[Interest] What's the purpose of Q_DECLARE_INTERFACE?

Christian Gagneraud chgans at gmail.com
Wed Sep 13 10:39:24 CEST 2017

On 13/09/2017 8:32 pm, "Elvis Stansvik" <elvstone at gmail.com> wrote:

(Adding back the interest@ list to recipients)

Thanks for doing that, I never reply in private without good reason. But I
used my phone to answer your message, so I guess something went wrong...


2017-09-13 10:15 GMT+02:00 Christian Gagneraud <chgans at gmail.com>:
> On 13/09/2017 7:26 pm, "Elvis Stansvik" <elvstone at gmail.com> wrote:
> I'm reading about extending applications with Qt plugins.
> What is the purpose of the Q_DECLARE_INTERFACE/Q_INTERFACES mechanism?
> The docs says:
>     "This macro associates the given Identifier (a string literal) to
> the interface class called ClassName." [1]
> and
>     "Use the Q_DECLARE_INTERFACE() macro to tell Qt's meta-object
> system about the interface." [2]
> The Plugins wiki page also has some more info [3]:
>     "Q_DECLARE_INTERFACE is a macro that defines helper functions that
> make qobject_cast<Tool*>(hammer) return a Tool pointer. qobject_cast
> is just a template function and you can think of Q_DECLARE_INTERFACE
> providing a template specialization for the interface. The macro
> itself just expands as a call to qt_metacast that moc generated above.
> So, Q_DECLARE_INTEFACE defines qobject_cast<Tool *>(object) which is a
> template specialization that translates to
> object->qt_metacall("in.forwardbias.tool/1.0");"
> (and also some info about the Q_INTERFACES macro: "When moc runs on
> the hammer.h code, it inspects Q_INTERFACES. It generates code for a
> function called qt_metacall - void *Hammer::qt_metacast(const char
> *iname). This goal of this 'casting' function is to return a pointer
> to an interface depending on iname. moc also verifies whether the
> interface names that you have put inside Q_INTERFACES have indeed been
> declared. It can do this by inspecting the header files and looking
> So from that I gather that the purpose of the mechanism is to make
> qobject_cast<SomeInterface*>(..) work, despite SomeInterface not being
> a QObject?
> But none of the docs explain why this is needed. What is this featured
> used for within Qt? Or is it provided for me as an application
> developer? If so, when would I use it?
> Sorry if the question is dumb, but I think all the docs explain what
> the macros do, not why you would want that or why it is needed.
> I think there are 2 things here:
> - the macro alows you to tag a QObject as implementing an interface with a
> symbolic name of your own (not a mangled c++ symbol) and a version of your
> own (which c++ doesn't provide at all).
> - qobject_cast is (way) faster than dynamic_cast
> The versioning and the symbolic naming gives host application way more
> freedom than dynamic_cast alone (fancy rename an interface class name each
> time you change it in a binary incompatible way?)
> Basically, with this mechanism, the linker doesn't interfer with how you
> want to cast your QObject. You are in full control to decide if it is safe
> or not to cast qobects.
> My 2 cents, I'm not an expert, but like you I have interest in all of
> please anyone, correct me if I'm wrong.

Thanks a lot Chris. That makes a lot of sense. I had not thought about
the versioning/greater control aspect.

With versioning, I guess you mean baking a version number of your
choosing into the IID string?


> Chris
> And a related question: Can I use Q_DECLARE_INTERFACE on a class that
> is not actually an interface, a class that does not have 100% pure
> virtual member functions? E.g. if I want some base functionality in my
> plugin "interface" class? (I realize of course that my plugins would
> then have to link against the lib that provides this "interface", so
> it would be a tighter coupling than just a shared header, but
> nonetheless).
> Thanks in advance,
> Elvis
> [1] http://doc.qt.io/qt-5/qtplugin.html#Q_DECLARE_INTERFACE
> [2]
> http://doc.qt.io/qt-5/plugins-howto.html#the-low-level-api-
> [3] https://wiki.qt.io/Plugins
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