[Qt-interest] How to clear QVector without releasing memory?

[Atlant Schmidt]

Nikos:

  (You switched from talking about resize() to reserve();
  I'll assume that's a typo.)

  Here's the description for resize() (from the Qt 4.7.1 docs):

    void QVector::resize ( int size )

    Sets the size of the vector to size. If size is
    greater than the current size, elements are added
    to the end; the new elements are initialized with
    a default-constructed value. If size is less than
    the current size, elements are removed from the end.

    See also size().


  It seems to me that there's no promise that this clears
  existing elements in the vector (except that elements
  at the end will be destroyed if you're shortening the
  vector). In fact, the behaviour when the new size ==
  the existing size is completely unspecified!

                             Atlant

-----Original Message-----
From: qt-interest-bounces+aschmidt=dekaresearch.com at qt.nokia.com [mailto:qt-interest-bounces+aschmidt=dekaresearch.com at qt.nokia.com] On Behalf Of Nikos Chantziaras
Sent: Monday, June 20, 2011 09:54
To: qt-interest at qt.nokia.com
Subject: Re: [Qt-interest] How to clear QVector without releasing memory?

I think even though it's not explicitly spelled out in the docs, it's
implied.  reserve() would have no meaning otherwise.  What's the point
of offering this function if Qt would then go ahead and lower the
capacity again?


On 06/20/2011 04:49 PM, Schimkowitsch Robert wrote:
> resize(0) does really seem to be an option here. The way I understood
> the source code, it really will not free up memory when I resize the
> vector. What bothers me a little is that I would rely on undocumented
> behaviour here, i.e. it might change without notice.
>
> But the way I see it, there IS no documented way to remove items without
> deallocating memory.
>
> Robert Schimkowitsch
>
>
> -----Original Message-----
> From: qt-interest-bounces+robert.schimkowitsch=andritz.com at qt.nokia.com
> [mailto:qt-interest-bounces+robert.schimkowitsch=andritz.com at qt.nokia.co
> m] On Behalf Of Nikos Chantziaras
> Sent: Friday, June 17, 2011 6:39 PM
> To: qt-interest at qt.nokia.com
> Subject: Re: [Qt-interest] How to clear QVector without releasing
> memory?
>
> On 06/17/2011 05:19 PM, Schimkowitsch Robert wrote:
>> Hi,
>>
>> I have a QVector that I want to re-use many times as a buffer. As the
>> application is long-running on a low memory device, memory
> fragmentation
>> might be an issue.
>>
>> To keep this in check, I would like to reserve() the QVector once,
> with
>> the maximum size I am going to need, and then re-use it without ever
>> freeing up memory.
>>
>> The only ways to clear all items from the vector I found will also
>> deallocate memory.
>>
>> What is the performace-optimal way to clear a QVector without
>> deallocating memory?
>
> QVector::resize(0).  It doesn't clear anything, which means it's as fast
>
> as it can possibly get.  The docs are hinting that this does not
> deallocate memory:
>
> "If you know in advance how large the vector will be, you can call this
> function, and if you call resize() often you are likely to get better
> performance."
>
> Dot not use QVector::clear().  This one will deallocate memory,
> according the docs.
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