Looking for a good way to make a widget use another widget

[jsulm]

@Gilboonet Rerun qmake/cmake after adding Q_OBJECT and rebuild.
In case you use CMake make sure you have this in your CMakeLists.txt file: https://cmake.org/cmake/help/latest/prop_tgt/AUTOMOC.html

[Gilboonet]

@jsulm I added

set_property(TARGET Deplieur PROPERTY AUTOMOC ON)

But it was not necessarily needed as there was already this parameter set :

set(CMAKE_AUTOMOC ON)

Anyway, it still doesn't compile, if I only add Q_OBJECT at the beginning of my class, I have errors starting from

error: 'staticMetaObject' is not a member of 'QGraphicsPolygonItem'

I tried to make my class co-inherit QObject

class TriangleItem : public QObject, public QGraphicsPolygonItem

Then it compiles but there's a linking error :

CMakeFiles/Deplieur.dir/Deplieur_autogen/mocs_compilation.cpp.obj: in function `QtPrivate::QMetaTypeForType<TriangleItem>::getDefaultCtr()::{lambda(QtPrivate::QMetaTypeInterface const*, void*)#1}::operator()(QtPrivate::QMetaTypeInterface const*, void*) const':

I'm surely doing it wrong.

[Gilboonet]

It's too bad that it's so hard to add signal/slot functionnalities to a widget, because it seem very interesting. For the moment I have another solution that works. I subclassed QGraphicsScene, add a pointer to my TableWidget on it, then on MouseDoubleClick event handler of my TriangleItem, I simply do a dynamic cast on its scene to and then use the pointer to the TableWidget to change its row correspondingly to the item's color id.

[JonB]

@Gilboonet
I forgot class TriangleItem : public QGraphicsPolygonItem means no Object so cannot itself emit signals.

Either persist with class TriangleItem : public QObject, public QGraphicsPolygonItem which is indeed what will be required (keep the Q_OBJECT, try cleaning everything out and rebuilding from scratch) or do it at the QGraphicsScene level, recognising the click/double-click on the item (e.g. https://stackoverflow.com/questions/26079807/function-getting-clicked-object-from-qgraphicsscene), that can emit a signal.

It's too bad that it's so hard to add signal/slot functionnalities to a widget

It's not. Anything that's a QWidget will have signals/slots. The issue here is because a QGraphicsItem is not a QObject. Most of the time signals/slots are easy, and they are very helpful. If you do UIs in Qt you will use signals/slots all the time.

[Gilboonet]

@JonB I tried to add QObject (it's on my reply) but then there's a linking error that made me conclude just like you said that QgraphicsItem is not a QObject. It could certainly be done but so I need to avoid using signals on my subclassed QGraphicsPolygonObject and I don't want to lose those abilities (dragging, group selection). I'm going to try at the QGraphicsScene level because I'm sure signals/slots are worth the effort.

[Gilboonet]

With my subclassed QGraphicsScene I have the same linking error :

[17/20 0,6/sec] Linking CXX executable Deplieur.exe
FAILED: Deplieur.exe 
C:\WINDOWS\system32\cmd.exe /C "cd . && C:\Qt\Tools\mingw1310_64\bin\g++.exe -DQT_QML_DEBUG -g -mwindows CMakeFiles/Deplieur.dir/Deplieur_autogen/mocs_compilation.cpp.obj CMakeFiles/Deplieur.dir/main.cpp.obj CMakeFiles/Deplieur.dir/mainwindow.cpp.obj CMakeFiles/Deplieur.dir/depliage.cpp.obj CMakeFiles/Deplieur.dir/facette.cpp.obj CMakeFiles/Deplieur.dir/mat4x4.cpp.obj CMakeFiles/Deplieur.dir/mesh.cpp.obj CMakeFiles/Deplieur.dir/triangleitem.cpp.obj CMakeFiles/Deplieur.dir/vec3d.cpp.obj CMakeFiles/Deplieur.dir/voisin.cpp.obj CMakeFiles/Deplieur.dir/triangle2d.cpp.obj CMakeFiles/Deplieur.dir/structures.cpp.obj CMakeFiles/Deplieur.dir/deplieurscene.cpp.obj CMakeFiles/Deplieur.dir/Deplieur_autogen/EWIEGA46WW/qrc_resources.cpp.obj -o Deplieur.exe -Wl,--out-implib,libDeplieur.dll.a -Wl,--major-image-version,0,--minor-image-version,0  C:/Qt/6.8.0/mingw_64/lib/libQt6Widgets.a  C:/Qt/6.8.0/mingw_64/lib/libQt6Gui.a  C:/Qt/6.8.0/mingw_64/lib/libQt6Core.a  -lmpr  -luserenv  -lmingw32  C:/Qt/6.8.0/mingw_64/lib/libQt6EntryPoint.a  -lshell32  -ld3d11  -ldxgi  -ldxguid  -ld3d12  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 && cd ."
C:/Qt/Tools/mingw1310_64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: CMakeFiles/Deplieur.dir/triangleitem.cpp.obj: in function `TriangleItem::mouseDoubleClickEvent(QGraphicsSceneMouseEvent*)':
C:/Users/gilbe/Documents/dev/C++/Deplieur/triangleitem.cpp:59: undefined reference to `DeplieurScene::ChangeToColor(int)'
collect2.exe: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

