[Qt-creator] Proposal: Adding std::(experimental::)optional

[Eike Ziller]

> On Aug 25, 2016, at 8:25 PM, Giuseppe D'Angelo <dangelog at gmail.com> wrote:
> 
> On Wed, Aug 24, 2016 at 1:57 PM, Eike Ziller <Eike.Ziller at qt.io> wrote:
>> - "optional<IEditor*> EditorManager::currentEditor() const” makes it explicit that not having a current editor is a reasonable expectation, and that you have to check this condition before doing anything with the potential IEditor instance.
>> - On the other hand, IEditor instances without an IDocument are not allowed, so if you are checking the return value of “IDocument *IEditor::document() const”, you should do that only as part of a QTC_ASSERT/QTC_GUARD style sanity check.
> 
> Playing devil's advocate here: what's the point of using an optional
> pointer? Pointers already have an optional value -- the null pointer
> constant. That seems totally redundant. Ideally one would use
> optional<IEditor&>, which doesn't compile for other reasons, but
> would have the semantics you describe.

Pointers do have an optional value, but actually often nullptr is not a valid value for a pointer semantically. So IMO it would add API clarity.

In an imaginary world where optional would be used consistently, I’d know that if I want to implement an IEditor, that
IDocument *MyEditor::document() const { return 0; }
is not correct, simply because it is not optional<IDocument*> ;)

And I would know that
if (IDocument *doc = someEditor->document()) ....
is senseless.

Also we do not check every pointer that we get from a method for nullptr. And actually I do not see the sense in doing it (e.g. for IEditor::document() we definitely don’t).
So even in the existing, non-ideal world I’d find out that
project()->activeTarget()->activeBuildConfiguration()->buildDirectory()
is just wrong, because both activeTarget and activeBuildConfiguration are optional, and the compiler doesn’t let me write it that way.

And, at the time that activeBuildConfiguration was made optional (it wasn’t all the time), all the code doing
target->activeBuildConfiguration()->buildDirectory()
which was perfectly right so far, would actually lead to a compile error because it is wrong now.

So I do see value of using optional even for pointers.

-- 
Eike Ziller
Principal Software Engineer

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