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0 pointspjmlp1mo ago0 comments

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.

0 comments

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.

pjmlpOP1mo 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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