[Interest] 64 bit capable QVector
Constantin Makshin
cmakshin at gmail.com
Wed Sep 4 01:55:47 CEST 2013
While I don't want to say that STL is a bad thing (it's not bad at all),
the fact that its allocators (and, as a consequence, everything that
uses them) are by design very friendly to memory fragmentation (the
"allocate new block -> copy data -> free old block" makes it completely
impossible even to shrink a block of memory in-place) makes me a bit sad...
On 09/04/2013 03:21 AM, Alex Malyushytskyy wrote:
> Forgot to add,
>
> I am not trying to offend performance or any other aspect of Qt container.
> Personally all my code related to displaying data use them.
>
> I just believe it is not replacement for STL.
>
> Regards, Alex
>
>
>
> On Tue, Sep 3, 2013 at 4:16 PM, Alex Malyushytskyy <alexmalvtk at gmail.com
> <mailto:alexmalvtk at gmail.com>> wrote:
>
> >> STL is first of all an interface and there are various
> implementations, hence your remark about performances does not make
> sense.
>
> It does. All implementations of STL I ever used are clear about
> their effectiveness, associated types and complexity guarantees.
>
> >> Qt containers are far more than "just convenience" classes
>
> They are designed to work within Qt only.
> As far as I understand they never meant to be one of implementation
> or replacement of STL, thus it is not provided if not counted rare
> exceptions .
> Thus these are 'convenience' classes which provide sufficient in
> terms of data size and performance support of tasks common for Qt.
>
> I would add that Qt containers are designed to work with Qt widgets
> and carry the same limitations.
> And until it is impossible for example to fill QCombobox the number
> of items which exceeds capabilities of Qt container, it does not
> make sense to change the containers.
>
> But I disagree with "99.9% of Qt programmers don't need 64 bit
> containers." statement.
> It might be true for mobile devices , but it is false even for home
> desktops.
>
> Even if simply counting % of software which have to handle data
> exceeding 32 bit limit on the home personal computer you will get
> higher %.
> Rising of interest in distributed computing including visualization
> probably does not meant to solve problems with low memory requirements.
>
> I would expect most of the scientific programs need to support 64
> bit containers even though sometimes they might need that support
> occasionally .
>
> Alex
>
>
>
> On Tue, Sep 3, 2013 at 2:55 PM, Philippe <philwave at gmail.com
> <mailto:philwave at gmail.com>> wrote:
>
> I could easily guess 99.9% of Qt programmers don't need 64 bit
> containers... Qt containers are far more than "just convenience"
> classes.
>
> STL is first of all an interface and there are various
> implementations, hence your remark about performances does not
> make sense.
>
> Philippe
>
> On Tue, 3 Sep 2013 14:44:58 -0700
> Alex Malyushytskyy <alexmalvtk at gmail.com
> <mailto:alexmalvtk at gmail.com>> wrote:
>
>
> This question appears on the mailing lists since Qt 3 at least .
>
> At one point I was disappointed with having signed int
> restriction, but then I decided
> that QT containers are just a convenience classes which are
> designed to work with either widgets or data of limited size
> displayed
> by that widgets.
>
> If guaranteed performance is needed you should use STL anyway.
>
> Regards,
> Alex
>
>
> On Tue, Sep 3, 2013 at 11:58 AM, Constantin Makshin
> <cmakshin at gmail.com <mailto:cmakshin at gmail.com>> wrote:
>
> Thanks for the explanation, although I still don't
> appreciate the choice
> (mostly regarding the size, not signedness). :)
>
> On 09/03/2013 10:48 PM, Thiago Macieira wrote:
> > On terça-feira, 3 de setembro de 2013 22:18:39,
> Constantin Makshin wrote:
> >> Could you please explain (or give a link to an article
> or something like
> >> that) the reasons Qt developers used to choose signed
> 32-bit integer for
> >> this purpose?
> >> Signed 32-bit container sizes, i.e. number of elements
> in a container,
> >> would be acceptable (considering the equation 'n *
> sizeof(T)' for the
> >> amount of memory consumed by the array alone) but why
> use them to
> >> calculate and store sizes of allocated memory blocks?
> >
> > For two reasons:
> >
> > 1) it's signed because we need negative values in
> several places in the API:
> > indexOf() returns -1 to indicate a value not found;
> many of the "from"
> > parameters can take negative values to indicate
> counting from the end. So even
> > if we used 64-bit integers, we'd need the signed
> version of it. That's the
> > POSIX ssize_t or the Qt qintptr.
> >
> > This also avoids sign-change warnings when you
> implicitly convert unsigneds to
> > signed:
> > -1 + size_t_variable => warning
> > size_t_variable - 1 => no warning
> >
> > 2) it's simply "int" to avoid conversion warnings or
> ugly code related to the
> > use of integers larger than int.
> >
> > io/qfilesystemiterator_unix.cpp:
> > size_t maxPathName =
> ::pathconf(nativePath.constData(), _PC_NAME_MAX);
> > if (maxPathName == size_t(-1))
> >
> > io/qfsfileengine.cpp:
> > if (len < 0 || len != qint64(size_t(len))) {
> >
> > io/qiodevice.cpp:
> > qint64 QIODevice::bytesToWrite() const
> > {
> > return qint64(0);
> > }
> >
> > return readSoFar ? readSoFar :
> qint64(-1);
> >
> >
> >>
> >> On 09/03/2013 08:42 PM, Thiago Macieira wrote:
> >>> On terça-feira, 3 de setembro de 2013 19:33:47,
> Mehmet Ipek wrote:
> >>>> Btw, size
> >>>> limit of QVector is 2^31 in 64 bit platforms too?
> >>>
> >>> Yes. All Qt container classes use a signed int for sizes.
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