I don't mind it not being default - but the section doesn't mention at all how to get wrapping behavior. Just one sentence like "use this different operator to get wrapping behavior" would be fine, but since they didn't mention it at all I kind of went away with the idea that it is not supported at all, which honestly would be a massive problem.
>defer is way more transparent than RAII, because your code is right there in the function call, not hidden in a pile of classes that might change underneath.
I mean, sort of? It seems way less composable though, if you have an object which contains 5 different resources, now you have to defer 5 things? How does this interact with return values, if you defer file.close() for example but return the file, does it still get closed? And if you have a heap allocated array of those objects, where objects get inserted at different places, defer doesn't work at all anymore, since it's just bound to function scope? Because that's usually the scenario that can be annoying to handle in C (opposed to just closing whatever was opened in the same function scope), so it seems a bit odd to me to kind of violate that "no hidden control flow" principle for something that doesn't even solve the primary issue that C resource handling has.
> I mean, sort of? It seems way less composable though, if you have an object which contains 5 different resources, now you have to defer 5 things?
Defer is used for scope-local resources. If you create five different things in the local scope that need to be cleaned up, then they are most likely not part of the same object; yes, you will need to clean them all up. If you have an object with five different thing, then you would defer that object's cleanup routine.
> How does this interact with return values, if you defer file.close() for example but return the file, does it still get closed?
Yes. Why would you defer close on a file that you are planning to return? That's by definition not some scope-local resource with scope lifetime you want to cleanup at the end of scope. That's like implementing malloc that frees the block before returning.
Note that there is errdefer, which you can use to defer the release of resources that would be returned normally but need to be cleaned up on error. https://ziglang.org/documentation/master/#defer
> And if you have a heap allocated array of those objects, where objects get inserted at different places, defer doesn't work at all anymore, since it's just bound to function scope?
I don't understand what you're asking about here. If you malloc() an array in C, then you free() it when you're done with it. The exact same mechanism works in Zig, but defer also helps you ensure free() actually gets called when you go out of scope, so you can still use early returns instead of ret = foo; goto err_bar;
> Because that's usually the scenario that can be annoying to handle in C (opposed to just closing whatever was opened in the same function scope), so it seems a bit odd to me to kind of violate that "no hidden control flow" principle for something that doesn't even solve the primary issue that C resource handling has.
I don't agree that it's violating the "no hidden control flow constraint." It's explicit, right there in the scope, not stashed away in a bunch of destructors. It's as transparent as return or goto or break.
Again, I don't really understand the rest of your complaints (or what's your primary issue with C's resource handling). Defer is just a mechanism to make it easier to release resources whose lifetime is tied to a scope.
>I think that piece would get too long if it described everything in depth.
I don't think one sentence more about a very fundamental operation in a page-long document would really be misplaced. But as far as the language itself goes, those operators seem fine to me.
>Note that there is errdefer
That is definitely more useful, and covers the situation I was thinking of.
>I don't understand what you're asking about here. If you malloc() an array in C, then you free() it when you're done with it.
This is about composed objects. So an allocated array of objects, which in turn also hold allocated objects, which might in turn also hold allocated objects.
>I don't agree that it's violating the "no hidden control flow constraint." It's explicit, right there in the scope, not stashed away in a bunch of destructors. It's as transparent as return or goto or break.
Not sure I can completely agree with that. Fundamentally, defer means that something happens at a point where there is no associated code. If you read the line "return 5;", you don't know if that triggers one (or multiple) function calls. Yes, you only have to look for those in function scope (which could be 5000+ lines in some cases) (you only have to look for destructors in function scope too btw), but it is still a "hidden code path". For me personally it doesn't really matter at that point if I have to first search in the function body (and then potentially in the cleanup function), or first in the function body and then potentially in destructors. It removes the ability to reason about code line by line either way. Which might be a price that is worth to pay for easier cleanup logic, but the way I'm seeing it I'm only getting like a quarter of the benefit of destructors but I'm paying essentially the same price.
> This is about composed objects. So an allocated array of objects, which in turn also hold allocated objects, which might in turn also hold allocated objects.
The way this is solved by calling the cleanup routine for the first object in the hierarchy. If that object is responsible for the lifetime of other objects, then it will also call their cleanup routine. From the user's perspective, those are invisible, just as any calls to free() or close() or fflush() inside an fclose() are.
> If you read the line "return 5;", you don't know if that triggers one (or multiple) function calls.
If you read the line "continue;" or "break;", you don't know if that triggers one (or multiple) function calls until you read the surrounding code.
I still find it much easier to follow code inside a function than to jump through classes.
But the bigger picture is that it makes lifetime management explicit and thus transparent, and you needn't complicate the language with copy constructors and move semantics.
>If you read the line "continue;" or "break;", you don't know if that triggers one (or multiple) function calls until you read the surrounding code.
Not sure what you mean exactly - you have to read the surrounding code to know where the control flow continues, but there's no hidden function call between that point and and the break. That's like arguing you need to understand the entire code base to know what return does - since it returns control flow to potentially a completely different file. But that's not really the point, the point is that it doesn't do anything else in between - if you know where control flow is going to, you know everything that is happening. More so with break and continue even, since for break and continue where control flow continues doesn't even depend on runtime state, whereas return could return to different places depending on where the function was called.
With break, control flow goes out of the loop or out of the switch. You may find function calls there. With continue, control flow goes to the start of the loop. You may find function calls there. With return, control goes through the defer blocks and then out of the function. Similar reasoning for goto. In every case, you know where the control flow goes by looking at the context inside the function.
I don't agree with the assessment that one of them is more hidden than the other.
>With break, control flow goes out of the loop or out of the switch. You may find function calls there.
Yeah but you don't have to look for anything in between. You look for where it goes, and that's it. That there could be a function call after the continue has executed isn't relevant at all, there could also be function call after a return has executed. Stepping through the code in your head is trivial because the execution never jumps anywhere without an explicit statement to jump.
With defers, you need to make sure you find every single defer that was executed up to the return (which might not be trivial if defers are executed conditionally). With a break there is exactly one, unconditional, position at which execution continues - and if you have found it, there is no reason to keep looking anywhere else.
I mean wanting to make the defer trade-off but not the destructor trade-off, fine, that is ultimately a matter of opinion. But defer being hidden control flow is just objectively true, execution jumps somewhere without a corresponding explicit jump in the code (return only explicitly jumps out of the function).
>defer is way more transparent than RAII, because your code is right there in the function call, not hidden in a pile of classes that might change underneath.
I mean, sort of? It seems way less composable though, if you have an object which contains 5 different resources, now you have to defer 5 things? How does this interact with return values, if you defer file.close() for example but return the file, does it still get closed? And if you have a heap allocated array of those objects, where objects get inserted at different places, defer doesn't work at all anymore, since it's just bound to function scope? Because that's usually the scenario that can be annoying to handle in C (opposed to just closing whatever was opened in the same function scope), so it seems a bit odd to me to kind of violate that "no hidden control flow" principle for something that doesn't even solve the primary issue that C resource handling has.