Re: [boost] [containers] Clarifications on incomplete type support?

Subject: Re: [boost] [containers] Clarifications on incomplete type support?
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2014-09-22 11:28:34

El 21/09/2014 23:19, Mikael Persson escribió:
> I cannot stress enough how important it is for data structure designs to be
> able to instantiate the iterators of containers of incomplete value types!
> And, in general, there is no inherent reason why, in general, incomplete
> type support could not extend to iterators as well. This is even more true
> in light of the SCARY iterator requirements (which boost containers purport
> to support, which cannot possibly be true given the compilation errors I
> have seen).
>
> So, my main question is this: Is the following code supposed to work when
> using Boost.Containers?
>
> boost::container::some_container<IncompleteType>::iterator it;

There was no requirement for that. Only for the container itself.

> The documentation only states that "all Boost.Containers containers [..] are
> designed to support incomplete types". This needs to be clarified when it
> comes to iterators, at least.

Sure.

> If the above code is not guaranteed to work, then may I request that it be
> added to the requirements? Because in my view, not having iterator support
> for incomplete types sort of defeats the whole purpose of supporting
> incomplete types, except for the most trivial use-cases (like those
> "recursive containers" examples). And moreover, without this support,
> workarounds are so major that they won't benefit from the container's
> support for incomplete types any more (and therefore, might as well use the
> standard ones instead).

I didn't have that need in the past, I used containers of incomplete to 
define recursive containers or like PIMPL-like idiom. If defining 
iterator type is desired, I see no problem in supporting that. The 
standard proposal:

http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3890.html

does not mention iterators.

>>From experience, I know that most Boost.Container containers had working
> iterators for incomplete value types up to and including version 1.54 (and
> it's a kind of natural consequence of supporting incomplete types for the
> container itself). But recently, changes to the boost::container::list
> class have made it impossible to use of boost::container::list iterators
> for incomplete types (due to some convoluted new iterator scheme that I can
> barely understand myself). If iterators are supposed to be included in the
> incomplete type support, then this is a bug in the list container (and btw,
> boost::unordered_set also has the same problem), and I would also recommend
> adding this to the container unit-tests.

I've seen your pull request, thanks. I will review it and try to add 
support for iterators to incomplete types. And yes, containers of 
incomplete types (including iterators) should be better tested in the 
unit-tests.

Best,

Ion