[Interest] QVectors vs QLists

Sean Harmer sean.harmer at kdab.com
Wed May 3 20:08:51 CEST 2017


On 03/05/2017 17:57, Jason H wrote:
> I have a app that does a lot of numeric processing,using quint32, qreal mostly (and occasional QVector3D). 10k-1M items in the data set. I generally do know the size of the dataset before I start adding to it, so I call reserve() on it. Given that, and that I've been using QMap and QVector, when I use QMap and want the keys() it gives me a QList, which I then have to call toVector() on.
> So this leads me to ask if I should be using QList all this time? I had thought that QList was considered harmful. [1][2][3]
> But "everything" seems to return to me a QList, rather than a QVector.
> What should I be using? (I'm Targeting 5.8 and later.)

Depends upon your usage patterns. Do you do a lot of map operations over 
the vectors/maps? If so cache coherency may well be the biggest factor, 
in which case QVector or even std::vector may give you better 
performance than QMap. Given the current discrepancy between CPU speeds 
and memory access latencies it's hard to do better than a vector for 
pure performance as the contiguous memory layout makes better use of the 
L2 cache.

If ease of API use is your main concern then QMap may be a better fit. 
It all depends what you are using this data for.



Dr Sean Harmer | sean.harmer at kdab.com | Managing Director UK
KDAB (UK) Ltd, a KDAB Group company
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