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

[Giuseppe D'Angelo]

Il 20/10/18 14:43, Elvis Stansvik ha scritto:
> In our application we added a helper like
> 
> template <typename T>
> const T moveToConst(T &&t)
> {
>      return std::move(t);
> }
> 
> that we use for these cases. It just moves the argument to a const
> value and returns it. With that we can do for (auto foo :
> moveToConst(returnsQtContainer()) { ... }.
> 
> Since it's such a common pattern to want to iterate a returned Qt
> container, what would you think of having such a helper
> (qMoveToConst?) in Qt?

Possibly... Note however that such a thing was already proposed for 
qAsConst itself, and shut down to avoid having qAsConst diverge from 
std::as_const (and I approve of not having Qt-specific behaviours). I 
can't find the relevant discussion on the mailing list right now.


For reference, std::as_const's original authors had doubts about the 
lifetime issues around this, saying that more investigations were needed:

> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0007r0.html#9.0


After a LEWG meeting it was reworded like this:

> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0007r1.html#8.0


I'm not entirely sure of what prompted the change for as_const -- if 
just a safety net while waiting for more investigation, or if something 
more profound was raised.


I can easily imagine however a scenario where moveToConst() can lead to 
worse code than the current solution.

Current solution:

// GCE may get applied, 0 moves
const QVector<Object> v = getVector();
for (auto &obj : v) ~~~

vs.

// Always 2 moves
for (auto &obj : qMoveToConst(getVector()) ~~~


And if the type is not even cheap to move (e.g. QVLA, std::array), 
qMoveToConst becomes a really unpleasant surprise.

How about asking LEWG about their reasoning too?


My 2 c,
-- 
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts

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