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<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="meet-safe-and-unsafe.html"><strong aria-hidden="true">1.</strong> Meet Safe and Unsafe</a></li><li><ol class="section"><li><a href="safe-unsafe-meaning.html"><strong aria-hidden="true">1.1.</strong> How Safe and Unsafe Interact</a></li><li><a href="what-unsafe-does.html"><strong aria-hidden="true">1.2.</strong> What Unsafe Can Do</a></li><li><a href="working-with-unsafe.html"><strong aria-hidden="true">1.3.</strong> Working with Unsafe</a></li></ol></li><li><a href="data.html"><strong aria-hidden="true">2.</strong> Data Layout</a></li><li><ol class="section"><li><a href="repr-rust.html"><strong aria-hidden="true">2.1.</strong> repr(Rust)</a></li><li><a href="exotic-sizes.html"><strong aria-hidden="true">2.2.</strong> Exotically Sized Types</a></li><li><a href="other-reprs.html"><strong aria-hidden="true">2.3.</strong> Other reprs</a></li></ol></li><li><a href="ownership.html"><strong aria-hidden="true">3.</strong> Ownership</a></li><li><ol class="section"><li><a href="references.html"><strong aria-hidden="true">3.1.</strong> References</a></li><li><a href="aliasing.html"><strong aria-hidden="true">3.2.</strong> Aliasing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.3.</strong> Lifetimes</a></li><li><a href="lifetime-mismatch.html"><strong aria-hidden="true">3.4.</strong> Limits of Lifetimes</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">3.5.</strong> Lifetime Elision</a></li><li><a href="unbounded-lifetimes.html"><strong aria-hidden="true">3.6.</strong> Unbounded Lifetimes</a></li><li><a href="hrtb.html"><strong aria-hidden="true">3.7.</strong> Higher-Rank Trait Bounds</a></li><li><a href="subtyping.html"><strong aria-hidden="true">3.8.</strong> Subtyping and Variance</a></li><li><a href="dropck.html"><strong aria-hidden="true">3.9.</strong> Drop Check</a></li><li><a href="phantom-data.html"><strong aria-hidden="true">3.10.</strong> PhantomData</a></li><li><a href="borrow-splitting.html"><strong aria-hidden="true">3.11.</strong> Splitting Borrows</a></li></ol></li><li><a href="conversions.html"><strong aria-hidden="true">4.</strong> Type Conversions</a></li><li><ol class="section"><li><a href="coercions.html"><strong aria-hidden="true">4.1.</strong> Coercions</a></li><li><a href="dot-operator.html"><strong aria-hidden="true">4.2.</strong> The Dot Operator</a></li><li><a href="casts.html"><strong aria-hidden="true">4.3.</strong> Casts</a></li><li><a href="transmutes.html"><strong aria-hidden="true">4.4.</strong> Transmutes</a></li></ol></li><li><a href="uninitialized.html"><strong aria-hidden="true">5.</strong> Uninitialized Memory</a></li><li><ol class="section"><li><a href="checked-uninit.html"><strong aria-hidden="true">5.1.</strong> Checked</a></li><li><a href="drop-flags.html"><strong aria-hidden="true">5.2.</strong> Drop Flags</a></li><li><a href="unchecked-uninit.html"><strong aria-hidden="true">5.3.</strong> Unchecked</a></li></ol></li><li><a href="obrm.html"><strong aria-hidden="true">6.</strong> Ownership Based Resource Management</a></li><li><ol class="section"><li><a href="constructors.html"><strong aria-hidden="true">6.1.</strong> Constructors</a></li><li><a href="destructors.html" class="active"><strong aria-hidden="true">6.2.</strong> Destructors</a></li><li><a href="leaking.html"><strong aria-hidden="true">6.3.</strong> Leaking</a></li></ol></li><li><a href="unwinding.html"><strong aria-hidden="true">7.</strong> Unwinding</a></li><li><ol class="section"><li><a href="exception-safety.html"><strong aria-hidden="true">7.1.</strong> Exception Safety</a></li><li><a href="poisoning.html"><strong aria-hidden="true">7.2.