Function core::ptr::copy_nonoverlapping

1.0.0 (const: unstable) · source · []
pub 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.


Behavior is undefined if any of the following conditions are violated:

  • src must be valid for reads of count * size_of::<T>() bytes.

  • dst must be valid for writes of count * size_of::<T>() bytes.

  • Both src and dst must be properly aligned.

  • The region of memory beginning at src with a size of count * size_of::<T>() bytes must not overlap with the region of memory beginning at dst 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.


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`.

    unsafe {
        // The call to offset is always safe because `Vec` will never
        // allocate more than `isize::MAX` bytes.
        let dst_ptr = dst.as_mut_ptr().offset(dst_len as isize);
        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.

        // 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']);