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

[Marc Mutz]

On Friday 10 July 2015 13:35:34 Smith Martin wrote:
> >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.;
> 
> This pressure on the memory allocator, what is that? My memory doesn't get
> fragmented, because I only build lists; I never destroy them. But I do
> have to put each C somewhere, and if I use QVector<C>, doesn't it allocate
> a lot of them up front? I don't know how many i will need, but the number
> is usually 0, 1, or 2 -- the number of parameters in a function signature.
> 
> >2) you make the compiler produce more inefficient code by introducing
> >the layer of indirection;
> 
> But it will process the list one time only, from beginning to end.
> 
> >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);
> 
> Why? All the entries in the list are created at the same time (well, during
> the parsing of the function signature)

http://www.akkadia.org/drepper/cpumemory.pdf Sections 3 and 6. Please, read 
it.

-- 
Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
Tel: +49-30-521325470
KDAB - The Qt Experts