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Qt-interest Archive, September 2007
cast QList from derived type to base type

Message 1 in thread

Hello,

I have a question, may be not a Qt specific one.

I have a QList from a derived type and want to cast this QList to a QList of a base type. 
for example:

QList<QLineEdit *> ledList;
ledList.append(new QLineEdit());

QList<QWidget*> widList;
widList = (QList<QWidget *>) ledList;

Have anyone an idea, how I can reach this, without creating a new QList by iterating and cast the objects one by one?
May be implementing a cast-operator?

Thanks,
Marcel
Message 2 in thread

I have no idea, but I am trying to figure out _why_ you want to do something like 
that. I simply cannot make sense of it. Could you enlighten us? Why not take a 
QList<QWidget *> to begin with?

Guido


On Tue, Sep 04, 2007 at 04:01:10PM +0200, Marcel Lehmann wrote:
> Hello,
> 
> I have a question, may be not a Qt specific one.
> 
> I have a QList from a derived type and want to cast this QList to a QList of a base type. 
> for example:
> 
> QList<QLineEdit *> ledList;
> ledList.append(new QLineEdit());
> 
> QList<QWidget*> widList;
> widList = (QList<QWidget *>) ledList;
> 
> Have anyone an idea, how I can reach this, without creating a new QList by iterating and cast the objects one by one?
> May be implementing a cast-operator?
> 
> Thanks,
> Marcel

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Message 3 in thread

QWidget and QLineEdit only be examples.
I ve some basic classes that work with the QList of the base type (eg. 
QWidget) and do some operations on each element of the QList. Then I have 
some classes that works with the special type (eg. QLineEdit) and do some 
other operations
If I make a QList of the base type (eg. QWidget), then I have to cast in 
some of my classes like dynamic_cast<QLineEdit *>(widget) to get the special 
type.

Hope you understand my goal.

----- Original Message ----- 
From: "Guido Seifert" <wargand@xxxxxx>
To: "TrolltechInterest" <qt-interest@xxxxxxxxxxxxx>
Sent: Tuesday, September 04, 2007 4:21 PM
Subject: Re: cast QList from derived type to base type


>I have no idea, but I am trying to figure out _why_ you want to do 
>something like
> that. I simply cannot make sense of it. Could you enlighten us? Why not 
> take a
> QList<QWidget *> to begin with?
>
> Guido
>
>
> On Tue, Sep 04, 2007 at 04:01:10PM +0200, Marcel Lehmann wrote:
>> Hello,
>>
>> I have a question, may be not a Qt specific one.
>>
>> I have a QList from a derived type and want to cast this QList to a QList 
>> of a base type.
>> for example:
>>
>> QList<QLineEdit *> ledList;
>> ledList.append(new QLineEdit());
>>
>> QList<QWidget*> widList;
>> widList = (QList<QWidget *>) ledList;
>>
>> Have anyone an idea, how I can reach this, without creating a new QList 
>> by iterating and cast the objects one by one?
>> May be implementing a cast-operator?
>>
>> Thanks,
>> Marcel
>
> --
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> "unsubscribe" in the subject or the body.
> List archive and information: http://lists.trolltech.com/qt-interest/
> 

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Message 4 in thread

On 9/4/07, Guido Seifert <wargand@xxxxxx> wrote:
>
> I have no idea, but I am trying to figure out _why_ you want to do
> something like
> that. I simply cannot make sense of it. Could you enlighten us? Why not
> take a
> QList<QWidget *> to begin with?


I can't answer for the original poster, but I've run into this situation
myself. I was constructing a list of the QObjects in an application. Once
you have a QObject, it is easy to use QObject::parent() and
QObject::children() to get all of the related QObjects. But to get a
starting place I used QApplication::topLevelWidgets(), but that returns a
QList<QWidget*> so I had to treat it as a special case... it would have been
useful for me to be able to cast it to a QList<QObject*>.

However, after seeing the one of the other posts I understand why this isn't
possible in general.
Tom
Message 5 in thread

Hi Marcel,

> QList<QLineEdit *> ledList;
> ledList.append(new QLineEdit());
> QList<QWidget*> widList;
> widList = (QList<QWidget *>) ledList;

I have no solution for you, but where is the point of doing this? If you 
want to hide the details, you should use a "QList<QWidget *> ledList" in 
first place ... but if you do know the specific type anyways, what's the 
point of that cast?

When accessing the members, of course you can write "QWidget * widget = 
ledList.at(0)" - no explicit cast needed, no matter if it is a 
"QList<QLineEdit *>" or "QList<QWidget *>".

OT: I am fighting a war against (type)-cast-operator in my own C++-code. I 
defenitely prefer static_cast<type>() and friends ;-)

Regards,
Malte

--
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Message 6 in thread

On Tuesday 04 September 2007 16:01:10 Marcel Lehmann wrote:
> Hello,
>
> I have a question, may be not a Qt specific one.
>
> I have a QList from a derived type and want to cast this QList to a QList
> of a base type. for example:
>
> QList<QLineEdit *> ledList;
> ledList.append(new QLineEdit());
>
> QList<QWidget*> widList;
> widList = (QList<QWidget *>) ledList;
>
> Have anyone an idea, how I can reach this, without creating a new QList by
> iterating and cast the objects one by one? May be implementing a
> cast-operator?

Setting aside Guido's question, you can do this by doing a reinterpret_cast. 
Using the reinterpret_cast will make sure to any readers of your code that 
it's an unsafe cast and you've assumed the risks that it brings along.

You'll probably get lots of warnings on that with recent compilers... 
(aliasing). There's no guarantee that it'll work everywhere.

With current Qt 4, since they're lists of pointers in both sides, the cast 
will *probably* work (the same cannot be said for non-pointer types). Just 
don't mail Trolltech support if it breaks :-)

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Message 7 in thread

The explanation on why it's not easily possible:

http://www.research.att.com/~bs/bs_faq2.html#conversion

Regards

Jarek

On Tuesday 04 September 2007 16:01, Marcel Lehmann wrote:
> Hello,
>
> I have a question, may be not a Qt specific one.
>
> I have a QList from a derived type and want to cast this QList to a QList
> of a base type. for example:
>
> QList<QLineEdit *> ledList;
> ledList.append(new QLineEdit());
>
> QList<QWidget*> widList;
> widList = (QList<QWidget *>) ledList;
>
> Have anyone an idea, how I can reach this, without creating a new QList by
> iterating and cast the objects one by one? May be implementing a
> cast-operator?
>
> Thanks,
> Marcel

--
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Message 8 in thread

I make it with a reinterpret_cast.
To have a type safety, i have done this:
--------------------------------------
template<class T>
class SpecialList: public QList<T> {
public:
 
 SpecialList()
  : QList()
 {
  int testVar = T::is_Not_A_BaseClass;
 }
 virtual ~SpecialList()
 {
 }
 
 operator QList<BaseClass > () 
 {
  return  reinterpret_cast< QList<BaseClass> &>(*this);
 }
};
-------------------------------------

In my BaseClass I have defined a static int is_Not_A_BaseClass. 

Thanks all
Marcel

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Message 9 in thread

> 
> The explanation on why it's not easily possible:
> 
> http://www.research.att.com/~bs/bs_faq2.html#conversion
> 

And another:

http://www.parashift.com/c++-faq-lite/proper-inheritance.html#faq-21.2


Cheers,

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