Q_DECLARE_METATYPE problem

[gib]

I'm now building with Qt5 code that compiled with no problems with Qt4, and getting an error message that is a bit obscure to me.

F:\Qt\5.13.0\msvc2017\include\QtCore\qmetatype.h:1 798: error: C2338: Type is not registered, please use the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object system
F:\Qt\5.13.0\msvc2017\include\QtCore\qmetatype.h:1 805: see reference to function template instantiation 'int qMetaTypeId<T>(void)' being compiled
with
[
T=QMouseEvent *
]
D:\testing\build-vspheroid-Qt3d\release\moc_myqgraphicsview.cpp:98: see reference to function template instantiation 'int qRegisterMetaType<QMouseEvent*>(void)' being compiled
F:\Qt\5.13.0\msvc2017\include\QtCore\qatomic.h:56: see reference to class template instantiation 'QBasicAtomicInteger<int>' being compiled
F:\Qt\5.13.0\msvc2017\include\QtCore\qatomic.h:154 : see reference to class template instantiation 'QAtomicInteger<int>' being compiled

I have extended QGraphicsView to allow image save on a mouse click like this:

myqgraphicsview.h:

#ifndef MYQGRAPHICSVIEW_H
#define MYQGRAPHICSVIEW_H
 
#include <QWidget>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QMouseEvent>
#include <QFileDialog>
 
#include "log.h"
LOG_USE();
 
class MyQGraphicsView : public QGraphicsView
{
    Q_OBJECT
public:
    explicit MyQGraphicsView(QWidget *parent = nullptr);
 
signals:
    void viewClicked();
public slots:
    void mouseReleaseEvent(QMouseEvent * e);
    void saveImage();
private:
    // member variable to store click position
    QPoint m_lastPoint;
    // member variable - flag of click beginning
    bool m_mouseClick;
};
 
#endif // MYQGRAPHICSVIEW_H

#ifndef MYQGRAPHICSVIEW_H
#define MYQGRAPHICSVIEW_H

#include <QWidget>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QMouseEvent>
#include <QFileDialog>

#include "log.h"
LOG_USE();

class MyQGraphicsView : public QGraphicsView
{
    Q_OBJECT
public:
    explicit MyQGraphicsView(QWidget *parent = nullptr);

signals:
    void viewClicked();
public slots:
    void mouseReleaseEvent(QMouseEvent * e);
    void saveImage();
private:
    // member variable to store click position
    QPoint m_lastPoint;
    // member variable - flag of click beginning
    bool m_mouseClick;
};

#endif // MYQGRAPHICSVIEW_H

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myqgraphicsview.cpp:

#include "myqgraphicsview.h"
 
MyQGraphicsView::MyQGraphicsView(QWidget *parent) :
    QGraphicsView(parent)
{
}
 
void MyQGraphicsView::mouseReleaseEvent(QMouseEvent * e)
{
    if (e->button() == Qt::RightButton) {
       saveImage();
    }
}
 
void MyQGraphicsView::saveImage()
{
    QGraphicsScene *ascene = this->scene();
    ascene->clearSelection();                                                  // Selections would also render to the file
    ascene->setSceneRect(ascene->itemsBoundingRect());                         // Re-shrink the scene to it's bounding contents
    QImage image(ascene->sceneRect().size().toSize(), QImage::Format_ARGB32);  // Create the image with the exact size of the shrunk scene
    image.fill(Qt::transparent);                                               // Start all pixels transparent
    QPainter painter(&image);
    ascene->render(&painter);
    QString fileName = QFileDialog::getSaveFileName(this, tr("Image File Name"), ".", tr("Image Files (*.png)"));
    if (fileName.compare("") != 0) {
        image.save(fileName);
    }
}

#include "myqgraphicsview.h"

