[Development] OK to add zero/bulk copy to QVector?

[Jiergir Ogoerg]

I would vouch for a non static method in addition or instead of
"static ...fromRawData(..)", because a non-static method
allows to set the buffer of a vector (needless to mention - with a
zero copy) at any time, not just at vector creation, otherwise,
as noted in my previous example, I'd end up having to create two
vectors - the 1st one is created automatically when the parent class
is created which holds the vector, and a second vector is created when
calling "fromRawData()" when I got the data on the heap to replace
the vector's buffer.
It's not a big deal, but I find it slightly better because it avoids a
vector creation as in my case, and you can replace the buffer at any
time
it suits you.


On Mon, Dec 30, 2013 at 2:45 AM, Konstantin Ritt <ritt.ks at gmail.com> wrote:
>
> There is a plan to add  "static QVector<T> QVector::fromRawData(const T
> *data, int size)". Though, I'm not sure if we can do this until 6.0.
>
> Regards,
> Konstantin
>
>
> 2013/12/30 Jiergir Ogoerg <f35f22fan at gmail.com>
>>
>> QVector is created before I can load the array on the heap, so I can't
>> init it to the desired
>> item count.
>> Particularly I'm talking about QVector<QComposeTableElement>
>> m_composeTable
>> from
>>
>> qt5/qtbase/src/plugins/platforminputcontexts/compose/generator/qtablegenerator.h
>>
>> And because of that I have to (can) resize() it, and resizing sucks
>> because it not just creates
>> more space for the internal buffer but also initializes each item to
>> the default value.
>>
>> The other option to resize() is reserve, but it doesn't also increase
>> the item count of the vector so
>> I still have to go over each element and push_back().
>>
>> Just setting the whole thing with one qvector->setBuffer(array) would
>> be quicker and faster.
>>
>> The example:
>> The actual source code is attached (as zip file) in the file
>> qtablegenerator.cpp, at static function loadCache(..),
>> the quickest approach I could figure out was to use reserve() and then
>> push_back() on each item,
>> which still does lots of copies:
>>
>> QComposeTableElement *elem;
>>     vec->reserve(kElemCount);
>>
>>     for(int i=0; i<kElemCount; i++) {
>>         elem = (QComposeTableElement*) (buf + (i * kElemSize));
>>         vec->push_back(*elem);
>>     }
>>
>>
>>
>>
>> On Sun, Dec 29, 2013 at 11:44 PM, Yves Bailly <yves.bailly at laposte.net>
>> wrote:
>> > On 29/12/2013 20:10, Jiergir Ogoerg wrote:
>> >> Hi,
>> >> there are 2 simple options to speed up setting/adding elements to a
>> >> vector when a heap buffer is filled in in advance:
>> >> 1) Bulk copy.
>> >> Assign a bunch of elements at once (from another vector or array),
>> >> like std::vector::assign() does.
>> >> A big extra copy happens, but avoids multiple calls to push_back().
>> >
>> > What about this, re-using your example:
>> > my_struct *arr = new my_struct[count];
>> > QVector<my_struct> v(count);
>> > memcpy(v.data(), arr, sizeof(my_struct)*count);
>> >
>> > Assuming "my_struct" doesn't contain self-referencing references or
>> > virtual table.
>> >
>> > Regards,
>> >
>> > --
>> > (o< | Yves Bailly                          | -o)
>> > //\ | Linux Dijon  : http://www.coagul.org | //\
>> > \_/ |                                      | \_/`
>> > _______________________________________________
>> > Development mailing list
>> > Development at qt-project.org
>> > http://lists.qt-project.org/mailman/listinfo/development
>>
>> _______________________________________________
>> Development mailing list
>> Development at qt-project.org
>> http://lists.qt-project.org/mailman/listinfo/development
>>
>
>
> _______________________________________________
> Development mailing list
> Development at qt-project.org
> http://lists.qt-project.org/mailman/listinfo/development
>