Unchecked -

# Unchecked Uninitialized Memory

One interesting exception to this rule is working with arrays. Safe Rust doesn't permit you to partially initialize an array. When you initialize an array, you can either set every value to the same thing with `let x = [val; N]`, or you can specify each member individually with `let x = [val1, val2, val3]`. Unfortunately this is pretty rigid, especially if you need to initialize your array in a more incremental or dynamic way.

Unsafe Rust gives us a powerful tool to handle this problem: `MaybeUninit`. This type can be used to handle memory that has not been fully initialized yet.

With `MaybeUninit`, we can initialize an array element-for-element as follows:

    #![allow(unused_variables)]
    fn main() {
    use std::mem::{self, MaybeUninit};

    // Size of the array is hard-coded but easy to change (meaning, changing just
    // the constant is sufficient). This means we can't use [a, b, c] syntax to
    // initialize the array, though, as we would have to keep that in sync
    // with `SIZE`!
    const SIZE: usize = 10;

    let x = {
        // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
        // safe because the type we are claiming to have initialized here is a
        // bunch of `MaybeUninit`s, which do not require initialization.
        let mut x: [MaybeUninit<Box<u32>>; SIZE] = unsafe {
            MaybeUninit::uninit().assume_init()
        };

        // Dropping a `MaybeUninit` does nothing. Thus using raw pointer
        // assignment instead of `ptr::write` does not cause the old
        // uninitialized value to be dropped.
        // Exception safety is not a concern because Box can't panic
        for i in 0..SIZE {
            x[i] = MaybeUninit::new(Box::new(i as u32));
        }

        // Everything is initialized. Transmute the array to the
        // initialized type.
        unsafe { mem::transmute::<_, [Box<u32>; SIZE]>(x) }
    };

    dbg!(x);
    }

This code proceeds in three steps:

1. Create an array of `MaybeUninit<T>`. With current stable Rust, we have to use unsafe code for this: we take some uninitialized piece of memory (`MaybeUninit::uninit()`) and claim we have fully initialized it (`assume_init()`). This seems ridiculous, because we didn't! The reason this is correct is that the array consists itself entirely of `MaybeUninit`, which do not actually require initialization. For most other types, doing `MaybeUninit::uninit().assume_init()` produces an invalid instance of said type, so you got yourself some Undefined Behavior.

2. Initialize the array. The subtle aspect of this is that usually, when we use `=` to assign to a value that the Rust type checker considers to already be initialized (like `x[i]`), the old value stored on the left-hand side gets dropped. This would be a disaster. However, in this case, the type of the left-hand side is `MaybeUninit<Box<u32>>`, and dropping that does not do anything! See below for some more discussion of this `drop` issue.

3. Finally, we have to change the type of our array to remove the `MaybeUninit`. With current stable Rust, this requires a `transmute`. This transmute is legal because in memory, `MaybeUninit<T>` looks the same as `T`.

   However, note that in general, `Container<MaybeUninit<T>>>` does not look the same as `Container<T>`! Imagine if `Container` was `Option`, and `T` was `bool`, then `Option<bool>` exploits that `bool` only has two valid values, but `Option<MaybeUninit<bool>>` cannot do that because the `bool` does not have to be initialized.

   So, it depends on `Container` whether transmuting away the `MaybeUninit` is allowed. For arrays, it is (and eventually the standard library will acknowledge that by providing appropriate methods).

It's worth spending a bit more time on the loop in the middle, and in particular the assignment operator and its interaction with `drop`. If we would have written something like

    *x[i].as_mut_ptr() = Box::new(i as u32); // WRONG!

we would actually overwrite a `Box<u32>`, leading to `drop` of uninitialized data, which will cause much sadness and pain.

The correct alternative, if for some reason we cannot use `MaybeUninit::new`, is to use the `ptr` module. In particular, it provides three functions that allow us to assign bytes to a location in memory without dropping the old value: `write`, `copy`, and `copy_nonoverlapping`.

- `ptr::write(ptr, val)` takes a `val` and moves it into the address pointed to by `ptr`.
- `ptr::copy(src, dest, count)` copies the bits that `count` T's would occupy from src to dest. (this is equivalent to memmove -- note that the argument order is reversed!)
- `ptr::copy_nonoverlapping(src, dest, count)` does what `copy` does, but a little faster on the assumption that the two ranges of memory don't overlap. (this is equivalent to memcpy -- note that the argument order is reversed!)

It should go without saying that these functions, if misused, will cause serious havoc or just straight up Undefined Behavior. The only things that these functions themselves require is that the locations you want to read and write are allocated and properly aligned. However, the ways writing arbitrary bits to arbitrary locations of memory can break things are basically uncountable!

It's worth noting that you don't need to worry about `ptr::write`-style shenanigans with types which don't implement `Drop` or contain `Drop` types, because Rust knows not to try to drop them. This is what we relied on in the above example.

However when working with uninitialized memory you need to be ever-vigilant for Rust trying to drop values you make like this before they're fully initialized. Every control path through that variable's scope must initialize the value before it ends, if it has a destructor. This includes code panicking. `MaybeUninit` helps a bit here, because it does not implicitly drop its content - but all this really means in case of a panic is that instead of a double-free of the not yet initialized parts, you end up with a memory leak of the already initialized parts.

Note that, to use the `ptr` methods, you need to first obtain a raw pointer to the data you want to initialize. It is illegal to construct a reference to uninitialized data, which implies that you have to be careful when obtaining said raw pointer:

- For an array of `T`, you can use `base_ptr.add(idx)` where `base_ptr: *mut T` to compute the address of array index `idx`. This relies on how arrays are laid out in memory.
- For a struct, however, in general we do not know how it is laid out, and we also cannot use `&mut base_ptr.field` as that would be creating a reference. Thus, it is currently not possible to create a raw pointer to a field of a partially initialized struct, and also not possible to initialize a single field of a partially initialized struct. (A solution to this problem is being worked on.)

One last remark: when reading old Rust code, you might stumble upon the deprecated `mem::uninitialized` function. That function used to be the only way to deal with uninitialized memory on the stack, but it turned out to be impossible to properly integrate with the rest of the language. Always use `MaybeUninit` instead in new code, and port old code over when you get the opportunity.

And that's about it for working with uninitialized memory! Basically nothing anywhere expects to be handed uninitialized memory, so if you're going to pass it around at all, be sure to be really careful.