On Tuesday, 15 November 2022 23:50:38 PST Marc Mutz via Development wrote:
> > in a thread-safe manner (such that if something in
> > the same thread or another thread-safely modifies that map, the original
> > user isn't affected).
>
> The above isn't thread-safe, it isn't even re-entrant, in the same way
> that iteration using iterators isn't. This is a known issue whenever you
> hand out references, and it's nothing that violates our
> const-is-thread-safe promise,
No, but it moves the responsibility for avoiding this problem to the user.
Right now, you can do:
for (auto elem : object.keyList()) {
operate(); // may recurse back into object and modify it
}
If you use a generator paradigm to return this key list, then the *user* must
know that they must create a local container with the items to be generated
and iterate over that. Performance-wise, this no different than if the Qt code
created the container and returned it, but it has two drawbacks:
1) the responsibility for knowing this
2) if the Qt object already has a QList with this, then using a generator
paradigm enforces the need of a deep copy, when implicit would have been
cheaper
> > Because you pointed to QStringTokenizer and that implicitly-
> > copies a QString.
>
> That's imprecise. QStringTokenizer extends rvalue lifetimes ("rvalue
> pinning") so's to make this safe:
>
> for (auto part : qTokenize(label->text(), u';'))
BTW, http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2012r2.pdf is
accepted for C++23 and moves the end of the temporaries' lifetimes to the end
of the full for statement.
Though we still need to work with C++17 and 20 for a while.
Also, sometimes I wonder if all the work you and I do to optimise these things
matter, in the end. We may save 0.5% of the CPU time, only for that to be
dwarfed by whatever QtGui, QtQml are doing.
--
Thiago Macieira - thiago.macieira (AT) intel.com
Cloud Software Architect - Intel DCAI Cloud Engineering
_______________________________________________
Development mailing list
Development@qt-project.org
https://lists.qt-project.org/listinfo/development
