I'm trying to trigger a signal when a double click happens in one of the draggable widgets on the fridge magnets example. Here's the changes I made to the example source:
DragLabel:
class DragLabel : public QLabel
{
public:
DragLabel(const QString &text, QWidget *parent);
QString labelText() const;
public slots:
void testSlot(){qDebug()<<"testSlot";} //<-- implemented this slot
protected:
void mouseDoubleClickEvent(QMouseEvent *ev){emit testSignal();} //<-- overriden this method
private:
QString m_labelText;
signals:
void testSignal(); //<-- added this signal
};
The only thing I changed in the implementation file is adding connect(this,SIGNAL(testSignal()),this,SLOT(testSlot())); to DragLabel's constructor.
Trying to compile the project resulted in 'undefined reference to `DragLabel::testSignal()' and 'collect2: ld returned 1 exit status' errors.
When I comment out the call to the signal, it compiles and runs, but gives off 'Object::connect: No such signal QLabel::testSignal() in draglabel.cpp' warning in the application output. Apparently testSignal() isn't being recognized as a signal.
I've tried to add the Q_OBJECT macro to DragLabel but it results in 4 'undefined reference to `vtable for DragLabel'' warnings and a 'collect2: ld returned 1 exit status' error.
What am I missing?
Put the Q_OBJECT macro at the top, (must be first thing in the class and no ";" )
Make sure you do a full rebuild, the VS-add-in especially doesn't always notice that a file has become qt-aware without a rebuild.
More good advice 20 ways to debug Qt signals and slots
It WAS the macro in the end. I had to reboot my PC for it to work though, cleaning and rebuilding the project didn't work out. Before rebooting Qt Creator kept giving the 'ld returned 1 exit status' error and the vtable warnings. Really weird. – David McDavidson
It's not weird, it's stupid.
I got the same error, but I make it after rearrange the .h files. Say:
1 classA.h include calssB.h ;
2 classB.h declared two class, classB and classC, (classB.h declared signals & slot)
I do three things,
seperate classC to another .h file
eliminate all forward class declaration about classB
classB.h included by classA.cpp, other than by classA.h
after that, QT compiled it. I'll test if it is working.
Related
I have inherited a class called GraphicsPixmapItem from QGraphicsPixmapItem in order to override/create some mouse events. The problem is that I want to emit a signal after some calculations are performed, but it looks like it's not possible unless some hacks are performed, since this class is not a QObject.
To do so, I have tried to inherit the aformentioned new class from QObject as well, but I keep getting compiler errors.
My attemp:
Header file (graphicspixmapitem.h):
#ifndef GRAPHICSPIXMAPITEM_H
#define GRAPHICSPIXMAPITEM_H
#include <QObject>
#include <QGraphicsPixmapItem>
#include <QGraphicsSceneMouseEvent>
class GraphicsPixmapItem : public QObject, public QGraphicsPixmapItem
{
Q_OBJECT
public:
explicit GraphicsPixmapItem(QGraphicsItem *parent = 0);
virtual void mousePressEvent(QGraphicsSceneMouseEvent * mouseEvent);
virtual void mouseReleaseEvent(QGraphicsSceneMouseEvent * mouseEvent);
signals:
void translationVector(QPointF info);
};
#endif // GRAPHICSPIXMAPITEM_H
Source file (graphicspixmapitem.cpp):
#include "graphicspixmapitem.h"
GraphicsPixmapItem::GraphicsPixmapItem(QGraphicsItem *parent) :
QGraphicsPixmapItem(parent)
{
}
void GraphicsPixmapItem::mousePressEvent(QGraphicsSceneMouseEvent * mouseEvent)
{
//Code
}
void GraphicsPixmapItem::mouseReleaseEvent(QGraphicsSceneMouseEvent * mouseEvent)
{
QPointF info;
//Code
emit(translationVector(info));
}
And I get the following linker errors:
undefined reference to `vtable for GraphicsPixmapItem'
undefined reference to
`GraphicsPixmapItem::translationVector(QPointF)'
Any clue about how to proceed accordingly?
Side note:
I am aware that QGraphicsObject may be a good alternative, but as discussed here, performance looks severely affected by the amount of signals that are emitted when operating with them, where most of them will not be used in my case. This is the reason why I prefer to create my own class with base QGraphicsItem, instead of QGraphicsObject.
