# Function std::ptr::write

1.0.0 (const: unstable) · source ·

    pub unsafe fn write<T>(dst: *mut T, src: T)

Expand description

Overwrites a memory location with the given value without reading or dropping the old value.

`write` does not drop the contents of `dst`. This is safe, but it could leak allocations or resources, so care should be taken not to overwrite an object that should be dropped.

Additionally, it does not drop `src`. Semantically, `src` is moved into the location pointed to by `dst`.

This is appropriate for initializing uninitialized memory, or overwriting memory that has previously been `read` from.

## Safety

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

- `dst` must be valid for writes.

- `dst` must be properly aligned. Use `write_unaligned` if this is not the case.

Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.

## Examples

Basic usage:

    let mut x = 0;
    let y = &mut x as *mut i32;
    let z = 12;

    unsafe {
        std::ptr::write(y, z);
        assert_eq!(std::ptr::read(y), 12);
    }

Run

Manually 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