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06:25 < jnewbery> This week's post incorrectly stated the date of the meeting as Sept 5th. It should have said tomorrow, Sept 2nd. Sorry for the confusion!
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08:29 < pinheadmz> all this back and forth... what is this, BIP340?
08:29  * pinheadmz ducks
09:18  * wumpus sheds a bike
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10:07 < michaelfolkson> #finalfinaldate
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15:47 < pinheadmz> If i have a struct A with members {A, B} and struct B with members {A, B, C} such that struct B is a subclass of struct A. And I start a function with a struct B but cast it to a struct A -- does member C just get gracefully ignored? What about the other direction? is member C uninitialized in that case?
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15:59 < gwillen> pinheadmz: as far as I know casting between unrelated structs or struct pointers is always illegal
15:59 < gwillen> 'subclass' as a concept does not exist in C
16:00 < gwillen> I think the only exception is casting between a pointer to a struct, and a pointer to its first element, which I believe is legal
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16:01 < gwillen> (so if you switched things up so that struct B started with a struct A as its first element, rather than just the same list of members as struct A, you could cast between _pointers_ to them, as long as the underlying object you started with was a struct B)
16:06 < robot-dreams> Great questions. I agree with gwillen that 'subclass' doesn't exist in the C programming language
16:07 < robot-dreams> Doing some experiments in C++:
16:07 < robot-dreams> struct A {
16:07 < robot-dreams>     int x;
16:07 < robot-dreams>     int y;
16:07 < robot-dreams> };     
16:07 < robot-dreams> struct B : A {
16:07 < gwillen> pinheadmz: actually I don't know why I assumed C rather than C++, I think I just saw the word struct, if you're talking about actual subclasses then the answer is something else (you can cast a subclass to the base class and it just throws members away, which is bad and you shouldn't; you can't cast base class to subclass.)
16:07 < robot-dreams>     int z;
16:07 < robot-dreams> };     
16:07 < gwillen> (the thing you actually always want to do is cast _pointers_, in which case everything behaves reasonably)
16:08 < robot-dreams> It seems like the following is allowed:
16:08 < robot-dreams> void foo(A a) { cout << a.x << endl; } // prints 1
16:08 < robot-dreams> int main() {
16:08 < robot-dreams>     B b = {1, 2, 3};
16:09 < robot-dreams>     foo(b); // casts a B into an A
16:09 < robot-dreams> }     
16:09 < gwillen> that behavior is called 'object slicing': https://en.wikipedia.org/wiki/Object_slicing
16:09 < robot-dreams> But casting in the other direction results in a compile error:
16:10 < gwillen> it is usually considered undesirable, but if your objects are pure structs with no methods it's no different from first casting a pointer, then making a copy (as far as I know)
16:14 < robot-dreams> Thanks gwillen for the explanation, that makes sense to me and seems to encompass the tests I tried
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16:15 < gwillen> for sure, I find that usually people only do this by accident, and you can almost always get what you want by casting pointers instead
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16:30 < pinheadmz> here's where i got "subclass" even though yeah I know thats more a C++ concept: https://github.com/libuv/libuv/blob/v1.x/include/uv.h#L488
16:31 < pinheadmz> sorry i should have explciitly said C not C++
16:31 < pinheadmz> thanks gwillen robot-dreams this is helpful
16:32 < gwillen> ok cool yeah, very welcome, I don't know how (or whether) libuv handles casting those without causing undefined behavior
16:33 < gwillen> (I expect you can do it safely using memcpy, anything cleverer than that contains dragons)
16:34 < gwillen> everything will be much better if you use the "base class is first member of subclass" pattern instead, which makes it legal to cast between pointers to them
16:34 < gwillen> with the only cost being the need for more dots when accessing things that are nested
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16:56 < pinheadmz> https://gist.github.com/pinheadmz/25cfc0b624e9a6dec3df0c12b56053ae
16:57 < pinheadmz> as expected I guess, members get sliced off and return unitialized garbage
16:57 < pinheadmz> going the other direction of course the member doesn exist, but the actual casting pointers didnt throw
16:58 < gwillen> a c-style cast will never throw, they are not defined to do so
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16:58 < pinheadmz> ah ok
16:58 < gwillen> but many possibly uses of c-style cases are undefined behavior and must not be used
16:58 < gwillen> possible*
16:58 < pinheadmz> makes sense, its just a pointer
16:59 < gwillen> as far as I'm aware casting between unrelated types or pointers to unrelated types, such as your structs here, is always undefined behavior
16:59 < pinheadmz> I think I like C better than C++ :-)
16:59 < robot-dreams> Line 24 specifically is not only uninitialized garbage, it's undefined behavior, right?
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16:59 < gwillen> line 20 is already undefined behavior
16:59 < gwillen> which means that the compiler is free to throw the whole program away or do whatever it wants (gcc is not known for being aggressive about this, but clang is)
17:00 < pinheadmz> oh interesting, so clang would throw on line 20?
17:00 < gwillen> no, there are no exceptions involved
17:00 < pinheadmz> ok     
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17:00 < gwillen> undefined behavior means "your program violates the C standard, and as such the compiler makes no promises about what happens when you compile it"
17:01 < gwillen> for example, if clang can see that a function performs undefined behavior, it is free to simply consider the entire function dead code and compile it out
17:01 < gwillen> the results may be obviously broken or they may just be weird, it is entirely up to clang
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17:02 < gwillen> (gcc is also free to do this, but I am picking on clang because it often actually _does_ things like this in practice, whereas people are used to gcc mostly not doing so)
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