MyQGraphicsView::MyQGraphicsView(QWidget *parent) :
    QGraphicsView(parent)
{
}

void MyQGraphicsView::mouseReleaseEvent(QMouseEvent * e)
{
    if (e->button() == Qt::RightButton) {
       saveImage();
    }
}

void MyQGraphicsView::saveImage()
{
    QGraphicsScene *ascene = this->scene();
    ascene->clearSelection();                                                  // Selections would also render to the file
    ascene->setSceneRect(ascene->itemsBoundingRect());                         // Re-shrink the scene to it's bounding contents
    QImage image(ascene->sceneRect().size().toSize(), QImage::Format_ARGB32);  // Create the image with the exact size of the shrunk scene
    image.fill(Qt::transparent);                                               // Start all pixels transparent
    QPainter painter(&image);
    ascene->render(&painter);
    QString fileName = QFileDialog::getSaveFileName(this, tr("Image File Name"), ".", tr("Image Files (*.png)"));
    if (fileName.compare("") != 0) {
        image.save(fileName);
    }
}

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I don't know where and how I should insert the Q_DECLARE_METATYPE line. I don't know what metatype it is referring to (or even what a metatype is).

[Fareanor]

I tried the code you posted and it compiles fine without any error (as expected). So to be honest I don't understand why you have this error message.

In fact you don't have to declare Qt's types as metatypes as Qt already knows them.
Normally you should not have to declare any metatype for overriding the mouseReleaseEvent().

Perhaps you can try to rebuild everything from scratch, from a clean base (remove every build-generated files, check your .pro file, run qmake and then run the compilation).

To answer your questioning about the metatypes, you need to use Q_DECLARE_METATYPE for user custom classes/structures if you want to use them in QVariant or in signals/slots connections. The related type must provide a public default constructor, a public copy constructor and a public destructor to be eligible to be declared as a metatype. Usually, we call the macro right below the class/struct declaration in the header file. If you use queued connections, you'll need to call qRegisterMetaType() too.
For more accurate information, look at the documentation about it.

[gib]

Thanks for looking at this. The situation now is even more confusing. I have removed MyQGraphicsView from the program, which eliminated that error. But after adding back most of the rest of the code (I started with a very stripped-down version of the program I am converting over from Qt4 to Qt5) I am getting the same Q_DECLARE_METATYPE error, but now I can't see where it is coming from. These are the 5 lines of error messages:

F:\Qt\5.13.0\msvc2017\include\QtCore\qmetatype.h:1 798: error: C2338: Type is not registered, please use the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object system
F:\Qt\5.13.0\msvc2017\include\QtCore\qmetatype.h:1 805: see reference to function template instantiation 'int qMetaTypeId<T>(void)' being compiled
with
[
T=QMouseEvent *
]
D:\testing\build-vspheroid-Qt3d\release\moc_mainwindow.cpp:686: see reference to function template instantiation 'int qRegisterMetaType<QMouseEvent*>(void)' being compiled
F:\Qt\5.13.0\msvc2017\include\QtCore\qatomic.h:56: see reference to class template instantiation 'QBasicAtomicInteger<int>' being compiled
F:\Qt\5.13.0\msvc2017\include\QtCore\qatomic.h:154 : see reference to class template instantiation 'QAtomicInteger<int>' being compiled

The relevant code in moc_mainwindow.cpp is:

case 73:
            switch (*reinterpret_cast<int*>(_a[1])) {
            default: *reinterpret_cast<int*>(_a[0]) = -1; break;
            case 0:
                *reinterpret_cast<int*>(_a[0]) = qRegisterMetaType< QMouseEvent* >(); break;
            }
            break;

case 73:
            switch (*reinterpret_cast<int*>(_a[1])) {
            default: *reinterpret_cast<int*>(_a[0]) = -1; break;
            case 0:
                *reinterpret_cast<int*>(_a[0]) = qRegisterMetaType< QMouseEvent* >(); break;
            }
            break;

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i.e. again reference to QMouseEvent. The file mainwindow.cpp is quite long, a couple of thousand lines, and I have no idea what part of the code is generating this error. There is no indication of what is causing the error. I have cleaned everything, run qmake and rebuilt (many times).

