[Interest] qobject_cast

[Igor Mironchik]

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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>>