no matching member function for call to 'connect'

t.vanbesien

Hello,

I am trying to use QSignalMapper to map many similar identifiable QLineEdit widgets in my program.
I am following instructions on this page: https://doc.qt.io/qt-5/qsignalmapper.html#details
However I get a compilation error (the title). I don't understand, QWidget inherits from QObject, so the member function exists, but I must be doing something wrong with the parameters. Help :)

The line that produces this error is: (in the .cpp file)

connect(lineEdit, &QLineEdit::editingFinished, signalMapper, &QSignalMapper::map);

Here is the source code and header of the class I'm implementing:

lineedityarnsetpoint.h

#ifndef LINEEDITYARNSETPOINT_H
#define LINEEDITYARNSETPOINT_H

#include <QWidget>
#include <QSignalMapper>
#include <QLineEdit>
#include <QGridLayout>
//#include <QObject>

class LineEditYarnSetpoint : public QWidget
{
	Q_OBJECT
public:
	LineEditYarnSetpoint(const int n, QWidget *parent = nullptr);

signals:
	void editingFinished(const int feeder_index);

private:
	QSignalMapper *signalMapper;

};

#endif // LINEEDITYARNSETPOINT_H

lineedityarnsetpoint.cpp

#include "lineedityarnsetpoint.h"

LineEditYarnSetpoint::LineEditYarnSetpoint(const int n, QWidget *parent)
	: QWidget(parent)
{
	signalMapper = new QSignalMapper(this);

	QGridLayout *gridLayout = new QGridLayout;
	for (int i = 0; i < n; ++i)
	{
		QLineEdit *lineEdit = new QLineEdit();
		/*ERROR*/connect(lineEdit, &QLineEdit::editingFinished, signalMapper, &QSignalMapper::map);
		signalMapper->setMapping(lineEdit, i);
		gridLayout->addWidget(lineEdit, 0, i);
	}
	connect(signalMapper, &QSignalMapper::mappedInt, this, &LineEditYarnSetpoint::editingFinished);
	setLayout(gridLayout);
}

Thanks in advance! :)

JonB

@t-vanbesien said in no matching member function for call to 'connect':

/*ERROR*/connect(lineEdit, &QLineEdit::editingFinished, signalMapper, &QSignalMapper::map);

void QSignalMapper::map(QObject *sender) expects a QObject *sender parameter. void QLineEdit::editingFinished() does not provide one.

OIC, void QSignalMapper::map() does not take any parameter. But now there are two slots of the same name which differ only by parameter. Then you may need to use qOverload<>()/QOverload<>::of() to specify which one? Maybe QOverload<>::of<>(&QSignalMapper::map) or QOverload<>::of<void>(&QSignalMapper::map)? (I don't know how you specify "the one with no parameter").

Ah, see https://stackoverflow.com/questions/68552698/connecting-qsignalmapper-qt-5-15-2 ?

Also, if you had copied and pasted the full error details from the compiler I think it would have said a bit more than just no matching member function for call to 'connect', does it not show information about the various possible slot method overloads?

t.vanbesien

@JonB

Here is the full error message:

/Users/thomasvanbesien/IHM_Bobink/lineedityarnsetpoint.cpp:14: error: no matching member function for call to 'connect'
../IHM_Bobink/lineedityarnsetpoint.cpp:14:3: error: no matching member function for call to 'connect'
                connect(lineEdit, &QLineEdit::editingFinished, signalMapper, &QSignalMapper::map);
                ^~~~~~~
/Users/thomasvanbesien/Qt/6.4.2/macos/lib/QtCore.framework/Headers/qobject.h:181:36: note: candidate function not viable: no known conversion from 'void (QLineEdit::*)()' to 'const char *' for 2nd argument
    static QMetaObject::Connection connect(const QObject *sender, const char *signal,
                                   ^
/Users/thomasvanbesien/Qt/6.4.2/macos/lib/QtCore.framework/Headers/qobject.h:184:36: note: candidate function not viable: no known conversion from 'void (QLineEdit::*)()' to 'const QMetaMethod' for 2nd argument
    static QMetaObject::Connection connect(const QObject *sender, const QMetaMethod &signal,
                                   ^
/Users/thomasvanbesien/Qt/6.4.2/macos/lib/QtCore.framework/Headers/qobject.h:432:41: note: candidate function not viable: no known conversion from 'void (QLineEdit::*)()' to 'const char *' for 2nd argument
inline QMetaObject::Connection QObject::connect(const QObject *asender, const char *asignal,
                                        ^
/Users/thomasvanbesien/Qt/6.4.2/macos/lib/QtCore.framework/Headers/qobject.h:201:43: note: candidate template ignored: couldn't infer template argument 'Func2'
    static inline QMetaObject::Connection connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal,
                                          ^
/Users/thomasvanbesien/Qt/6.4.2/macos/lib/QtCore.framework/Headers/qobject.h:242:13: note: candidate template ignored: couldn't infer template argument 'Func2'
            connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, const QObject *context, Func2 slot,
            ^
/Users/thomasvanbesien/Qt/6.4.2/macos/lib/QtCore.framework/Headers/qobject.h:287:13: note: candidate template ignored: couldn't infer template argument 'Func2'
            connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, const QObject *context, Func2 slot,
            ^
/Users/thomasvanbesien/Qt/6.4.2/macos/lib/QtCore.framework/Headers/qobject.h:233:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
            connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, Func2 slot)
            ^
/Users/thomasvanbesien/Qt/6.4.2/macos/lib/QtCore.framework/Headers/qobject.h:276:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
            connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, Func2 slot)
            ^

