Reinventing From Scratch — Box<T>

Chapter 6 — Drop Order & Safety

6.1 The rule

Drop the value in place, then deallocate the bytes.

use std::alloc::{dealloc, Layout};

impl<T> Drop for MyBox<T> {
    fn drop(&mut self) {
        unsafe {
            if self.ptr.is_null() { return; } // consumed via into_raw
            std::ptr::drop_in_place(self.ptr);          // 1) run T's destructor
            dealloc(self.ptr.cast(), Layout::new::<T>()); // 2) free storage
        }
    }
}

6.2 What goes wrong if reversed?

Dealloc first → drop_in_place runs on freed memory → use-after-free → UB.

6.3 Panic behavior

Destructors should not panic. If they do during unwinding, the runtime aborts. Design Drop to be “no-panic.”

Checklist

Never drop after dealloc.
Handle the “disarmed” (null) state from into_raw.
Use the correct Layout for dealloc.

Exercise

Explain why calling drop_in_place twice is UB and how your API prevents it.

Deep Dive: Ownership Proofs, Drop Order, and DST Considerations

A. Formal Invariants for `MyBox<T>` (Sized)

B1 (Pointer Validity): ptr is either null only after into_raw or a valid, properly aligned pointer to initialized T.
B2 (Single Drop): The destructor of T is invoked exactly once if and only if ptr is non-null at Drop time.
B3 (Dealloc after Drop): dealloc(layout_of::<T>()) is called exactly once, and only after drop_in_place.
B4 (From/Into Raw Consistency): from_raw only accepts pointers produced by into_raw of the same type/allocator; mixing allocators is UB.
B5 (No References to Uninit): No &/&mut references are created before ptr::write initializes the allocation.

B.1 Single Drop — Drop checks for null and calls drop_in_place once; into_raw nulls out ptr and forgets self, preventing Drop from running on a live value.
B.2 No Use-After-Free — Deallocation happens only after the destructor; references returned by Deref are derived from a live ptr and never stored beyond the box’s lifetime.
B.3 Panic Safety — If constructor panics before publishing, no ownership is established; if Drop panics (should not), process aborts, avoiding double-unwind corruption.

C. DST Box Notes

Slices (Box<[T]>): store length; the fat pointer (data, len) enables correct deallocation.
Box<str>: same as [u8] with UTF‑8 invariant; length in metadata.
Box<dyn Trait>: fat pointer (data, vtable); the vtable encodes drop and size/alignment; std uses compiler magic for correct layout.

D. Interop Patterns

FFI Ownership Transfer: into_raw -> C takes ownership; C must call back into Rust with from_raw or a custom free.
Leaking Globals: leak returns 'static reference, acceptable for process lifetime singletons; document intent.

E. Debugging

Double Drop: look for *p assignment instead of ptr::write on uninitialized memory.
Mismatched Layout: using dealloc with wrong Layout causes heap corruption; keep Layout::new::<T>() paired.

F. Exercises

Implement try_new returning Result<MyBox<T>, AllocError>.
Add into_inner(self) -> T by ptr::read and skipping dealloc? Explain why you must still dealloc after moving T.
Implement MyVec::into_boxed_slice that hands RawVec buffer to a Box<[T]> safely.

FAQ (Extended)

Q: Does Box<T> guarantee a stable address? A: Yes, the pointee’s address is stable for the life of the box; moving the box moves only the handle.
Q: Why ptr::write not *p = value? A: The latter reads/drops the previous contents (uninitialized), which is UB.
Q: Can Box<T> be null? A: By design, standard Box<T> is non-null; our MyBox may set ptr = null only as a consumed sentinel post-into_raw.
Q: Is Pin<Box<T>> needed for stable address? A: Not for stability; Pin is for forbidding moves via the API.

Drop Order and Safety