On 2018-03-04 13:20, Christian Ehrlicher wrote:
Am 04.03.2018 um 10:03 schrieb Christian Ehrlicher:
Am 03.03.2018 um 23:22 schrieb Martins, Sérgio:
On 2018-03-03 20:38, Christian Ehrlicher wrote:
But it looks like reserve()
allocates *exactly* the amount of elements given.
Actually that qpainterpath code is off-by-one, it should be:
d_func()->elements.reserve(d_func()->elements.size() + polygon.size()
- 1);
which also fixes the performance problem (by luck, in that specific
benchmark). Please test.
You're correct - this fixes the problem. Looks like the growth
strategy is only applied when the current capacity == current size and
not when the current capacity is slightly higher... good to know (and
should maybe be documented?)
The off-by-one idea was wrong. moveTo() will add another point to
elements - therefore reserve(d_func()->elements.size() +
polygon.size()) is correct.
Maybe the best idea here would be to simply remove the reserve() calls
and let QVector do what it is made for... :)
If you're sure you're not pessimizing any other case, then removing
reserve() is fine.
But maybe there's a third option:
vec.reserve(qMax(needed,
what_the_capacity_would_have_been_without_reserve))
this way it's good for short containers, as growth is bootstrapped and
also good for big containers as reserving becomes a no-op, as there's
already capacity.
Regards,
--
Sérgio Martins | sergio.mart...@kdab.com | Senior Software Engineer
Klarälvdalens Datakonsult AB, a KDAB Group company
Tel: Sweden (HQ) +46-563-540090, USA +1-866-777-KDAB(5322)
KDAB - The Qt, C++ and OpenGL Experts
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