JonB

@t-vanbesien said in no matching member function for call to 'connect':

couldn't infer template argument 'Func2'

I'm not sure, but this is probably the one that relates to your situation.
In any case you need to do the "qoverload" I suggested and is in the SO post.

t.vanbesien

@JonB

Thank you for the help. I was able to make it work, I changed the erroneous line to:

connect(lineEdit, &QLineEdit::editingFinished, signalMapper, QOverload<>::of<>(&QSignalMapper::map));

I'm not completely sure why it works. C++ templates are still a bit fuzzy to me, but I'll post an explanation when I understand exactly what the problem was.

JonB

@t-vanbesien said in no matching member function for call to 'connect':

connect(lineEdit, &QLineEdit::editingFinished, signalMapper, QOverload<>::of<>(&QSignalMapper::map));

I don't believe that is a working copy/paste, I think you mean QOverload<>::of(&QSignalMapper::map)?

C++ templates are still a bit fuzzy to me, but I'll post an explanation when I understand exactly what the problem was.

There are 2 possible overloads: QSignalMapper::map() & QSignalMapper::map(QObject *).
QOverload<>::of(...) picks the first one (no parameters), QOverload<QObject *>::of(...) picks the second one. The bit inside <...> specifies the parameters. It's kind of like a cast.

t.vanbesien

@JonB said in no matching member function for call to 'connect':

I don't believe that is a working copy/paste, I think you mean QOverload<>::of(&QSignalMapper::map)?

No, the line I copy/pasted does compile and run and has the expected behavior.

JonB

@t-vanbesien
Interesting. Could you tell me/give me a reference to where you got an example or documentation which uses QOverload<...>::of<...>(...) rather than QOverload<...>::of(...), please? Note the bit I am asking about is the ::of<> having a <...> segment?

t.vanbesien

@JonB
I don't know ? I'm not sure I understand the question. I used

QOverload<...>::of<...>()

because you wrote it in your first answer to this post. I can't find any documentation about it. If you want a working example I can share the class .cpp and .h of the class and the small debug function I made to make sure it is working properly. You can put it in an empty qt project to check.

JonB

@t-vanbesien said in no matching member function for call to 'connect':

because you wrote it in your first answer to this post.

Damn! So I did, I was guessing at that point! :( Now I know better, from the SO link I gave you :)

I suggest that you try copy/paste the following line to replace yours:

connect(lineEdit, &QLineEdit::editingFinished, signalMapper, QOverload<>::of(&QSignalMapper::map));

Even though your ::of<>( apparently works, it should be ::of(. Don't confuse yourself by using my guessed-syntax, for when you come back to it.

t.vanbesien

@JonB

I replaced the line so that now it is:

connect(lineEdit, &QLineEdit::editingFinished, signalMapper, QOverload<>::of(&QSignalMapper::map));

And everything is still working properly.

By the way, you wrote 'IOC' in one of your answers, what does it mean ?

JonB

@t-vanbesien said in no matching member function for call to 'connect':

By the way, you wrote 'IOC' in one of your answers, what does it mean ?

I wrote OIC => O-I-C => "Oh I See" :)