[Interest] qobject_cast

[Igor Mironchik]

Hi,

All is ok with Q_OBJECT in real code. This is just pseudo code...


20.04.2017 9:05, nikita baryshnikov пишет:
> Hi, Igor
>
> Your example is incorrect, cause you forget about Q_OBJECT macro in
> Channel class
>
> On Thu, Apr 20, 2017 at 8:56 AM, Igor Mironchik
> <igor.mironchik at gmail.com> wrote:
>>
>> 20.04.2017 8:47, Ch'Gans пишет:
>>
>>> On 20 April 2017 at 17:03, Igor Mironchik <igor.mironchik at gmail.com>
>>> wrote:
>>>> Hi,
>>>>
>>>>
>>>> 20.04.2017 7:52, Ch'Gans пишет:
>>>>> On 20 April 2017 at 16:38, Igor Mironchik <igor.mironchik at gmail.com>
>>>>> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> What the benefits of qobject_cast<> on simple static_cast<>? Or there
>>>>>> is
>>>>>> no
>>>>> qobject_cast
>>>>> - works only on QObject (obviously)
>>>>> - returns nullptr if the object cannot be casted to the required type
>>>>
>>>> And it can fail if QObject derived pointer was returned from plugin.
>>> Is this a statement or a question?
>>>
>>> if I'm not wrong, qobject_cast relies on QMetaObject
>>> (object->metaObject() and Class::staticMetaObject()).
>>>
>>> Actually, i've just checked the sources and qobject_cast<T*>(object)
>>> returns "static_cast<T*>(T::staticMetaObject().cast(object))" and
>>> QMetaObject::cast(obj) returns "(obj &&
>>> obj->metaObject()->inherits(this)) ? obj : nullptr"
>>>
>>> QMetaObject allows you to walk the class inheritance hierarchy, and
>>> inherits() does exactly this.
>>> I don't think that the fact that the object comes from a plugin can
>>> make inherits() fail.
>>> If you are able to load the plugin successfully, it means that you
>>> have no unresolved symbols.
>>
>> Exactly! qobject_cast<> will fail.
>>
>> Look in application you have:
>>
>> class Channel
>>    :  public QObject
>> {};
>>
>> In plugin you have:
>>
>> class MyChannel
>>    :  public Channel
>> {};
>>
>> Channel * create();
>>
>> In application you call:
>>
>> Channel * ch = create();
>>
>> And use this object in your application.
>>
>> Then in one slot in application you can have:
>>
>> void mySlot()
>> {
>>    Channel * ch = qobject_cast< Channel* > ( sender() );
>>    // You are 100% sure that sender() is Channel!!!
>>    // And this cast will fail!!!
>>
>> }
>>
>>> AFAIK, the only problems with plugins is meta-type registration that
>>> has to be done explicitly by calling qRegisterMataType(...);
>>> I don't think that QMetaObject suffers from this. Maybe someone can
>>> confirm this.
>>>
>>>>> - doesn't relies on RTTI, and is way faster than dynamic cast
>>>>
>>>> This is understandable. But with comparison with static cast there are no
>>>> benefits?
>>> My point was: dynamic_cast is safer than static_cast, and qobject_cast
>>> is faster than dynamic_cast.
>>>
>>> So qobject_cast is a "double win" over static_cast and dynamic_cast.
>>>
>>> Chris
>>>
>>>>> Chris
>>>>>
>>>>>> one?
>>>>>>
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>>>>
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