[Development] leak in QMetaObject?

André Somers andre at familiesomers.nl
Thu Jul 14 17:18:50 CEST 2016

Op 14/07/2016 om 17:10 schreef Olivier Goffart:
> On Donnerstag, 14. Juli 2016 16:33:26 CEST Thomas Senyk wrote:
>> Hi,
>> I lately wanted to validate that a connecting&disconnecting to a lambda
>> will not leak the lambda object .. and found that more then that is leaked.
>> Here is my example: http://paste.ofcode.org/36dmWCT8ddF6Zaqb9csKZNv
>> After the 10million connections are executed and disconnected (all of
>> them a successful), I assume a drop in memory consumption and more
>> importantly a drop of cpu load to close to 0%.
>> I see neither one nor the other. I see continues 100% consume with no
>> memory consumption drop what so ever.
>> I did a quick peak into QMetaObject::activate and it seems no clean up
>> happened .. the list returned in
>>    list = &connectionLists->at(signal_index);
>>    (line #3660 in 5.6 checkout from today)
>> seems rather long! (10million?)
>> However all "if (!c->receiver)" shows no receiver and therefor will
>> continue right away .. at least
>> Is this expected? A bug? ... I'm happy to fill a bug report, I just
>> wanted to get some feedback, maybe I'm just doing something wrong
> The lambda object and its capture is destroyed by the call to
>    slotObj->destroyIfLastRef()
> The internal data structre (QObjectPrivate::Connection) is deleted by
> QObjectPrivate::cleanConnectionLists which is actually only called when
> a new connection is made to this object.
> I don't think there is a leak, but it is true that some memory is only free'ed
> when you add new connection or that the object is destroyed. (but the lambda
> object is destroyed right after the disconnection)
>> Some background:
>> I wanted to check if a single-shot-connection (connection to a signal
>> and disconnect at first execute, using a lambda as slot) would be a
>> feasible thing to do.
> You can do that already:
>   QTimer::sigleShot(100, [] { qDebug() << "hello"; });
That's not the same thing.
This looks like my earlier suggestion 


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