[Development] Qt5 API change coming up: QObject::connectNotify(const char *) --> connectNotify(QMetaMethod)

Kent Hansen kent.hansen at nokia.com
Tue May 15 09:28:03 CEST 2012

The const char *-based API is now gone 
Thanks to all who helped get the qt5 submodule porting changes through.

Best regards,

Den 30. april 2012 14:14, skrev ext Kent Hansen:
> Hi all,
> Quick links: https://bugreports.qt-project.org/browse/QTBUG-25541 and
> https://codereview.qt-project.org/#change,24282
> (TL;DR; skip down to "IMPACT ON EXISTING connectNotify() REIMPLEMENTATIONS")
> Yes, it's late, but it's also one of the final nails in the const char
> *-based Q(Meta)Object introspection API coffin.
> We've had a nice template-based connect() in Qt for some time already.
> But the old connectNotify() hook was biased towards the original
> string-based connect(). To get connectNotify() to be called as expected
> by client code, the template-based connect() had to reconstruct the
> signal signature string (and prepend the darn QSIGNAL_CODE+'0'
> character). In Qt5, this is an even costlier operation, since the method
> signature is no longer stored verbatim in the QMetaObject data; it must
> be dynamically constructed from the name and parameter type information.
> Constructing a QMetaMethod, however, is very cheap; it's just a wrapper
> around a QMetaObject* and an index (which the QObject implementation has
> readily available when making/breaking connections). Once you have the
> QMetaMethod, you can also query all aspects of the method (such as the
> name, and parameter count), including its full signature
> (QMetaMethod::methodSignature()).
> The other (public) shortcoming of the const char *-based connectNotify()
> is that if you were comparing the signal string to something else, you
> had to ensure that that "something else" was fully normalized. Otherwise
> the string comparison would fail (due to an extra space character, for
> example). In Qt5, there is an operator== for QMetaMethods, and there is
> the new QMetaMethod::fromSignal() function to get a QMetaMethod that
> corresponds to a C++ signal function. So you can easily compare methods
> without resorting to fragile, raw string comparisons.
> This is why we're migrating away from the const char *-based API (yes,
> I'm also looking at you, QObject::receivers(const char *) ;-) ).
> Existing reimplementations must be ported over to the new API in order
> to build with Qt5. I'm preparing patches for the places in Qt that I'm
> aware of. For a basic example, you can have a look at the QBuffer patch:
> https://codereview.qt-project.org/#change,24284.
> For modules like qtsystems, which has dozens of connectNotify()
> reimplementations and performs signal proxying, it's more involved (WIP
> qtsystems patch: https://codereview.qt-project.org/#change,24817).
> However, we are keeping support for the old API for some time (obviously
> it will have to be removed before Qt5.0 final), to allow places outside
> of qtbase to be ported incrementally.
> If there are places outside of the qt5 modules that need to be ported, I
> can help you with that.
> In addition to the API change itself: We now call (dis)connectNotify()
> directly from the internal index-based
> QMetaObjectPrivate::(dis)connect() function. The nice thing about this
> is that connectNotify() reimplementations are called consistently "out
> of the box" (e.g., when you call connectSlotsByName(), or when QML
> connects to your signals -- and we could get rid of the hack that calls
> it from qtscript: https://codereview.qt-project.org/#change,24558). So
> the only remaining bug is that, as with the old API, disconnectNotify()
> is not called when the receiver is destroyed. But I'll propose a patch
> for fixing that (we will need to store the signal index in the internal
> Connection object, for fast retrieval of the QMetaMethod).
> Best regards,
> Kent
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