Edit: I have discovered that the error is triggered by this function:

void MainWindow::clickedGraph(QMouseEvent *event)
{
    if (event->button() == Qt::RightButton) {
        LOG_QMSG("Right button click");
        QString fileName = QFileDialog::getSaveFileName(this, tr("Select image file"), ".",
            tr("Image files (*.png)"));
        if (fileName.compare("") == 0) {
            return;
        }
        colony_plot->savePng(fileName);
    }
}

void MainWindow::clickedGraph(QMouseEvent *event)
{
    if (event->button() == Qt::RightButton) {
        LOG_QMSG("Right button click");
        QString fileName = QFileDialog::getSaveFileName(this, tr("Select image file"), ".",
            tr("Image files (*.png)"));
        if (fileName.compare("") == 0) {
            return;
        }
        colony_plot->savePng(fileName);
    }
}

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specifically, it appears, by the function argument. There doesn't seem to be anything unusual about this.

[Fareanor]

It seems strange indeed.

When you "cleaned everything", did you also remove the moc files ? (in order to let qmake regenerate them properly).

If this problem still appears, I think it comes either from the way you do the porting from Qt4 to Qt5 or from some code that becomes wrong from a version to another by generating side effects (which makes it harder to track).
I do not pretend it is the case but if we could have a complete, minimal, reproducible (compilable and runnable) example that illustrates your problem, it would be much better to find the problem (and this way, we will avoid uncertain conjectures).

[d_stranz]

Any errors from Qt's metatype systems are usually due to MOC (the "meta-object compiler"), which reads your header files and generates a "moc" file with metatype information for every class where the Q_OBJECT macro is declared.

If, in doing your porting, you are simply copying over moc_*.cpp and ui_*.h files, then this is incorrect. These files are generated during the build process from the corresponding *.h and *.ui files by the MOC and UIC programs. You should remove them then rebuild the whole thing so they can be regenerated against the proper set of Qt 5 headers and libraries.

[gib]

No, I started with just the source files, certainly no moc_ files and no ui_*.h files. I'm on the trail of the bug again this morning.

Edit: If I comment out clickedGraph I can build a working version of the program (not yet complete, but displaying the UI). If I comment out just the contents of the function, so I have just:

void MainWindow::clickedGraph(QMouseEvent *event)
{
}

I get the metatype error.

There is something very odd about this line:

void MainWindow::clickedGraph(QMouseEvent *event)

In Qt Creator 'MainWindow' normally shows as pink, the rest of the line a dark blue. On this line 'inWindow::cl' was pink, the rest dark blue. I made a copy of the line (commenting out the original line), and this showed all as dark blue. After a couple of cycles of commenting and uncommenting, the anomalous pink disappeared, now what I see is either:

//void MainWindow::clickedGraph(QMouseEvent *event)
void MainWindow::clickedGraph(QMouseEvent *event)

//void MainWindow::clickedGraph(QMouseEvent *event)
void MainWindow::clickedGraph(QMouseEvent *event)

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or

void MainWindow::clickedGraph(QMouseEvent *event)
//void MainWindow::clickedGraph(QMouseEvent *event)

void MainWindow::clickedGraph(QMouseEvent *event)
//void MainWindow::clickedGraph(QMouseEvent *event)

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In the first case, the uncommented line is all black, while in the second case it is all dark blue (I don't know if these colours show up here.)

Oh, now after some commenting and uncommenting, the uncommented line is all black in both cases. So I have a moving target, and I don't even know what it is.
It is looking increasingly like bad behaviour on the part of Qt Creator.

[d_stranz]

Then are you running qmake after adding new files to the project? Qmake is necessary in order to ensure that the moc_*.cpp and ui*.h files are added to the project when you add new source files.