[Development] qMoveToConst helper for rvalue references to movable Qt containers?

[Thiago Macieira]

On Monday, 29 October 2018 04:43:09 PDT Olivier Goffart wrote:
> > 1) it will copy containers. For Qt containers, that's rather cheap (two
> > atomic refcount operations), but it's not free. And for Standard Library
> > containers, that is likely very expensive.
> 
> But using for(:) with a Qt container will cause a detach in the most common
> case, so I'd say is even worse.
> Which is why i'm saying using this reason is a bit ironic.

True, which is why we didn't deprecate, but did add qAsConst.

> > 2) it's implemented by way of a for loop inside a for loop, which is known
> > to throw optimisers out, generating slightly worse code
> 
> I would consider that the missed optimization is quite small, if not
> negligible. And it can be solved in C++17:
> https://codereview.qt-project.org/243984/

I'd solve this the other way around, by making the macro:

    if (const auto &_container_ = (container); true) \
        for (variable : _container_)

This creates a scope-only const reference to the original container and then 
uses the ranged for on it.

We should be able to depend on this for Qt 6.4 or something.

> > I disagree. The optimisation problem is not solved.
> 
> I'm ok with discouraging Q_FOREACH, but deprecating such a function need
> more thinking.

I'm with you.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center