rust/raii.md

% The Perils Of RAII

Ownership Based Resource Management (AKA RAII: Resource Acquisition is Initialization) is something you'll interact with a lot in Rust. Especially if you use the standard library.

Roughly speaking the pattern is as follows: to acquire a resource, you create an object that manages it. To release the resource, you simply destroy the object, and it cleans up the resource for you. The most common "resource" this pattern manages is simply memory. Box, Rc, and basically everything in std::collections is a convenience to enable correctly managing memory. This is particularly important in Rust because we have no pervasive GC to rely on for memory management. Which is the point, really: Rust is about control. However we are not limited to just memory. Pretty much every other system resource like a thread, file, or socket is exposed through this kind of API.

So, how does RAII work in Rust? Unlike C++, Rust does not come with a slew on builtin kinds of constructor. There are no Copy, Default, Assignment, Move, or whatever constructors. This largely has to do with Rust's philosophy of being explicit.

Move constructors are meaningless in Rust because we don't enable types to "care" about their location in memory. Every type must be ready for it to be blindly memcopied to somewhere else in memory. This means pure on-the-stack-but-still-movable intrusive linked lists are simply not happening in Rust (safely).

Assignment and copy constructors similarly don't exist because move semantics are the default in rust. At most x = y just moves the bits of y into the x variable. Rust does provide two facilities for going back to C++'s copy-oriented semantics: Copy and Clone. Clone is our moral equivalent of copy constructor, but it's never implicitly invoked. You have to explicitly call clone on an element you want to be cloned. Copy is a special case of Clone where the implementation is just "duplicate the bitwise representation". Copy types are implicitely cloned whenever they're moved, but because of the definition of Copy this just means not treating the old copy as uninitialized; a no-op.

While Rust provides a Default trait for specifying the moral equivalent of a default constructor, it's incredibly rare for this trait to be used. This is because variables aren't implicitely initialized (see [working with uninitialized memory][uninit] for details). Default is basically only useful for generic programming.

More often than not, in a concrete case a type will provide a static new method for any kind of "default" constructor. This has no relation to new in other languages and has no special meaning. It's just a naming convention.

What the language does provide is full-blown automatic destructors through the Drop trait, which provides the following method:

fn drop(&mut self);

This method gives the type time to somehow finish what it was doing. After drop is run, Rust will recursively try to drop all of the fields of the self struct. This is a convenience feature so that you don't have to write "destructor boilerplate" dropping children. There is no way to prevent this in Rust 1.0. Also note that &mut self means that even if you could supress recursive Drop, Rust will prevent you from e.g. moving fields out of self. For most types, this is totally fine: they own all their data, there's no additional state passed into drop to try to send it to, and self is about to be marked as uninitialized (and therefore inaccessible).

For instance, a custom implementation of Box might write Drop like this:

struct Box<T>{ ptr: *mut T }

impl<T> Drop for Box<T> {
	fn drop(&mut self) {
		unsafe {
			(*self.ptr).drop();
			heap::deallocate(self.ptr);
		}
	}
}

and this works fine because when Rust goes to drop the ptr field it just sees a *mut that has no actual Drop implementation. Similarly nothing can use-after-free the ptr because the Box is completely gone.

However this wouldn't work:

struct Box<T>{ ptr: *mut T }

impl<T> Drop for Box<T> {
	fn drop(&mut self) {
		unsafe {
			(*self.ptr).drop();
			heap::deallocate(self.ptr);
		}
	}
}

struct SuperBox<T> { 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
			heap::deallocate(self.box.ptr);
		}
	}
}

because after we deallocate the box'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.

Note that the recursive drop behaviour applies to all structs and enums regardless of whether they implement Drop. Therefore something like

struct Boxy<T> {
	data1: Box<T>,
	data2: Box<T>,
	info: u32,
}

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 needs Drop, even though it is not itself Drop.

Similarly,

enum Link {
	Next(Box<Link>),
	None,
}

will have its inner Box field dropped if and only if a value stores the Next variant.

In general this works really nice because you don't need to worry about adding/removing dtors when you refactor your data layout. Still there's certainly many valid usecases for needing to do trickier things with destructors.

The classic safe solution to blocking recursive drop semantics and allowing moving out of Self is to use an Option:

struct Box<T>{ ptr: *mut T }

impl<T> Drop for Box<T> {
	fn drop(&mut self) {
		unsafe {
			(*self.ptr).drop();
			heap::deallocate(self.ptr);
		}
	}
}

struct SuperBox<T> { 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`
			// fields as `None` to prevent Rust from trying to Drop it.
			heap::deallocate(self.box.take().unwrap().ptr);
		}
	}
}

However this has fairly odd semantics: you're saying that a field that should always be Some may be None, just because that happens in the dtor. 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.

On balance this is an ok choice. Certainly if you're just getting started.

In the future, we expect there to be a first-class way to announce that a field should be automatically dropped.

[uninit]: