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I'm using a QGraphicsScene widget and showing upon it some points as QGraphicsRectItem.

This means calling lots of new + addItem() when showing up, and removeItem() + delete to get rid of unused points.

Of course, for performance issues, I've implemented my own new and delete operators, which basically recycle pre-allocated memory chunks.

Those operators works very well with anything, except for Qt classes (I mean QObject derived classes). The error raised is different every time but always occurs during an internal call after all my slot functions have been executed with no errors.

I also tried them with some instances of QWidget added to some layouts etc.

So the question is, does Qt allready have its own new and delete operators, which I am totally skipping defining my own ones?

nyarlathotep108
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  • can you give an example please? – 4pie0 Sep 19 '14 at 10:24
  • Cool! But we need code. See also: [Must I replace global operators new and delete to change memory allocation strategy in third party code?](http://stackoverflow.com/questions/16377991/must-i-replace-global-operators-new-and-delete-to-change-memory-allocation-strat) – Ivan Aksamentov - Drop Sep 19 '14 at 10:37
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    As far as I know (exploring the Qt's code) there is no operator replacement in Qt. – Ivan Aksamentov - Drop Sep 19 '14 at 10:38
  • Instead of using custom new and delete, have you considered using `addItem` and `removeItem` of `QGraphicsScene` to add and remove and re-add items without destroying them? That sounds like it might be the most efficient solution for your performance bottleneck. – hyde Sep 19 '14 at 17:29

1 Answers1

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It's trivial enough to inspect Qt's headers to ascertain whether such custom operators exist. They don't.

The QGraphicsRectItem does not derive from QObject, so that's not an issue anyway. You're not getting as much savings from that as you think you are, since Qt uses the PIMPL idiom extensively, and QGraphicsItem is allocating its PIMPL on the heap.

You'd also need to show the implementation of your operator - for all I know, you're not maintaining proper alignment. What compiler(s) are you targeting? What version of Qt?

To do what you intend, you must modify Qt. Declare and implement:

void* QGraphicsItem::operator new (size_t sz)
void* QGraphicsItem::operator new[] (std::size_t count)
void QGraphicsItem::operator delete (void* ptr, std::size_t sz)
void QGraphicsItem::operator delete[] (void* ptr, std::size_t sz)
void* QGraphicsItemPrivate::operator new (size_t sz)
void QGraphicsItemPrivate::operator delete (void* ptr, std::size_t sz)

Make sure that your allocator properly aligns the storage.

Kuba hasn't forgotten Monica
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    It is not too much trivial since if they are redefined as "global", new and delete operators have a special linkage rule that permits them to never appear in any header file. Anyway, I totally agree with you about PIMPL making almost useless this kind of approach. – nyarlathotep108 Sep 23 '14 at 10:02