The docs mention that the following is undefined behaviour: let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! Is this really always undefined, even when b is never read or written to? If so, what's the reason for that? What could break in this example? And if so…

[image] konsumlamm: Is this really always undefined, even when b is never read or written to? It doesn't really matter whether a value is read or written to. A bool can only ever have the bit patterns 0x01 and 0x00, whereas you're creating a bool from uninitialized memory (which can have any …

Creating a bool with a non-true/false state is UB. It's an invariant that must be upheld, and creating a book that is not either true or false is UB. But let's say you did do this, and prayed the compiler didn't do anything about it. If you're referring to std::ptr::drop_in_place then that's exactl…

[image] OptimisticPeach: If you're referring to std::ptr::drop_in_place Whoops, i actually meant std::ptr::write, edited the original post.

[image] OptimisticPeach: Is not UB, as long as you call std::men::forget on it. So my example wouldn't be UB anymore if i added std::mem::forget(b); or std::ptr::write(&mut b, true); ?

[image] OptimisticPeach: Is not UB, as long as you call std::men::forget on it. Since the String is in an invalid state, and you now must pass ownership of it to forget , there's really no point of doing this. Sure, you could argue that std::ptr::write is viable for just writing into here, but y…

Thank you everyone for your help! [image] Michael-F-Bryan: You may also like to read RalfJung's "What The Hardware Does" is not What Your Program Does: Uninitialized Memory . That was a very informative article, thanks.

Question on uninitalized memory

help

TomP March 20, 2020, 12:36am 6

See this succinct explanation of UB by @alice.

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Question on uninitalized memory

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