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

[Atlant Schmidt]

Robert:

  Well, if I'm reading this right, on initial construction, Qt just delivers
  zeroed memory to you so that suggests one way you could reinitialize
  an already-allocated QVector.

  Depending on how much safety you're willing to dispense with, you
  can probably memset() much less area than the full p->array data
  area. On theother hand, if your data area is a multiple of some large
  natural size (4, 8, 16, or 32 bytes) and especially if the data area is
  itself naturally-aligned, memset() is usually a highly-optimized
  routine so I would expect qMemSet() to also be highly-optimized.

                                     Atlant


>From qvector.h:

template <typename T>
QVector<T>::QVector(int asize)
{
    d = malloc(asize);
    d->ref = 1;
    d->alloc = d->size = asize;
    d->sharable = true;
    d->capacity = false;
    if (QTypeInfo<T>::isComplex) {
        T* b = p->array;
        T* i = p->array + d->size;
        while (i != b)
            new (--i) T;
    } else {
        qMemSet(p->array, 0, asize * sizeof(T));
    }
}

________________________________
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 Schimkowitsch Robert
Sent: Friday, June 17, 2011 10:19
To: qt-interest at qt.nokia.com
Subject: [Qt-interest] How to clear QVector without releasing memory?


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?


Robert Schimkowitsch

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