[Interest] C'tor member initialisation with 'this'
Hi,
when looking at the code of the qt solutions (from git), I realized following
snippet:
class QT_QTSOAP_EXPORT QtSoapHttpTransport : public QObject
{
...
private:
QNetworkAccessManager networkMgr;
...
};
QtSoapHttpTransport::QtSoapHttpTransport(QObject *parent)
: QObject(parent), networkMgr(this)
{
...
}
Is there any reason why the QNetworkAccessManager needs to be initialized with
the 'this' reference?
Seems like a wrong approach to me - maybe because it used to be a pointer at
some point in time?
I found this when analyzing some weird and not really reproducible stack
traces ...
Frank
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Re: [Interest] C'tor member initialisation with 'this'
Everything is OK unless QNetworkAccessManager's constructor (or anything
further in the call chain) tries to use that pointer for anything beyond
copying it somewhere else. What exactly is looking suspicious to you?
On Jan 10, 2014 5:32 PM, Frank Hemer fr...@hemer.org wrote:
Hi,
when looking at the code of the qt solutions (from git), I realized
following
snippet:
class QT_QTSOAP_EXPORT QtSoapHttpTransport : public QObject
{
...
private:
QNetworkAccessManager networkMgr;
...
};
QtSoapHttpTransport::QtSoapHttpTransport(QObject *parent)
: QObject(parent), networkMgr(this)
{
...
}
Is there any reason why the QNetworkAccessManager needs to be initialized
with
the 'this' reference?
Seems like a wrong approach to me - maybe because it used to be a pointer
at
some point in time?
I found this when analyzing some weird and not really reproducible stack
traces ...
Frank
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Re: [Interest] C'tor member initialisation with 'this'
On Friday 10 January 2014 20:51:35 Constantin Makshin wrote:
Everything is OK unless QNetworkAccessManager's constructor (or anything
further in the call chain) tries to use that pointer for anything beyond
copying it somewhere else. What exactly is looking suspicious to you?
When the transports parent object gets deleted, it will delete the transport.
Thus I wonder whether it could cause a duplicate delete ('this' gets deleted
and will subsequently delete the networkMgr but it is a member var and thus
will be deleted twice?
Frank
On Jan 10, 2014 5:32 PM, Frank Hemer fr...@hemer.org wrote:
Hi,
when looking at the code of the qt solutions (from git), I realized
following
snippet:
class QT_QTSOAP_EXPORT QtSoapHttpTransport : public QObject
{
...
private:
QNetworkAccessManager networkMgr;
...
};
QtSoapHttpTransport::QtSoapHttpTransport(QObject *parent)
: QObject(parent), networkMgr(this)
{
...
}
Is there any reason why the QNetworkAccessManager needs to be initialized
with
the 'this' reference?
Seems like a wrong approach to me - maybe because it used to be a pointer
at
some point in time?
I found this when analyzing some weird and not really reproducible stack
traces ...
Frank
___
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Interest@qt-project.org
http://lists.qt-project.org/mailman/listinfo/interest
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Re: [Interest] C'tor member initialisation with 'this'
No, it won't. 'this' is just a pointer and copying it anywhere (except a
free-memory-in-destructor kind of smartpointer, of course, but using these
to store a pointer to the parent object is a weird idea anyway) doesn't
affect the object's lifetime in any way; neither does the object's
destructor gets called when a pointer to it is destroyed by going out of
scope or any other means.
On Jan 10, 2014 9:42 PM, Frank Hemer fr...@hemer.org wrote:
On Friday 10 January 2014 20:51:35 Constantin Makshin wrote:
Everything is OK unless QNetworkAccessManager's constructor (or anything
further in the call chain) tries to use that pointer for anything beyond
copying it somewhere else. What exactly is looking suspicious to you?
When the transports parent object gets deleted, it will delete the
transport.
Thus I wonder whether it could cause a duplicate delete ('this' gets
deleted
and will subsequently delete the networkMgr but it is a member var and thus
will be deleted twice?
Frank
On Jan 10, 2014 5:32 PM, Frank Hemer fr...@hemer.org wrote:
Hi,
when looking at the code of the qt solutions (from git), I realized
following
snippet:
class QT_QTSOAP_EXPORT QtSoapHttpTransport : public QObject
{
...
private:
QNetworkAccessManager networkMgr;
...
};
QtSoapHttpTransport::QtSoapHttpTransport(QObject *parent)
: QObject(parent), networkMgr(this)
{
...
}
Is there any reason why the QNetworkAccessManager needs to be
initialized
with
the 'this' reference?
Seems like a wrong approach to me - maybe because it used to be a
pointer
at
some point in time?
I found this when analyzing some weird and not really reproducible
stack
traces ...
Frank
___
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http://lists.qt-project.org/mailman/listinfo/interest
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Interest mailing list
Interest@qt-project.org
http://lists.qt-project.org/mailman/listinfo/interest
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Re: [Interest] C'tor member initialisation with 'this'
On Friday 10 January 2014 21:58:13 Constantin Makshin wrote:
No, it won't. 'this' is just a pointer and copying it anywhere (except a
free-memory-in-destructor kind of smartpointer, of course, but using these
to store a pointer to the parent object is a weird idea anyway) doesn't
affect the object's lifetime in any way; neither does the object's
destructor gets called when a pointer to it is destroyed by going out of
scope or any other means.
