Merge pull request #130 from vorner/uninit-improvements

Various unchecked-uninit improvements
pull/134/head
Alexis Beingessner 6 years ago committed by GitHub
commit e4042e69a1
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@ -8,10 +8,10 @@ Unfortunately this is pretty rigid, especially if you need to initialize your
array in a more incremental or dynamic way. array in a more incremental or dynamic way.
Unsafe Rust gives us a powerful tool to handle this problem: Unsafe Rust gives us a powerful tool to handle this problem:
`mem::uninitialized`. This function pretends to return a value when really [`mem::uninitialized`][uninitialized]. This function pretends to return a value
it does nothing at all. Using it, we can convince Rust that we have initialized when really it does nothing at all. Using it, we can convince Rust that we have
a variable, allowing us to do trickier things with conditional and incremental initialized a variable, allowing us to do trickier things with conditional and
initialization. incremental initialization.
Unfortunately, this opens us up to all kinds of problems. Assignment has a Unfortunately, this opens us up to all kinds of problems. Assignment has a
different meaning to Rust based on whether it believes that a variable is different meaning to Rust based on whether it believes that a variable is
@ -24,9 +24,9 @@ longer safely use normal assignment.
This is also a problem if you're working with a raw system allocator, which This is also a problem if you're working with a raw system allocator, which
returns a pointer to uninitialized memory. returns a pointer to uninitialized memory.
To handle this, we must use the `ptr` module. In particular, it provides To handle this, we must use the [`ptr`] module. In particular, it provides
three functions that allow us to assign bytes to a location in memory without three functions that allow us to assign bytes to a location in memory without
dropping the old value: `write`, `copy`, and `copy_nonoverlapping`. dropping the old value: [`write`], [`copy`], and [`copy_nonoverlapping`].
* `ptr::write(ptr, val)` takes a `val` and moves it into the address pointed * `ptr::write(ptr, val)` takes a `val` and moves it into the address pointed
to by `ptr`. to by `ptr`.
@ -80,6 +80,19 @@ Every control path through that variable's scope must initialize the value
before it ends, if it has a destructor. before it ends, if it has a destructor.
*[This includes code panicking](unwinding.html)*. *[This includes code panicking](unwinding.html)*.
Not being careful about uninitialized memory often leads to bugs and it has been
decided the [`mem::uninitialized`][uninitialized] function should be deprecated.
The [`MaybeUninit`] type is supposed to replace it as its API wraps many common
operations needed to be done around initialized memory. This is nightly only for
now.
And that's about it for working with uninitialized memory! Basically nothing 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 anywhere expects to be handed uninitialized memory, so if you're going to pass
it around at all, be sure to be *really* careful. it around at all, be sure to be *really* careful.
[uninitialized]: ../std/mem/fn.uninitialized.html
[`ptr`]: ../std/ptr/index.html
[`write`]: ../std/ptr/fn.write.html
[`copy`]: ../std/ptr/fn.copy.html
[`copy_nonoverlapping`]: ../std/ptr/fn/copy_nonoverlapping.html
[`MaybeUninit`]: ../std/mem/union.MaybeUninit.html

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