Function std::ptr::copy_nonoverlapping
1.0.0 (const: 1.63.0) · source · pub const unsafe fn copy_nonoverlapping<T>(
src: *const T,
dst: *mut T,
count: usize
)
Expand description
Copies count * size_of::<T>()
bytes from src
to dst
. The source
and destination must not overlap.
For regions of memory which might overlap, use copy
instead.
copy_nonoverlapping
is semantically equivalent to C’s memcpy
, but
with the argument order swapped.
The copy is “untyped” in the sense that data may be uninitialized or otherwise violate the
requirements of T
. The initialization state is preserved exactly.
§Safety
Behavior is undefined if any of the following conditions are violated:
-
src
must be valid for reads ofcount * size_of::<T>()
bytes. -
dst
must be valid for writes ofcount * size_of::<T>()
bytes. -
Both
src
anddst
must be properly aligned. -
The region of memory beginning at
src
with a size ofcount * size_of::<T>()
bytes must not overlap with the region of memory beginning atdst
with the same size.
Like read
, copy_nonoverlapping
creates a bitwise copy of T
, regardless of
whether T
is Copy
. If T
is not Copy
, using both the values
in the region beginning at *src
and the region beginning at *dst
can
violate memory safety.
Note that even if the effectively copied size (count * size_of::<T>()
) is
0
, the pointers must be non-null and properly aligned.
§Examples
Manually implement Vec::append
:
use std::ptr;
/// Moves all the elements of `src` into `dst`, leaving `src` empty.
fn append<T>(dst: &mut Vec<T>, src: &mut Vec<T>) {
let src_len = src.len();
let dst_len = dst.len();
// Ensure that `dst` has enough capacity to hold all of `src`.
dst.reserve(src_len);
unsafe {
// The call to add is always safe because `Vec` will never
// allocate more than `isize::MAX` bytes.
let dst_ptr = dst.as_mut_ptr().add(dst_len);
let src_ptr = src.as_ptr();
// Truncate `src` without dropping its contents. We do this first,
// to avoid problems in case something further down panics.
src.set_len(0);
// The two regions cannot overlap because mutable references do
// not alias, and two different vectors cannot own the same
// memory.
ptr::copy_nonoverlapping(src_ptr, dst_ptr, src_len);
// Notify `dst` that it now holds the contents of `src`.
dst.set_len(dst_len + src_len);
}
}
let mut a = vec!['r'];
let mut b = vec!['u', 's', 't'];
append(&mut a, &mut b);
assert_eq!(a, &['r', 'u', 's', 't']);
assert!(b.is_empty());
Run