[Qt-interest] Qt memory management.

[Ramesh]

Fine andres..
I do agree.. but what about this case.

QString a = "foo"; // "QStringPrivate" has a refcount of 1
QString b = a;// Shallow copy of QStringPrivate, that now has a refcount of
2
a = "chock" // in this case?

If we change the value of a, then what about b?

 b should point to "foo" right. So what happens in this case, I guess
Deepcopy occurs right?


-----Original Message-----
From: qt-interest-bounces at trolltech.com
[mailto:qt-interest-bounces at trolltech.com] On Behalf Of Andre Somers
Sent: Wednesday, August 18, 2010 4:20 PM
To: qt-interest at trolltech.com
Subject: Re: [Qt-interest] Qt memory management.

  Op 18-8-2010 12:29, Ramesh schreef:
> Hi arnold,
> Thanks for the reply...
>
> With copy constructor I can copy one widget to another widget?..
Copying widgets (or objects) should not even compile, AFAIK. Their copy
and assignment constructors are private. Of course, you can pass around
pointers to them freely.
> And what about the reference count means?. I saw the QString class
> destructor, there before deleting the object they check for reference
count.
> But QString is not a QObject right.
>
> So it means, assignment of one Qstring to another does copy or not?
Copying a QString makes a shallow copy. That means, that it will point
to the same data structure that contains the actual data, and that the
refcount of that internal structure will be increased. So, there is a
copy going on, but a very cheap one (just a pointer). As soon as an
operation starts that would alter data on an object with a refcount > 1,
the copies will detach. That means that the copy of the actual data will
take place at that time, which is way more data than just that one pointer.

So, in your example with QString:

QString a = "foo"; // "QStringPrivate" has a refcount of 1
a = "foo2"; //no copy, still a refcount of 1
QString b = a; // Shallow copy of QStringPrivate, that now has a
refcount of 2
QString b = "bar"; // detach! A deep copy will occur, so we have two
individual QStringPrivate's each with a refcount of 1
b = "bar2"; //no copy needed anymore.

André

>
>
> -----Original Message-----
> From: qt-interest-bounces at trolltech.com
> [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Arnold Krille
> Sent: Wednesday, August 18, 2010 3:40 PM
> To: qt-interest at trolltech.com
> Subject: Re: [Qt-interest] Qt memory management.
>
> Hi,
>
> On Wednesday 18 August 2010 11:15:19 Ramesh wrote:
>> Qt widgets uses shared memory so  we can't copy a widget to another
>> widget right?
> No, QWidgets (and all QObjects) are organized in a tree. And copying
within
> a tree or from one tree to another needs copy-constructors and -operators
> with two arguments (original and new parent).
>
>> But in this case
>> p2 = p1;
>> both are Qpixmap object, and Qpixmap is not inherited from Qobject so
>> assigning a pixamp to another leads to copy operation right?.
> Yes.
>
> Have fun,
>
> Arnold
>
>
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