# Function std::intrinsics::copy

1.0.0 (const: 1.63.0) · source ·

    pub const unsafe fn copy<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 may overlap.

If the source and destination will never overlap, `copy_nonoverlapping` can be used instead.

`copy` is semantically equivalent to C’s `memmove`, but with the argument order swapped. Copying takes place as if the bytes were copied from `src` to a temporary array and then copied from the array to `dst`.

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 of `count * size_of::<T>()` bytes, and must remain valid even when `dst` is written for `count * size_of::<T>()` bytes. (This means if the memory ranges overlap, the two pointers must not be subject to aliasing restrictions relative to each other.)

- `dst` must be valid for writes of `count * size_of::<T>()` bytes, and must remain valid even when `src` is read for `count * size_of::<T>()` bytes.

- Both `src` and `dst` must be properly aligned.

Like `read`, `copy` 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

Efficiently create a Rust vector from an unsafe buffer:

    use std::ptr;

    /// # Safety
    ///
    /// * `ptr` must be correctly aligned for its type and non-zero.
    /// * `ptr` must be valid for reads of `elts` contiguous elements of type `T`.
    /// * Those elements must not be used after calling this function unless `T: Copy`.
    unsafe fn from_buf_raw<T>(ptr: *const T, elts: usize) -> Vec<T> {
        let mut dst = Vec::with_capacity(elts);

        // SAFETY: Our precondition ensures the source is aligned and valid,
        // and `Vec::with_capacity` ensures that we have usable space to write them.
        ptr::copy(ptr, dst.as_mut_ptr(), elts);

        // SAFETY: We created it with this much capacity earlier,
        // and the previous `copy` has initialized these elements.
        dst.set_len(elts);
        dst
    }

Run