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

[Giuseppe D'Angelo]

Il 10/07/2015 13:21, Smith Martin ha scritto:
> But I'm still not clear on one point. If I use QList<C>, and it is implemented as QList<C*> because my C is not a "good" C, why is this inefficent if the only copy of each C is the one that gets allocated on the heap? I just create the list to store things. It won't ever be moved. I will just process the list when it's time comes.
>
> Why is that inefficient?

Because

1) you put pressure on the memory allocator, by asking it to allocate on 
the heap each individual C object you put in the list. Every single 
allocation has a cost, plus the overhead you need for bookkeeping, plus 
the costs of getting your memory fragmented, etc.;

2) you make the compiler produce more inefficient code by introducing 
the layer of indirection;

3) you kill caching, as potentially every single access will result in a 
cache miss (even in the common scenario of a simple forward iteration 
over your list);

and so on.

Cheers,

-- 
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Software Engineer
KDAB (UK) Ltd., a KDAB Group company | Tel: UK +44-1625-809908
KDAB - The Qt Experts

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