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0 pointsAr-Curunir7mo ago0 comments

And that is safer... how?

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   Foo foo{}; init(*(Bar *)foo);

is UB in most cases (alignment aside, if Bar is not unsigned char, char, std::byte or a base class of Foo). This is obvious why, Foo and Bar may have constructors and destructors. You should use construct_at if you mean to;

For implicit-lifetimes types (iirc types with trivial default constructors (or are aggregates) plus trivial destructors), you can use memcpy, bit_cast and soon std::start_lifetime_as (to get a pointer) when it is implemented.

If I'm not mistaken, in C, the lifetime rules are more or less equivalent to implicitly using C++'s start_lifetime_as

Measter7mo ago

Ironically, Rust doesn't need any of that, you literally can just cast to a different pointer type between arbitrary types and start using it without it inherently being UB (you know, as long as your access patterns are more generally valid).

TuxSH7mo ago

That's because Rust doesn't have constructors/assignment operators, is it not? Because of that, all objects are trivially relocatable (sort of, because Rust allows destructors for these objects, I guess).

And strict aliasing is not a concern due to Rust's aliasing models, thus the combination of the two makes it safe to type-pun like that. But Rust's models has its downsides/is a tradeoff, so...

I don't particularly mind the C++ object model (since C++20), it makes sense after all: construct your object if it needs to, or materalize it through memcpy/bit_cast. std::start_lifetime_as should fix the last remaining usability issue about the model.

Measter7mo ago

You're right that Rust doesn't have constructors or assignment operators, but its abstract machine also views memory as simply being a bag of bytes, rather than inherently typed like C/C++'s model.

j / k navigate · click thread line to collapse

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TuxSH7mo ago

   Foo foo{}; init(*(Bar *)foo);

If I'm not mistaken, in C, the lifetime rules are more or less equivalent to implicitly using C++'s start_lifetime_as

Measter7mo ago

TuxSH7mo ago

And strict aliasing is not a concern due to Rust's aliasing models, thus the combination of the two makes it safe to type-pun like that. But Rust's models has its downsides/is a tradeoff, so...

Measter7mo ago

You're right that Rust doesn't have constructors or assignment operators, but its abstract machine also views memory as simply being a bag of bytes, rather than inherently typed like C/C++'s model.

j / k navigate · click thread line to collapse