Function std::ptr::drop_in_place

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

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., when writing Box/Rc/Vec), as the compiler doesn't need to prove that it's sound to elide the copy.

Unaligned values cannot be dropped in place, they must be copied to an aligned location first using ptr::read_unaligned.

Safety

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

• to_drop must be valid for reads.

• to_drop must be properly aligned.

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.

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

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());

Notice that the compiler performs this copy automatically when dropping packed structs, i.e., you do not usually have to worry about such issues unless you call drop_in_place manually.