# Function std::ptr::drop_in_place

1.8.0 · source ·

    pub unsafe fn drop_in_place<T>(to_drop: *mut T)where
        T: ?Sized,

Expand description

Executes the destructor (if any) of the pointed-to value.

This is semantically equivalent to calling `ptr::read` and discarding the result, but has the following advantages:

- It is required to use `drop_in_place` to drop unsized types like trait objects, because they can’t be read out onto the stack and dropped normally.

- It is friendlier to the optimizer to do this over `ptr::read` when dropping manually allocated memory (e.g., in the implementations of `Box`/`Rc`/`Vec`), as the compiler doesn’t need to prove that it’s sound to elide the copy.

- It can be used to drop pinned data when `T` is not `repr(packed)` (pinned data must not be moved before it is dropped).

Unaligned values cannot be dropped in place, they must be copied to an aligned location first using `ptr::read_unaligned`. For packed structs, this move is done automatically by the compiler. This means the fields of packed structs are not dropped in-place.

## Safety

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

- `to_drop` must be valid for both reads and writes.

- `to_drop` must be properly aligned, even if `T` has size 0.

- `to_drop` must be nonnull, even if `T` has size 0.

- The value `to_drop` points to must be valid for dropping, which may mean it must uphold additional invariants. These invariants depend on the type of the value being dropped. For instance, when dropping a Box, the box’s pointer to the heap must be valid.

- While `drop_in_place` is executing, the only way to access parts of `to_drop` is through the `&mut self` references supplied to the `Drop::drop` methods that `drop_in_place` invokes.

Additionally, if `T` is not `Copy`, using the pointed-to value after calling `drop_in_place` can cause undefined behavior. Note that `*to_drop = foo` counts as a use because it will cause the value to be dropped again. `write()` can be used to overwrite data without causing it to be dropped.

## Examples

Manually remove the last item from a vector:

    use std::ptr;
    use std::rc::Rc;

    let last = Rc::new(1);
    let weak = Rc::downgrade(&last);

    let mut v = vec![Rc::new(0), last];

    unsafe {
        // Get a raw pointer to the last element in `v`.
        let ptr = &mut v[1] as *mut _;
        // Shorten `v` to prevent the last item from being dropped. We do that first,
        // to prevent issues if the `drop_in_place` below panics.
        v.set_len(1);
        // Without a call `drop_in_place`, the last item would never be dropped,
        // and the memory it manages would be leaked.
        ptr::drop_in_place(ptr);
    }

    assert_eq!(v, &[0.into()]);

    // Ensure that the last item was dropped.
    assert!(weak.upgrade().is_none());

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