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

[Giuseppe D'Angelo]

Il 29/10/18 12:43, Olivier Goffart ha scritto:
> Deprecating means you will force user to port all their codebase away from it,
> which is a huge work. If the rationale is just that they will save a couple of
> atomic operations, i do not think it is worth it.
> 
> Deprecating it only for non-shared container seems more logical, since we then
> warn only when there is actually a problem.
> 
> And for the Qt shared container case, using foreach is less typing, but also
> less error prone. (do i have to use qAsConst? or make a copy? or even return a
> const object which even lead to more problems)


Not really, my main argument is teachability. I want to stop teaching 
foreach to Qt users (which are also C++ users; why do we keep forgetting 
that there's a C++ world out there that doesn't use Qt?), and tell them that

* foreach is supposed to be used only on Qt containers, not STL / Boost ones
* actually, only on _certain_ Qt containers,
* but if you use it on a "wrong" container it still works, no warnings 
or anything, just a tremendous price hit,
* why? because it's unconditionally doing a copy, which is cheap only 
for the certain containers in Qt,
* and because of this copy it can't do mutating iteration, so you need 
to learn ranged-based for *anyhow*,
* but thanks to this copy it is perfectly safe to modify the original 
container, because it's copied anyhow (and you can rely on it) (and 
documentation was encouraging this) (and Qt's OWN SOURCE CODE had places 
that relied on that) ---

Well, no. :(


I want to teach users:

* if you have a container and you want to do mutating iteration, use

for (auto &element : container)


* if you have a container and you want to do non-mutating iteration, use

for (const auto &c = container; const auto &element : c)


And that's it; this is a pure C++ solution that works for any container 
(Qt, STL, Boost, ...), does never take any copy (you're iterating on the 
original), it is const-correct (can't accidentally modify the container 
through the element in a non-mutating iteration, can't accidentally 
modify a const container), never detaches a Qt container unless you need 
to anyhow (mutating iteration), and in general it is not safe to modify 
the container's structure from within the loop's body so don't do it.

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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