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

[Giuseppe D'Angelo]

Il 10/07/2015 11:54, Smith Martin ha scritto:
> Then I don't see why it is so inherently inefficient. The QList entry is allocated on the heap anyway. Doesn't QList<C> just allocate a bigger entry? And if I don't have the C object stored anywhere else, it has to be somewhere, so why not keep it in the QList entry?

Because for a "wrong" type C (*) QList will allocate an array of 
pointers to C (not an array of Cs!), then proceed to allocate on the 
heap every single C object you put in it (by new'ing them); the array 
will then store the pointer to the allocated object.

In other words:

struct C {};
QList<C> list;
for (auto i = 0; i < 100; ++i)
	list << C{};

Calls operator new at least 101 times.

(*) not movable, or bigger than a void*. And user-defined types are not 
movable by default (they're complex). And if we forget 
Q_DECLARE_TYPEINFO on public types, we can't add it back because it's 
binary incompatible.

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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