core::ptr::drop_in_place - Rust
Description: API documentation for the Rust `drop_in_place` fn in crate `core`.
Keywords: rust, rustlang, rust-lang, drop_in_place

# 1.8.0[−][src]Function core::ptr::drop_in_place

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

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.

# 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. See the example below for how to drop an unaligned pointer.

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());Run

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

    use std::ptr;
    use std::mem::{self, MaybeUninit};

    unsafe fn drop_after_copy<T>(to_drop: *mut T) {
        let mut copy: MaybeUninit<T> = MaybeUninit::uninit();
        ptr::copy(to_drop, copy.as_mut_ptr(), 1);
        drop(copy.assume_init());
    }

    #[repr(packed, C)]
    struct Packed {
        _padding: u8,
        unaligned: Vec<i32>,
    }

    let mut p = Packed { _padding: 0, unaligned: vec![42] };
    unsafe {
        drop_after_copy(&mut p.unaligned as *mut _);
        mem::forget(p);
    }Run

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.

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