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by amluto4 hours ago |
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upvoteby Metasyntactic1 hours ago|
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Hi there! One of the C# language designers here, working on unions. We're interesting in both forms! We decided to go with this first as felt there was the most value here, and we could build the named form on top of this. In no way are we thinking the feature is done in C#15. But it's part of our ongoing evolution.

If you're interested, i can point you to specs i'm writing that address the area you care about :)

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upvoteby Shalomboy1 hours ago|
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Not OP but I would love to check that out
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upvoteby Metasyntactic1 hours ago|
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Sure, as an example: https://github.com/dotnet/csharplang/blob/main/meetings/work...

Again, very rough. We go round and round on things. Loving to decompose, debate and determine how we want to tackle all these large and interesting areas :)

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upvoteby repelsteeltje3 hours ago|
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Not sure, but I think C++ actually does allow std::variant with multiple choices using the same type. You might not be able to distinguish between them by type (using get<Type>()), but you can by position (get<0>(), get<1>(), ...)
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upvoteby amluto1 hours ago|
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I haven’t tried this, and I don’t intend to, because visitors and similar won’t work (how could they?) and I don’t want to have to think about which is choice 2 and which is choice 7.
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upvoteby NooneAtAll32 hours ago|
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I think GP is talking about name-of-field access, not index access or name-of-type
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upvoteby tialaramex3 hours ago|
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The C and Rust union types are extremely sharp blades, enough so that I expect the average Rust beginner doesn't even know Rust has unions (and I assume you were thinking of Rust's enum not union)

I've seen exactly one Rust type which is actually a union, and it's a pretty good justification for the existence of this feature, but one isn't really enough. That type is MaybeUninit<T> which is a union of a T and the empty tuple. Very, very, clever and valuable, but I didn't run into any similarly good uses outside that.

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upvoteby amluto1 hours ago|
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> The C and Rust union types are extremely sharp blades

Sure, but the comparable Rust feature is enum, not union.

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upvoteby masklinn3 hours ago|
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Unions can be used as a somewhat safer (not safe by any means but safer), more flexible, and less error-prone form of transmute. Notably you can use unions to transmute between a large type and a smaller type.

That is essentially the motivation, primarily in the context of FFI where matching C's union behaviour using transmute is tricky and error-prone.

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upvoteby randomNumber73 hours ago|
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There are rare cases where all attributes of the C union are valid at the same time. Say you have a 32-bit RGBA color value and you want to access the individual 8 bit values. You can make a union of an 32 bit int and a struct that contains 4x 8 bit integers.

Also you can manually tag them and get s.th. more like other high level languages. It will just look ugly.

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upvoteby Animats13 minutes ago|
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Yes. I once wanted C unions limited to fully mapped type conversions, where any bit pattern in either type is a valid bit pattern in the other. Then you can map two "char" to "int". Even "float". But pointer types must match exactly.

If you want disjoint types, something like Pascal's discriminated variants or Rust's enums is the way to go. It's embarrassing that C never had this.

Many bad design decision in C come from the fact that originally, separate compilation was really dumb, so the compiler would fit in small machines.

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