Learning that you can use unions in C for grouping things into namespaces • Hacker News Board

Re: Learning that you can use unions in C for grouping things into namespaces

#21

Post by 10000truths » Sun, Aug 01, 2021, 5:49 PM UTC

nyanpasu64 wrote: post #19

10000truths wrote: post #15

Anonymous nested structs are also quite useful for creating struct fields with explicit offsets: #include #include #define YDUMMY(suffix, size) char dummy##suffix[size] #define XDUMMY(suffix, size) YDUMMY(suffix, size) #define PAD(size) XDUMMY(__COUNTER__, size) struct ExplicitLayoutStruct { union { struct __attribute__((packed)) { PAD(3); uint32_t foo; }; struct __attribute__((packed)) { PAD(5); uint16_t bar; }; str…

Do foo and bar deliberately overlap?

Yes, I was looking to demonstrate the flexibility of the approach by including overlapping fields.

Re: Learning that you can use unions in C for grouping things into namespaces

#22

Post by PostThisTooFast » Sun, Aug 01, 2021, 5:52 PM UTC

Subsentient wrote: post #20

Bleurgh. I have a deep soft spot for C, and I'm known to get twisted pleasure from using obscure language features in new ways to annoy people, but this is a level of abuse that even I can't get behind. If you need namespacing, use C++. As much as I love C, it's terrible for large projects.

I will point out, however, that you can abuse "struct" in a similar way to simulate namespaces in Swift.

Re: Learning that you can use unions in C for grouping things into namespaces

#23

Post by kevin_thibedeau » Sun, Aug 01, 2021, 6:05 PM UTC

The result is uglier and less maintainable than a pair of macros. Or just stop trying to hide syntax. This is ultimately on the same level as typedefing pointers.

Re: Learning that you can use unions in C for grouping things into namespaces

#24

Post by cjaybo » Sun, Aug 01, 2021, 6:51 PM UTC

gumby wrote: post #18

10000truths wrote: post #15

Anonymous nested structs are also quite useful for creating struct fields with explicit offsets: #include #include #define YDUMMY(suffix, size) char dummy##suffix[size] #define XDUMMY(suffix, size) YDUMMY(suffix, size) #define PAD(size) XDUMMY(__COUNTER__, size) struct ExplicitLayoutStruct { union { struct __attribute__((packed)) { PAD(3); uint32_t foo; }; struct __attribute__((packed)) { PAD(5); uint16_t bar; }; str…

One of the very few things from C that I miss in C++ is anonymous structs and enums. I really don’t understand why they are not allowed. That is, C style enums don’t have to have a name but “type safe” (enum class) ones do. One classic use is to name an otherwise boolean option in a function signature; there’s typically no need to otherwise name it. C++ incompatibly requires a name for all struct and class declaratio…

> C++ incompatibly requires a name for all struct and class declarations

You're right about "enum class", but anonymous classes and structs are perfectly valid in C++:

https://godbolt.org/z/7MbcqhnoK

Re: Learning that you can use unions in C for grouping things into namespaces

#25

Post by vbezhenar » Sun, Aug 01, 2021, 6:55 PM UTC

Subsentient wrote: post #20

Bleurgh. I have a deep soft spot for C, and I'm known to get twisted pleasure from using obscure language features in new ways to annoy people, but this is a level of abuse that even I can't get behind. If you need namespacing, use C++. As much as I love C, it's terrible for large projects.

Linux kernel is large project and clearly C is sufficient for it, given the fact that migrating to C++ would probably be very easy (not using all C++ features, but just selected ones), yet it did not happen.

I think that C++ is better than C, but C is not that bad, even for large projects.

Re: Learning that you can use unions in C for grouping things into namespaces

#26

Post by bruce343434 » Sun, Aug 01, 2021, 6:55 PM UTC

Imo this is not “perverse”. In my vector library I alias a vec3 as float x,y,z and float[3] using this technique.

Re: Learning that you can use unions in C for grouping things into namespaces

#27

Post by adtac » Sun, Aug 01, 2021, 7:16 PM UTC

vbezhenar wrote: post #25

Subsentient wrote: post #20

Bleurgh. I have a deep soft spot for C, and I'm known to get twisted pleasure from using obscure language features in new ways to annoy people, but this is a level of abuse that even I can't get behind. If you need namespacing, use C++. As much as I love C, it's terrible for large projects.

Linux kernel is large project and clearly C is sufficient for it, given the fact that migrating to C++ would probably be very easy (not using all C++ features, but just selected ones), yet it did not happen. I think that C++ is better than C, but C is not that bad, even for large projects.

the kernel has to live with the choices it made in the 90s, you don't

Re: Learning that you can use unions in C for grouping things into namespaces

#28

Post by dataflow » Sun, Aug 01, 2021, 7:23 PM UTC

cjaybo wrote: post #24

gumby wrote: post #18

Earlier quoted context omitted.

One of the very few things from C that I miss in C++ is anonymous structs and enums. I really don’t understand why they are not allowed. That is, C style enums don’t have to have a name but “type safe” (enum class) ones do. One classic use is to name an otherwise boolean option in a function signature; there’s typically no need to otherwise name it. C++ incompatibly requires a name for all struct and class declaratio…

> C++ incompatibly requires a name for all struct and class declarations You're right about "enum class", but anonymous classes and structs are perfectly valid in C++: https://godbolt.org/z/7MbcqhnoK

Try

  struct S { struct { int x; }; };

under -pedantic and you'll get

  warning: ISO C++ prohibits anonymous structs [-Wpedantic]

Re: Learning that you can use unions in C for grouping things into namespaces

#29

Post by thaumasiotes » Sun, Aug 01, 2021, 7:29 PM UTC

tialaramex wrote: post #5

pjmlp wrote: post #3

Earlier quoted context omitted.

In C++ we have namespaces for 30 years now, no need for such tricks.

Hmm. How do C++ namespaces help with the structure naming problem in this example? They seem completely orthogonal. C++ namespaces are a way to avoid library A's symbol "cow" clashing with library B's symbol "cow" without everything being named library_a_cow and library_b_cow all over the place which is annoying. I agree C would be nicer with such a namespace feature. However this technique is about what happens when…

Based on the writeup, this technique isn't really about enabling you to start writing `s.position.x` where the old code would have written `s.x`. If that were all you wanted, you'd just keep writing `s.x`. It's about enabling you to write `s.x` everywhere, in old code and new code, while also being able to pass `s.position` to memcpy calls. You're never supposed to write `s.position.x`.

Re: Learning that you can use unions in C for grouping things into namespaces

#30

Post by midjji » Sun, Aug 01, 2021, 7:32 PM UTC

This is probably a terrible idea, remember that if you have written one member of a union, all other members remain public, yet accessing any of them in any way is undefined behaviour. This is made way worse by most compilers mostly choosing to let you do what you think it will. They just dont guarantee they always will or in all cases.