[Qt-interest] QList, copy problems

[Thiago Macieira]

On Sunday, 16 de October de 2011 10:32:58 Andre Somers wrote:
> It might be smarter to use some kind of smart pointer as the thing that
> you store in your list, instead of a raw pointer. In the case of
> QObject*s, you could store a QPointer<QObject> instead, and in the
> general case you would probably either store a shared pointer or a weak
> pointer. The actual choices you make, depend on where the ownership of
> the objects in your list resides, of course.

Using QList<QPointer<QObject> > is not a good idea, for two reasons:

1) QPointer is not movable, so QList cannot optimise reallocations by memcpy 
(or, more simply, do nothing after realloc). Each QObject keeps a list of all 
QPointers pointing to it. Copying a QPointer involves a function call into 
QtCore.

2) QPointer is, for that reason, considered deprecated in Qt 4. Do not use it.

A better approach would be to use QSharedPointer and QWeakPointer, which are 
movable. However, both of them run into another limitation of QList in Qt 4 
(to be lifted in Qt 5): they are considered "large". These objects are always 
twice the size of a pointer.

The best solution is QVector<QSharedPointer<OQObject> > or 
QVector<QWeakPointer<QObject> >.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
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