[Development] HEADS UP: Don't use QList, use Q_DECLARE_TYPEINFO

Kevin Kofler kevin.kofler at chello.at
Sun Aug 9 13:58:12 CEST 2015

Thiago Macieira wrote:

> On Sunday 09 August 2015 03:26:33 Kevin Kofler wrote:
>> Marc Mutz wrote:
>> > On Friday 07 August 2015 19:41:07 Kuba Ober wrote:
>> >> How about making such traits explicitly necessary? People would then
>> >> have to learn about them. As a user, I wouldn't mind - the compiler
>> >> would pick it all up for me.
>> > 
>> > An option for Qt 6, but for Qt 5, it would be a source-incompatible
>> > change.
>> Or, since we're already discussing source-incompatible changes, how about
>> defaulting to Q_MOVABLE_TYPE? It's the common case, and if you screw up,
>> you'll notice it crash and burn pretty quickly.
> No can do.

Care to elaborate? This would be a source-incompatible change for a Qt 6. It 
would apply to Qt containers and Qt containers only. Nothing says that Qt 
needs to follow STL conventions and contracts. And as I wrote, a complex 
type erroneously treated as movable will trigger a noticeable crash, whereas 
the opposite will just be slow and go unnoticed.

For the lifetime of Qt 5 (which I hope will be long!), this change is of 
course not possible due to the source incompatibility.

> Please see the C++17 proposal of destructive moves instead.

It took 4 years for Qt to agree to require a limited subset of C++11, so, 
realistically, C++17 features are not likely to end up used (everywhere, at 
least) before at least 2021.

I guess it will also require good compiler optimization to be as effective 
as a memmove operating on the whole vector. (Basically, the compiler needs 
to detect that you are memmoving a whole vector and convert it to a 

        Kevin Kofler

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