The undefined reference points to the signal

class DeplieurScene : public QGraphicsScene
{
public:
    explicit DeplieurScene(QObject *parent = nullptr);
    QTableWidget * twC;

signals:
    ChangeToColor(int);
};

[Christian Ehrlicher]

You forgot Q_OBJECT

[Gilboonet]

@Christian-Ehrlicher I don't think so, I added it first and also added public QObject as ancester and it causes an error saying that I used 2 classes with Q_OBJECT and it didn't compile, Then only adding Q_OBJECT as you suggest causes compilation error on a MOC generated file. I can try again to add Q_OBJECT but if it is needed there is also something else to add.

[0/4 0,0/sec] Re-checking globbed directories...
[1/20 0,5/sec] Automatic MOC and UIC for target Deplieur
[2/20 1,0/sec] Automatic RCC for resources.qrc
[3/20 0,2/sec] Building CXX object CMakeFiles/Deplieur.dir/mat4x4.cpp.obj
[4/20 0,2/sec] Building CXX object CMakeFiles/Deplieur.dir/facette.cpp.obj
[5/20 0,3/sec] Building CXX object CMakeFiles/Deplieur.dir/Deplieur_autogen/mocs_compilation.cpp.obj
FAILED: CMakeFiles/Deplieur.dir/Deplieur_autogen/mocs_compilation.cpp.obj 
C:\Qt\Tools\mingw1310_64\bin\g++.exe -DMINGW_HAS_SECURE_API=1 -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NEEDS_QMAIN -DQT_WIDGETS_LIB -DUNICODE -DWIN32 -DWIN64 -D_ENABLE_EXTENDED_ALIGNED_STORAGE -D_UNICODE -D_WIN64 -IC:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/include -isystem C:/Qt/6.8.0/mingw_64/include/QtCore -isystem C:/Qt/6.8.0/mingw_64/include -isystem C:/Qt/6.8.0/mingw_64/mkspecs/win32-g++ -isystem C:/Qt/6.8.0/mingw_64/include/QtWidgets -isystem C:/Qt/6.8.0/mingw_64/include/QtGui -DQT_QML_DEBUG -g -std=gnu++20 -fdiagnostics-color=always -MD -MT CMakeFiles/Deplieur.dir/Deplieur_autogen/mocs_compilation.cpp.obj -MF CMakeFiles\Deplieur.dir\Deplieur_autogen\mocs_compilation.cpp.obj.d -o CMakeFiles/Deplieur.dir/Deplieur_autogen/mocs_compilation.cpp.obj -c C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/mocs_compilation.cpp
In file included from C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/mocs_compilation.cpp:2:
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:79:71: error: type/value mismatch at argument 1 in template parameter list for 'template<class T, class ForceComplete_> struct QtPrivate::TypeAndForceComplete'
   79 |         QtPrivate::TypeAndForceComplete<ChangeToColor, std::false_type>,
      |                                                                       ^
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:79:71: note:   expected a type, got 'DeplieurScene::ChangeToColor'
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:75:5: error: template argument 3 is invalid
   75 |     qt_incomplete_metaTypeArray<qt_meta_stringdata_CLASSDeplieurSceneENDCLASS_t,
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   76 |         // Q_OBJECT / Q_GADGET
      |         ~~~~~~~~~~~~~~~~~~~~~~
   77 |         QtPrivate::TypeAndForceComplete<DeplieurScene, std::true_type>,
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   78 |         // method ''
      |         ~~~~~~~~~~~~
   79 |         QtPrivate::TypeAndForceComplete<ChangeToColor, std::false_type>,
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   80 |         QtPrivate::TypeAndForceComplete<int, std::false_type>
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   81 |     >,
      |     ~
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp: In static member function 'static void DeplieurScene::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)':
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:91:32: error: expected ';' before '_r'
   91 |         case 0: { ChangeToColor _r = _t->((*reinterpret_cast< std::add_pointer_t<int>>(_a[1])));
      |                                ^~~
      |                                ;
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:91:19: error: statement cannot resolve address of overloaded function
   91 |         case 0: { ChangeToColor _r = _t->((*reinterpret_cast< std::add_pointer_t<int>>(_a[1])));
      |                   ^~~~~~~~~~~~~
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:92:43: error: 'ChangeToColor' does not name a type
   92 |             if (_a[0]) *reinterpret_cast< ChangeToColor*>(_a[0]) = std::move(_r); }  break;
      |                                           ^~~~~~~~~~~~~
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:92:56: error: expected '>' before '*' token
   92 |             if (_a[0]) *reinterpret_cast< ChangeToColor*>(_a[0]) = std::move(_r); }  break;
      |                                                        ^
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:92:56: error: expected '(' before '*' token
   92 |             if (_a[0]) *reinterpret_cast< ChangeToColor*>(_a[0]) = std::move(_r); }  break;
      |                                                        ^
      |                                                        (
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:92:57: error: expected primary-expression before '>' token
   92 |             if (_a[0]) *reinterpret_cast< ChangeToColor*>(_a[0]) = std::move(_r); }  break;
      |                                                         ^
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:92:78: error: '_r' was not declared in this scope; did you mean 'tr'?
   92 |             if (_a[0]) *reinterpret_cast< ChangeToColor*>(_a[0]) = std::move(_r); }  break;
      |                                                                              ^~
      |                                                                              tr
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:92:81: error: expected ')' before ';' token
   92 |             if (_a[0]) *reinterpret_cast< ChangeToColor*>(_a[0]) = std::move(_r); }  break;
      |                                                                                 ^
      |                                                                                 )
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:98:24: error: expected type-specifier before 'ChangeToColor'
   98 |             using _t = ChangeToColor (DeplieurScene::*)(int );
      |                        ^~~~~~~~~~~~~
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:99:17: error: '_t' was not declared in this scope; did you mean 'Qt'?
   99 |             if (_t _q_method = &DeplieurScene::; *reinterpret_cast<_t *>(_a[1]) == _q_method) {
      |                 ^~
      |                 Qt
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:99:68: error: '_t' does not name a type
   99 |             if (_t _q_method = &DeplieurScene::; *reinterpret_cast<_t *>(_a[1]) == _q_method) {
      |                                                                    ^~
C:/Users/gilbe/Documents/dev/C++/Deplieur/build/Desktop_Qt_6_8_0_MinGW_64_bit-Debug/Deplieur_autogen/EWIEGA46WW/moc_deplieurscene.cpp:99:71: error: expected '>' before '*' token

