QMetaObject::invokeMethod and SLOT macro: No such method

[candy76041820]

Qt5:

class C: public QObject{
    Q_OBJECT
public slots:
  void F(void);
};

C c;
QMetaObject::invokeMethod(&c, "F");//OK
QMetaObject::invokeMethod(&c, SLOT(F()));//QMetaObject::invokeMethod: No such method C::1F()()

Are QMetaObject::invokeMethod and SLOT macro incompatible?

WORKAROUND (for this particular scenario):
To be precise, what I need is invocation of slots connect()ed to signals of another class (we don't want to emit another object's signal even if they are public, do we?), but all I have would be the SLOT string. So a springboard should suffice:

class Signaller: public QObject{
    Q_OBJECT
signals:
    void S(void);
}s;
class Forwarder: public QObject{
    Q_OBJECT
public slots:
    void F(void){
        emit this->FS();
    }
signals:
    void FS(void);
};

class C; C c;//As defined above
const char *slot_str;//Its value comes from SLOT(F())
Forwarder *fwd=new Forwarder();
fwd->setParent(&c);
connect(&s, &Signaller::S, fwd, &Forwarder::F);
connect(fwd, SIGNAL(FS()), &c, slot_str);

Now fwd->F() will do the trick.

[JKSH]

Hi, and welcome!

@candy76041820 said in QMetaObject::invokeMethod and SLOT macro: No such method:

Are QMetaObject::invokeMethod and SLOT macro incompatible?

The SLOT() macro is for the old version of QObject::connect(). It is not for QMetaObject::invokeMethod().

[candy76041820]

So this means I have to make a workaround if the string SLOT returned is the only piece of information I have?

[JKSH]

@candy76041820 said in QMetaObject::invokeMethod and SLOT macro: No such method:

So this means I have to make a workaround if the string SLOT returned is the only piece of information I have?

Sounds like it.

How do you get the string returned by SLOT()?

[candy76041820]

@JKSH Isn't it just a plain const char *? Or will it invalidate after subsequent calls?

[SGaist]

Hi,

What the macro generates is however it's not literally what you pass in that is returned hence the error you got.

[candy76041820]

@SGaist Just thinking that maybe I'll parse the string and take the function name.

[mpergand]

@candy76041820

Here examples of some macros of mine:

#define InvokeLater(slot)  QMetaObject::invokeMethod(this, #slot,Qt::QueuedConnection)

#define InvokeLaterWithArg(slot,argType,arg)  QMetaObject::invokeMethod(this, #slot,Qt::QueuedConnection,Q_ARG(argType,arg))

[candy76041820]

@mpergand Problem is that invokeMethod doesn't recognize what SLOT says.

[JKSH]

@candy76041820 said in QMetaObject::invokeMethod and SLOT macro: No such method:

Problem is that invokeMethod doesn't recognize what SLOT says.

I'm still trying to understand why this is a problem.

Can you please explain your use-case? How come the SLOT() string is the only piece of information that you have?

Isn't it just a plain const char *?

Yes, it's a const char*, but it's in the wrong format for invokeMethod().

Anyway, my question was: How do you obtain your slot function name?

[candy76041820]

@JKSH Well, the scenario:

namespace Helper{
    class QFileDialogSaveCanceller: public QObject{
        Q_OBJECT
    public slots:
        void Cancel(void){
            emit this->Cancelled(QString());
        }
    signals:
        void Cancelled(const QString &empty);
    }
    void InvokeQFileDialogSave(QWidget *window, const char *slot/*void(const QString &)*/, .../*filters, default name & extension, etc*/){
        QFileDialog *dlg=new QFileDialog();//Can't use the static getSaveFileName(), since I want the default extension auto-added
        QFileDialogSaveCanceller *canceller=new QFileDialogSaveCanceller;
        canceller->setParent(dlg);
        connect(canceller, SIGNAL(Cancelled(const QString &)), window, slot); 
        connect(dlg, &QDialog::rejected, canceller, &QFileDialogSaveCanceller::Cancel);
        /*connect(dlg, &QDialog::rejected, [window](void)->void{
            QMetaObject::invokeMethod(window, slot, Q_ARG(QString()));//No such method
        });*/
        dlg->open(window, slot);
    }
}

class MainWindow: public QMainWindow{};
MainWindow mw;
Helper::InvokeQFileDialogSave(&mw, SLOT(FileSelected(const QString &)), ...);

I'm just too lazy to template-ize the helper function, neither do I wanna write another "MainWindow::Cancelled(void)" slot to do the forwarding. Tried invokeMethod but no luck, thus this post.

[JKSH]

@candy76041820 said in QMetaObject::invokeMethod and SLOT macro: No such method:

I'm just too lazy to template-ize the helper function, neither do I wanna write another "MainWindow::Cancelled(void)" slot to do the forwarding. Tried invokeMethod but no luck, thus this post.

That's an interesting setup.

You could use function pointers instead of strings:

using MySlot = void(QWidget::*)(const QString&);

void InvokeQFileDialogSave(QWidget *window, MySlot slot) {
    auto dlg = new QFileDialog;
    auto canceller = new QFileDialogSaveCanceller;
    
    connect(canceller, &QFileDialogSaveCanceller::Cancelled,
            window, slot); 

    connect(dlg, &QDialog::rejected, [window, slot] {
        (window->*slot)( QString() );
    });

    // ...
}

Or, you could use string manipulation to find the slot name.

//Can't use the static getSaveFileName(), since I want the default extension auto-added

The selectedFilter parameter of getSaveFileName() lets you set the default extension

[candy76041820]

@JKSH Not applicable because I have other event sink classes.
Also, I use the SLOT string because (for the sake of keeping this post clear, I didn't demonstrate that) I have file-open scenarios too: void OnSingleSelect(const QString &file) and void OnMultiSelect(const QStringList &files). They don't share signatures, so either I need to template-ize (which doesn't work well w/ signals&slots), or to repeat and tweak the helper function.
Wait: will selectedFilter trigger setDefaultSuffix along with selectNameFilter? Didn't find evidence in the source tree.
Also again: Linux Qt 5.6.1, selectedFilter doesn't setDefaultSuffix. Call chain according to https://code.qt.io/cgit/qt/qtbase.git/plain/src/widgets/dialogs/qfiledialog.cpp :
QFileDialog::getSaveFileName -> QFileDialog::getSaveFileUrl -> QFileDialog::selectNameFilter -> QFileDialogPrivate::_q_useNameFilter, which doesn't seem to be doing the work (at least, in my case) after the user clicked OK.