmp);
// `tmp` has been moved (`write` takes ownership of its second argument),
// so nothing is dropped implicitly here.
}
}
let mut foo = "foo".to_owned();
let mut bar = "bar".to_owned();
swap(&mut foo, &mut bar);
assert_eq!(foo, "bar");
assert_eq!(bar, "foo");
Run§Ownership of the Returned Value
read
creates a bitwise copy of T
, regardless of whether T
is Copy
.
If T
is not Copy
, using both the returned value and the value at
*src
can violate memory safety. Note that assigning to *src
counts as a
use because it will attempt to drop the value at *src
.
write()
can be used to overwrite data without causing it to be dropped.
use std::ptr;
let mut s = String::from("foo");
unsafe {
// `s2` now points to the same underlying memory as `s`.
let mut s2: String = ptr::read(&s);
assert_eq!(s2, "foo");
// Assigning to `s2` causes its original value to be dropped. Beyond
// this point, `s` must no longer be used, as the underlying memory has
// been freed.
s2 = String::default();
assert_eq!(s2, "");
// Assigning to `s` would cause the old value to be dropped again,
// resulting in undefined behavior.
// s = String::from("bar"); // ERROR
// `ptr::write` can be used to overwrite a value without dropping it.
ptr::write(&mut s, String::from("bar"));
}
assert_eq!(s, "bar");
Run