[Qt-interest] QQueue / QList memory allocation behaviour

[Thiago Macieira]

On Monday, 16 de May de 2011 08:13:09 Schimkowitsch Robert wrote:
> I have a QQueue<MyClass*>, or a QList<MyClass*> where I mainly append at
> the end, and remove in front.

QQueue is implemented by inheriting a QList.

> Let's assume I start out with the maximum size of Queue or List, and
> then let it shrink and grow as needed, but never beyond the original
> size.
> 
> Will it perform new and delete operations continuously, once in a while,
> or will it only re-use already allocated memory?

It will, sometimes. Since you're using a pointer as the type contained, QList 
will store the pointers in its own pointer array, so it will avoid an 
indirection to the pointer.

If you keep the container at the same size and never exceed the maximum, it 
will probably not reallocated. If you exceed the allocated size, it will 
reallocate and memcpy all the pointers. If you shrink too much, it might 
reallocate and discard unused memory.

> The question behind this: Will I create memory fragmentation when using
> QQueue / QList in this fashion in multiple threads?

That's a very hard question to answer, as memory fragmentation depends on what 
the memory allocator does.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Senior Product Manager - Nokia, Qt Development Frameworks
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