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

[Jiergir Ogoerg]

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 | //\
> \_/ |                                      | \_/`
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