Using std::function as parameter for slot and signal

[CrazySiberianScientist]

Hi!
How to set object of std::function as parameter for signal and slot?

Example:

signals:
    void sendSetProcFunc(const std::function<void (const QImage &,
                                               QList<QPolygon> &,
                                               QList<QImage> &)>
                     &procFunc);

...................................
public slots:
    void onSetProcFunc(const std::function<void (const QImage &,
                                               QList<QPolygon> &,
                                               QList<QImage> &)>
                     &procFunc);

Building in Qt 5.7 with MinGW_32bit gives next errors:

---

debug\moc_QtCVGUI.cpp: In static member function 'static void QtCVGUI::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)':
debug\moc_QtCVGUI.cpp:131:128: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
case 2: _t->sendSetProcFunc((reinterpret_cast< const std::function<const void( QImage&,QList<QPolygon>&,QList<QImage>&)>()>(_a[1]))); break;
^
debug\moc_QtCVGUI.cpp:131:142: error: no matching function for call to 'QtCVGUI::sendSetProcFunc(const std::function<const void(QImage&, QList<QPolygon>&, QList<QImage>&)>&)'
case 2: _t->sendSetProcFunc((reinterpret_cast< const std::function<const void( QImage&,QList<QPolygon>&,QList<QImage>&)>()>(_a[1]))); break;
^
In file included from debug\moc_QtCVGUI.cpp:9:0:
****/QtCVGUI.h:49:10: note: candidate: void QtCVGUI::sendSetProcFunc(const std::function<void(const QImage&, QList<QPolygon>&, QList<QImage>&)>&)
void sendSetProcFunc(const std::function<void (const QImage &,
^
****/QtCVGUI.h:49:10: note: no known conversion for argument 1 from 'const std::function<const void(QImage&, QList<QPolygon>&, QList<QImage>&)>' to 'const std::function<void(const QImage&, QList<QPolygon>&, QList<QImage>&)>&'

---

Removing of all const and setting of std::function as usual variable (NOT reference) solve this problem (i.e. make succesful build).
Why are there such restrictions and how to bypass them?

void sendSetProcFunc(std::function<void (QImage &,
                                               QList<QPolygon> &,
                                               QList<QImage> &)>
                     procFunc);

[jsulm]

@CrazySiberianScientist said in Using std::function as parameter for slot and signal:

reinterpret_cast< const std::function<const void( QImage&,QList<QPolygon>&,QList<QImage>&)>

Shouldn't your cast (why do you use reinterpret_cast actually? What type is _a?) be

reinterpret_cast< std::function<void(const QImage&,QList<QPolygon>&,QList<QImage>&)>

So, no const in front of std::function, no const in front of void and const for the first parameter.

[VRonin]

the problem is here QtCVGUI.h:49:10: note: no known conversion for argument 1 from 'const std::function<const void(QImage&, QList<QPolygon>&, QList<QImage>&)>' to 'const std::function<void(const QImage&, QList<QPolygon>&, QList<QImage>&)>&'

notice std::function<const void vs std::function<void

Difficult to tell you exactly what went wrong without seeing your emit call but my guess is you are trying to pass a function that has a signature like this: void MyClass::MyFunction(const QImage&, QList<QPolygon>&, QList<QImage>&) const and the final const screws you

[CrazySiberianScientist]

@VRonin
In my code there isn't any emit with sendSetProcFunc .

---

@jsulm said in Using std::function as parameter for slot and signal:

Shouldn't your cast (why do you use reinterpret_cast actually? What type is _a?) be

This isn't my cast, this cast of MOC.

---

On bugtracker I created issue : https://bugreports.qt.io/browse/QTBUG-60539

[kshegunov]

Probably a deficiency in moc's parsing abilities (no wonder with all the introduced additional complexity in c++11, I might add). Try to typedef and use the alias instead.
E.g:

typedef std::function<void (const QImage &, QList<QPolygon> &, QList<QImage> &)> UnnecessaryWrapper;

signals:
    void sendSetProcFunc(const UnnecessaryWrapper & procFunc);

public slots:
    void onSetProcFunc(const UnnecessaryWrapper & procFunc);

Edit:
I see that Thiago already triaged it as a moc issue. Okay, for the sake of argument try to typedef your argument type.

[CrazySiberianScientist]

@kshegunov

I didn't properly check all combinations.
This builds successfull even without typedef:

void sendSetProcFunc(const std::function<void (/*const*/ QImage &,
                                               QList<QPolygon> &,
                                               QList<QImage> &)>
                     &procFunc);


void onSetProcFunc(const std::function<void (/*const*/ QImage &,
                                               QList<QPolygon> &,
                                               QList<QImage> &)>
                     &procFunc);

[kshegunov]

@CrazySiberianScientist said in Using std::function as parameter for slot and signal:

This builds successfull even without typedef

Because the moc won't have a const to place at the inappropriate place. My suggestion holds as a possible workaround nonetheless, as const QImage & is profoundly different from QImage &.

[CrazySiberianScientist]

@kshegunov

Thank you. Your solution works.
But don't forget about (may be this is obvious for someone, but I will put this for others):

int main(){
...
   // May be this isn't good place for it
   qRegisterMetaType<UnnecessaryWrapper>("UnnecessaryWrapper");
...
}

[kshegunov]

@CrazySiberianScientist said in Using std::function as parameter for slot and signal:

   // May be this isn't good place for it
   qRegisterMetaType<UnnecessaryWrapper>("UnnecessaryWrapper");

This is exactly correct and also an appropriate place, but it is unneeded unless you're going to pass that callback across threads.
Anyway, I'm glad it worked.