Challenge: write a reference counted slice that works as much as possible like a built-in slice (page 6)

On Wednesday, 10 November 2021 at 13:52:26 UTC, deadalnix wrote:

On Tuesday, 9 November 2021 at 18:33:01 UTC, Atila Neves wrote:

I think that C++'s greatest gift to the world was the destructor. We have those too! Let's use them.

Indeed, but I have unfortunate news. We broke the gift. In D, object can be destroyed without being constructed first.

Consider: https://godbolt.org/z/EdW75jWGn

This is the first problem we need to fix here, if we don't want to have to plaster our code with runtime checks.

The struct is instantiated [1], it just doesn't run the constructor in that case. You can always disable the default constructor. Doesn't C++ have an implicit default constructor also?

[1] https://dlang.org/spec/struct.html#struct-instantiation

November 10, 2021

Re: Challenge: write a reference counted slice that works as much as possible like a built-in slice

Posted by Paul Backus
in reply to deadalnix

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Paul Backus

Posted in reply to deadalnix

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On Wednesday, 10 November 2021 at 13:48:53 UTC, deadalnix wrote:

On Tuesday, 9 November 2021 at 17:04:29 UTC, Paul Backus wrote:

Here's a basic sketch of the scheme I had in mind: https://gist.github.com/run-dlang/d1982a29423b2cb545bc9fa452d94c5e

It's entirely possible I've overlooked something and this is actually unsound. As Andrei says: "destroy!"

There are a few problems with the code, indeed.
1/ You need to add runtime check, because in D, destructed objects may not have been constructed, so you need to be able to destroy .init . in your case, pointers will be null and you'll segfault.

Destroying .init works fine here. free(null) is guaranteed by the C standard to be a no-op.

2/ For a struct, you cannot guarantee that the struct's method don't escape the struct pointer, so none of this can be safe in any way.

Good point. I guess you'd need transitive scope for this, at minimum.

On Wed, Nov 10, 2021 at 01:52:26PM +0000, deadalnix via Digitalmars-d wrote: > On Tuesday, 9 November 2021 at 18:33:01 UTC, Atila Neves wrote: > > I think that C++'s greatest gift to the world was the destructor. We have those too! Let's use them. > > > > Indeed, but I have unfortunate news. We broke the gift. In D, object can be destroyed without being constructed first. > > Consider: https://godbolt.org/z/EdW75jWGn > > This is the first problem we need to fix here, if we don't want to have to plaster our code with runtime checks. Isn't that what @disable this() is for? And to be pedantic, the object *was* constructed -- by the default ctor. If you didn't want that, you should @disable this(). T -- "The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts." -- Bertrand Russell. "How come he didn't put 'I think' at the end of it?" -- Anonymous

On Wednesday, 10 November 2021 at 17:09:05 UTC, H. S. Teoh wrote: > Isn't that what @disable this() is for? > > And to be pedantic, the object *was* constructed -- by the default ctor. If you didn't want that, you should @disable this(). > > > T No. Everything that is destructed must have been constructed. It is the only way you can enforce invariant within a type.

On Wednesday, 10 November 2021 at 17:24:03 UTC, deadalnix wrote: > On Wednesday, 10 November 2021 at 17:09:05 UTC, H. S. Teoh wrote: >> Isn't that what @disable this() is for? >> >> And to be pedantic, the object *was* constructed -- by the default ctor. If you didn't want that, you should @disable this(). >> >> >> T > > No. > > Everything that is destructed must have been constructed. It is the only way you can enforce invariant within a type. Invariants must hold also with structures initialised from '.init', if you don't want to @disable this()...

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