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by pjmlp 115 days ago
In Rust's case, union types should only be used for FFI with a C ABI.

As for C, it is a sharp blade on its own.
1 comments
tialaramex 115 days ago
I do not agree. MaybeUninit<T> is without any doubt more valuable than the C FFI use

I can't even think of any prominent C FFI problems where I'd reach for the union's C representation. Too many languages can't handle that so it seems less useful at an FFI edge.

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pjmlp 115 days ago
OS APIs for one, at least there are some Win32 calls that take unions if I remember correctly.

One of the reasons .NET had Managed C++, replaced by C++/CLI (nowadays C++20 compliant, minus modules), is exactly that P/Invoke (and RCW/CCW) cannot represent everything.

Which they don't want to expose on .NET type system directly.

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