Well, I was confused by the QObject d'tor doc stating:
Warning: All child objects are deleted. If any of these objects are on the
stack or global, sooner or later your program will crash. We do not recommend
holding pointers to child objects from outside the parent. If you still do,
the destroyed() signal gives you an opportunity to detect when an object is
destroyed.
And the networkMgr is on the stack.
Frank
On Jan 10, 2014 9:42 PM, Frank Hemer fr...@hemer.org wrote:
On Friday 10 January 2014 20:51:35 Constantin Makshin wrote:
Everything is OK unless QNetworkAccessManager's constructor (or anything
further in the call chain) tries to use that pointer for anything beyond
copying it somewhere else. What exactly is looking suspicious to you?
When the transports parent object gets deleted, it will delete the
transport.
Thus I wonder whether it could cause a duplicate delete ('this' gets
deleted
and will subsequently delete the networkMgr but it is a member var and
thus
will be deleted twice?
Frank
On Jan 10, 2014 5:32 PM, Frank Hemer fr...@hemer.org wrote:
Hi,
when looking at the code of the qt solutions (from git), I realized
following
snippet:
class QT_QTSOAP_EXPORT QtSoapHttpTransport : public QObject
{
...
private:
QNetworkAccessManager networkMgr;
...
};
QtSoapHttpTransport::QtSoapHttpTransport(QObject *parent)
: QObject(parent), networkMgr(this)
{
...
}
Is there any reason why the QNetworkAccessManager needs to be
initialized
with
the 'this' reference?
Seems like a wrong approach to me - maybe because it used to be a
pointer
at
some point in time?
I found this when analyzing some weird and not really reproducible
stack
traces ...
Frank
___
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Interest@qt-project.org
http://lists.qt-project.org/mailman/listinfo/interest
___
Interest mailing list
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http://lists.qt-project.org/mailman/listinfo/interest
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Re: [Interest] C'tor member initialisation with 'this'
On 10 January 2014 19:09, Frank Hemer fr...@hemer.org wrote: On Friday 10 January 2014 21:58:13 Constantin Makshin wrote: No, it won't. 'this' is just a pointer and copying it anywhere (except a free-memory-in-destructor kind of smartpointer, of course, but using these to store a pointer to the parent object is a weird idea anyway) doesn't affect the object's lifetime in any way; neither does the object's destructor gets called when a pointer to it is destroyed by going out of scope or any other means. Well, I was confused by the QObject d'tor doc stating: Warning: All child objects are deleted. If any of these objects are on the stack or global, sooner or later your program will crash. We do not recommend holding pointers to child objects from outside the parent. If you still do, the destroyed() signal gives you an opportunity to detect when an object is destroyed. And the networkMgr is on the stack. It's not on the stack; it's a member. This means it will be properly destroyed at the end of the QtSoapHttpTransport's destructor, which will run before QObject's destructor. The destruction code of QNAM (actually, of its QObject subobject) will remove it from the parent children list. So when the QObject destructor (for the QObject sub-object of QtSoapHttpTransport), it won't double delete anything. -- Giuseppe D'Angelo ___ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
Re: [Interest] C'tor member initialisation with 'this'
The C++'s order of destruction is, in this case: 1) ~QtSoapHttpTransport(), which does more or less nothing. By the time it returns, the object is no longer a QSHT (which is the reason why you must not call virtual methods from destructors). 2) Any member variables are destroyed. In this case, one if them is a QNAM which derives from QObject, so at some point the ~QObject() of the QNAM runs, and at that time the QNAM is removed from the list of children of its parent QObject, which is the former QSHT. This is safe, because QSHT::~QObject has not run yet, so the former QSHT is still an QObject. 3) QSHT::~QObject runs, finally. It checks the list of its children, but that list does not include the already destroyed QNAM, so the former QNAM is not deleted twice and everything is OK. Yup, took me some time to understand this as well -- thanks for ab excellent question. Cheers, Jan -- Trojitá, a fast Qt IMAP e-mail client -- http://trojita.flaska.net/ ___ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
Re: [Interest] C'tor member initialisation with 'this'
On Friday 10 January 2014 20:35:56 Jan Kundrát wrote: The C++'s order of destruction is, in this case: 1) ~QtSoapHttpTransport(), which does more or less nothing. By the time it returns, the object is no longer a QSHT (which is the reason why you must not call virtual methods from destructors). 2) Any member variables are destroyed. In this case, one if them is a QNAM which derives from QObject, so at some point the ~QObject() of the QNAM runs, and at that time the QNAM is removed from the list of children of its parent QObject, which is the former QSHT. This is safe, because QSHT::~QObject has not run yet, so the former QSHT is still an QObject. 3) QSHT::~QObject runs, finally. It checks the list of its children, but that list does not include the already destroyed QNAM, so the former QNAM is not deleted twice and everything is OK. Yup, took me some time to understand this as well -- thanks for an excellent question. Thanks for the comprehensive explanation:-) @Thiago: Good hint - never thought of that. Frank ___ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
