[winternewt]

Destructors are vastly superior to the finally keyword because they only require us to remember a single time to release resources (in the destructor) as opposed to every finally clause. For example, a file always closes itself when it goes out of scope instead of having to be explicitly closed by the person who opened the file. Syntax is also less cluttered with less indentation, especially when multiple objects are created that require nested try... finally blocks. Not to mention how branching and conditional initialization complicates things. You can often pair up constructors with destructors in the code so that it becomes very obvious when resource acquisition and release do not match up.

[yoshuaw]

I couldn't agree more. And in the rare cases where destructors do need to be created inline, it's not hard to combine destructors with closures into library types.

To point at one example: we recently added `std::mem::DropGuard` [1] to Rust nightly. This makes it easy to quickly create (and dismiss) destructors inline, without the need for any extra keywords or language support.

[1]: https://doc.rust-lang.org/nightly/std/mem/struct.DropGuard.h...

[WalterBright]

The scope guard statement is even better!

https://dlang.org/articles/exception-safe.html

https://dlang.org/spec/statement.html#ScopeGuardStatement

Yes, D also has destructors.

[indiosmo]

I use this library a lot for scope guards in C++ https://github.com/Neargye/scope_guard, especially for rolling back state on errors, e.g.

In a function that inserts into 4 separate maps, and might fail between each insert, I'll add a scope exit after each insert with the corresponding erase.

Before returning on success, I'll dismiss all the scopes.

I suppose the tradeoff vs RAII in the mutex example is that with the guard you still need to actually call it every time you lock a mutex, so you can still forget it and end up with the unreleased mutex, whereas with RAII that is not possible.

[Defletter]

Scope guards are neat, particularly since D has had them since 2006! (https://forum.dlang.org/thread/dtr2fg$2vqr$4@digitaldaemon.c...) But they are syntactically confusing since they look like a function invocations with some kind of aliased magic-value passed in.

[sigwinch28]

A writable file closing itself when it goes out of scope is usually not great, since errors can occur when closing the file, especially when using networked file systems.

https://github.com/isocpp/CppCoreGuidelines/issues/2203

[mort96]

You need to close it and check for errors as part of the happy path. But it's great that in the error path (be that using an early return or throwing an exception), you can just forget about the file and you will never leak a file descriptor.

You may need to unlink the file in the error path, but that's best handled in the destructor of a class which encapsulates the whole "write to a temp file, rename into place, unlink on error" flow.

[leni536]

Any fallible cleanup function is awkward, regardless of error handling mechanism.

[mandarax8]

The entire point of the article is that you cannot throw from a destructor. Now how do you signal that closing/writing the file in the destructor failed?

[winternewt]

You are allowed to throw from a destructor as long as there's not already an active exception unwinding the stack. In my experience this is a total non-issue for any real-world scenario. Propagating errors from the happy path matters more than situations where you're already dealing with a live exception.

For example: you can't write to a file because of an I/O error, and when throwing that exception you find that you can't close the file either. What are you going to do about that other than possibly log the issue in the destructor? Wait and try again until it can be closed?

If you really must force Java semantics into it with chains of exception causes (as if anybody handled those gracefully, ever) then you can. Get the current exception and store a reference to the new one inside the first one. But I would much rather use exceptions as little as possible.

[EPWN3D]

Just panic. What's the caller realistically going to do with that information?

[locknitpicker]

> The entire point of the article is that you cannot throw from a destructor.

You need to read the article again because your assertion is patently false. You can throw and handle exceptions in destructors. What you cannot do is not catch those exceptions, because as per the standard uncaught exceptions will lead the application to be immediately terminated.

[DonHopkins]

That tastes like leftover casserole instead of pizza.

[raverbashing]

But they're addressing different problems

Sure destructors are great but you still want a "finally" for stuff you can't do in a destructor

[dist-epoch]

Python has that too, it's called a context manager, basically the same thing as C++ RAII.

You can argue that RAII is more elegant, because it doesn't add one mandatory indentation level.

[logicchains]

It's not the same thing at all because you have to remember to use the context manager, while in C++ the user doesn't need to write any extra code to use the destructor, it just happens automatically.

[mort96]

How do you return a file in the happy path when using a context manager?

If you can't, it's not remotely "basically the same as C++ RAII".

[jchw]

Destructors and finally clauses serve different purposes IMO. Most of the languages that have finally clauses also have destructors.

> Syntax is also less cluttered with less indentation, especially when multiple objects are created that require nested try... finally blocks.

I think that's more of a point against try...catch/maybe exceptions as a whole, rather than the finally block. (Though I do agree with that. I dislike that aspect of exceptions, and generally prefer something closer to std::expected or Rust Result.)

[mort96]

> Most of the languages that have finally clauses also have destructors.

Hm, is that true? I know of finally from Java, JavaScript, C# and Python, and none of them have proper destructors. I mean some of them have object finalizers which can be used to clean up resources whenever the garbage collector comes around to collect the object, but those are not remotely similar to destructors which typically run deterministically at the end of a scope. Python's 'with' syntax comes to mind, but that's very different from C++ and Rust style destructors since you have to explicitly ask the language to clean up resources with special syntax.

Which languages am I missing which have both try..finally and destructors?