</strong> Poisoning</a></li></ol></li><li><a href="concurrency.html"><strong aria-hidden="true">8.</strong> Concurrency</a></li><li><ol class="section"><li><a href="races.html"><strong aria-hidden="true">8.1.</strong> Races</a></li><li><a href="send-and-sync.html"><strong aria-hidden="true">8.2.</strong> Send and Sync</a></li><li><a href="atomics.html"><strong aria-hidden="true">8.3.</strong> Atomics</a></li></ol></li><li><a href="vec.html"><strong aria-hidden="true">9.</strong> Implementing Vec</a></li><li><ol class="section"><li><a href="vec-layout.html"><strong aria-hidden="true">9.1.</strong> Layout</a></li><li><a href="vec-alloc.html"><strong aria-hidden="true">9.2.</strong> Allocating</a></li><li><a href="vec-push-pop.html"><strong aria-hidden="true">9.3.</strong> Push and Pop</a></li><li><a href="vec-dealloc.html"><strong aria-hidden="true">9.4.</strong> Deallocating</a></li><li><a href="vec-deref.html"><strong aria-hidden="true">9.5.</strong> Deref</a></li><li><a href="vec-insert-remove.html"><strong aria-hidden="true">9.6.</strong> Insert and Remove</a></li><li><a href="vec-into-iter.html"><strong aria-hidden="true">9.7.</strong> IntoIter</a></li><li><a href="vec-raw.html"><strong aria-hidden="true">9.8.</strong> RawVec</a></li><li><a href="vec-drain.html"><strong aria-hidden="true">9.9.</strong> Drain</a></li><li><a href="vec-zsts.html"><strong aria-hidden="true">9.10.</strong> Handling Zero-Sized Types</a></li><li><a href="vec-final.html"><strong aria-hidden="true">9.11.</strong> Final Code</a></li></ol></li><li><a href="arc-and-mutex.html"><strong aria-hidden="true">10.</strong> Implementing Arc and Mutex</a></li><li><a href="ffi.html"><strong aria-hidden="true">11.</strong> FFI</a></li></ol>
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<a class="header" href="destructors.html#destructors" id="destructors"><h1>Destructors</h1></a>
<p>What the language <em>does</em> provide is full-blown automatic destructors through the
<code>Drop</code> trait, which provides the following method:</p>
<pre><code class="language-rust ignore">fn drop(&mut self);
</code></pre>
<p>This method gives the type time to somehow finish what it was doing.</p>
<p><strong>After <code>drop</code> is run, Rust will recursively try to drop all of the fields
of <code>self</code>.</strong></p>
<p>This is a convenience feature so that you don't have to write "destructor
boilerplate" to drop children. If a struct has no special logic for being
dropped other than dropping its children, then it means <code>Drop</code> doesn't need to
be implemented at all!</p>
<p><strong>There is no stable way to prevent this behavior in Rust 1.0.</strong></p>
<p>Note that taking <code>&mut self</code> means that even if you could suppress recursive
Drop, Rust will prevent you from e.g. moving fields out of self. For most types,
this is totally fine.</p>
<p>For instance, a custom implementation of <code>Box</code> might write <code>Drop</code> like this:</p>
<pre><pre class="playpen"><code class="language-rust">#![feature(ptr_internals, allocator_api, unique)]
use std::alloc::{Alloc, Global, GlobalAlloc, Layout};
use std::mem;
use std::ptr::{drop_in_place, NonNull, Unique};
struct Box<T>{ ptr: Unique<T> }
impl<T> Drop for Box<T> {
fn drop(&mut self) {
unsafe {
drop_in_place(self.ptr.as_ptr());
let c: NonNull<T> = self.ptr.into();
Global.dealloc(c.cast(), Layout::new::<T>())
}
}
}
# fn main() {}
</code></pre></pre>
<p>and this works fine because when Rust goes to drop the <code>ptr</code> field it just sees
a <a href="phantom-data.html">Unique</a> that has no actual <code>Drop</code> implementation. Similarly nothing can
use-after-free the <code>ptr</code> because when drop exits, it becomes inaccessible.</p>
