Function tracing::stdlib::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());