[Gilboonet]

What I missed was also void before the signal definition, it works now.

[Gilboonet]

Here's what I did :

• Subclass QGraphicsScene
• add Q_OBJECT to the subclass class definition
• add a signal with a parameter to transmit the color
• add a slot to my MainWindow class that also uses a parameter of same type
• connect somewhere on my MainWindow all that
• add an emit of the signel into my Item subclass MouseDoubleClickEvent() with the item color id as parameter

It's very light, I will try to use that mechanism whenever possible as it eliminates need for extra data

[Gilboonet]

Here is the implementation of the solution using Slots/Signals to make a event on a subclassed QGraphicsItem send a signal that will activate a slot on another widget. As GraphicsItem is not QObject, the signal could not be emited directly from the Item, but instead from its Scene. That is possible because GraphicsScene is QObject.

To add a signal to my Scene class, I needed to subclass it and add Q_OBJECT at the beginning of its definition. Then the signal is added to a section of the definition called Signals. It is a void function definition that can have parameters, but don't need to have an impementation.

The Q_OBJECT can certainly be added when you use menu option "File|New File|C++ class" and select it.

#ifndef DEPLIEURSCENE_H
#define DEPLIEURSCENE_H

#include <QGraphicsScene>

class DeplieurScene : public QGraphicsScene
{
    Q_OBJECT
public:
    explicit DeplieurScene(QObject *parent = nullptr);

signals:
    void changeCouleur(int);
};

#endif // DEPLIEURSCENE_H

Then, I think there are two possibilites, if your target widget class is a subclass you can add a slot to its definition on the private slots section, use the right click option to refactor and add its implementation. This slot must have the same parameter as your signal. Or if your target widget is a basic widget, you can simply add the slot to you MainWindow class. On the slot you use the parameter to do what you need to do.
my slot is in my MainWindow class and is :

void MainWindow::changeCouleur(int couleur)
{
    ui->tableCouleurs->selectRow(couleur);
}

Last, you must create the connection between the signal and the slot, somewhere on your MainWindow, mine is :

            dep->dessineModele();
            connect(dep->scene3d, &DeplieurScene::changeCouleur, this, &MainWindow::changeCouleur);
            dep->creeFaces2d();
            connect(dep->scene2d, &DeplieurScene::changeCouleur, this, &MainWindow::changeCouleur);