<p>However this wouldn't work:</p>
<pre><pre class="playpen"><code class="language-rust">#![feature(allocator_api, ptr_internals, unique)]
use std::alloc::{Alloc, Global, GlobalAlloc, Layout};
use std::ptr::{drop_in_place, Unique, NonNull};
use std::mem;
struct Box<T>{ ptr: Unique<T> }
impl<T> Drop for Box<T> {
fn drop(&mut self) {
unsafe {
drop_in_place(self.ptr.as_ptr());
let c: NonNull<T> = self.ptr.into();
Global.dealloc(c.cast(), Layout::new::<T>());
}
}
}
struct SuperBox<T> { my_box: Box<T> }
impl<T> Drop for SuperBox<T> {
fn drop(&mut self) {
unsafe {
// Hyper-optimized: deallocate the box's contents for it
// without `drop`ing the contents
let c: NonNull<T> = self.my_box.ptr.into();
Global.dealloc(c.cast::<u8>(), Layout::new::<T>());
}
}
}
# fn main() {}
</code></pre></pre>
<p>After we deallocate the <code>box</code>'s ptr in SuperBox's destructor, Rust will
happily proceed to tell the box to Drop itself and everything will blow up with
use-after-frees and double-frees.</p>
<p>Note that the recursive drop behavior applies to all structs and enums
regardless of whether they implement Drop. Therefore something like</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct Boxy<T> {
data1: Box<T>,
data2: Box<T>,
info: u32,
}
#}</code></pre></pre>
<p>will have its data1 and data2's fields destructors whenever it "would" be
dropped, even though it itself doesn't implement Drop. We say that such a type
<em>needs Drop</em>, even though it is not itself Drop.</p>
<p>Similarly,</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
enum Link {
Next(Box<Link>),
None,
}
#}</code></pre></pre>
<p>will have its inner Box field dropped if and only if an instance stores the
Next variant.</p>
<p>In general this works really nicely because you don't need to worry about
adding/removing drops when you refactor your data layout. Still there's
certainly many valid usecases for needing to do trickier things with
destructors.</p>
<p>The classic safe solution to overriding recursive drop and allowing moving out
of Self during <code>drop</code> is to use an Option:</p>
<pre><pre class="playpen"><code class="language-rust">#![feature(allocator_api, ptr_internals, unique)]
use std::alloc::{Alloc, GlobalAlloc, Global, Layout};
use std::ptr::{drop_in_place, Unique, NonNull};
use std::mem;
struct Box<T>{ ptr: Unique<T> }
impl<T> Drop for Box<T> {
fn drop(&mut self) {
unsafe {
drop_in_place(self.ptr.as_ptr());
let c: NonNull<T> = self.ptr.into();
Global.dealloc(c.cast(), Layout::new::<T>());
}
}
}
struct SuperBox<T> { my_box: Option<Box<T>> }
impl<T> Drop for SuperBox<T> {
fn drop(&mut self) {
unsafe {
// Hyper-optimized: deallocate the box's contents for it
// without `drop`ing the contents. Need to set the `box`
// field as `None` to prevent Rust from trying to Drop it.
let my_box = self.my_box.take().unwrap();
let c: NonNull<T> = my_box.ptr.into();
Global.dealloc(c.cast(), Layout::new::<T>());
mem::forget(my_box);
}
}
}
# fn main() {}
</code></pre></pre>
<p>However this has fairly odd semantics: you're saying that a field that <em>should</em>
always be Some <em>may</em> be None, just because that happens in the destructor. Of
course this conversely makes a lot of sense: you can call arbitrary methods on
self during the destructor, and this should prevent you from ever doing so after
deinitializing the field. Not that it will prevent you from producing any other
arbitrarily invalid state in there.</p>
<p>On balance this is an ok choice. Certainly what you should reach for by default.
However, in the future we expect there to be a first-class way to announce that
a field shouldn't be automatically dropped.</p>
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