Many thanks in advance.
It looks like the meta object compiler (moc) wasn't run over the code, or that moc's result wasn't included when linking.
Have you added graphicspixmapitem.h to qmake's HEADERS variable?
Have you re-run qmake and tried a clean build in general?
Is moc run on graphicspixmapitem.h? Check your compile log.
Is graphicspixmapitem_moc.o included when linking? Check your compile log.
I have finally found out the problem involving the linkage error. In this sense, I must give my thanks to Thomas McGuire for pointing out the key idea to look for the source of the problem.
The reason is that few days ago I attempted to subclass QGraphicsPixmapItem (for other purposes) with the exact same name than this one, namely, GraphicsPixmapItem (with header file graphicspixmapitem.h and source file graphicspixmapitem.cpp).
When I did that, I finally figured out that I could do things in a different way and I no longer needed that inherited subclass, hence I removed these files from the project. But doing this is a major mistake if you do not make sure to clean the project before removing the files from the project, because the files generated by qmake and moc (*.o, moc_*.cpp, moc_*.o) will remain in the build/debug directory unless you remove them by hand, because they will not be deleted by cleaning the project.
Therefore, it looks like in that situation, qmake detects that the files already exist and it does not generate the correct ones from the updated class, causing the linking error expounded above.
In summary, if you are going to delete some files from your project, make sure to clean it previously, especially if you are going to remove a class with the Q_OBJECT macro.
There are numerous threads on this all over. None seems to fit my bill. I'm getting the following linker errors in my code:
Undefined symbols for architecture x86_64:
"vtable for MSFSPlugin::MSFSPluginImpl", referenced from:
MSFSPlugin::MSFSPluginImpl::MSFSPluginImpl(QObject*) in MSFSPlugin.o
MSFSPlugin::MSFSPluginImpl::~MSFSPluginImpl() in MSFSPlugin.o
NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
It should have been something obvious as - a missing vtable usually means the first non-inline virtual member function has no definition. However, I don't see what I'm missing:
I have this class declation in MSFSPlugin.h:
class MSFSPlugin
:
public QObject,
public IMediaSource
{
Q_OBJECT
Q_INTERFACES(IMediaSource)
...
protected:
class MSFSPluginImpl;
MSFSPluginImpl* mImpl;
}
Then in MSFSPlugin.cpp, I have the following:
class MSFSPlugin::MSFSPluginImpl : public QThread
{
Q_OBJECT
public:
MSFSPluginImpl(QObject *parent = 0);
virtual ~MSFSPluginImpl();
QString getSourceDirectory() const;
void setSourceDirectory(QString sourceDirectory);
signals:
void loadDirectoryFinished(bool success);
protected:
QString mSourceDirectory;
};
Followed by definitions:
MSFSPlugin::MSFSPluginImpl::MSFSPluginImpl(QObject *parent) : QThread(parent)
{
}
MSFSPlugin::MSFSPluginImpl::~MSFSPluginImpl()
{
}
QString MSFSPlugin::MSFSPluginImpl::getSourceDirectory() const
{
return mSourceDirectory;
}
void MSFSPlugin::MSFSPluginImpl::setSourceDirectory(QString sourceDirectory)
{
mSourceDirectory = sourceDirectory;
}
...
In short, I don't think I'm missing any non-inline virtual member function definition. I'm using:
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
Additional Info:
In my moc_MSFSPlugin.cpp I don't see the autogenerated Q_OBJECT related code for class MSFSPlugin::MSFSPluginImpl which indirectly (via QThread) derives from QObject. Specifically, I don't see code generated for the signal, declared in that class (loadDirectoryFinished). Could this be the problem?
EDIT 1:
The error goes away if I comment out Q_OBJECT from the declation of MSFSPlugin::MSFSPluginImpl, but then I lose the signal functionality.
EDIT 2:
I see moc operates on header files only. Could this be related to the fact that my QObject derived class is declared & defined in a .cpp file?
Assuming we're dealing with qmake.
The golden rules are:
Make sure the Q_OBJECT macro is present in the definition of all QObject-derived classes.
Make sure you declare your QObject-derived classes in your header files only.
Make sure all of your header files are listed in your .pro file in the HEADERS= list.
Run qmake every time you add Q_OBJECT to one of your classes or modify your .pro file.
Explanation
EDIT 1: The error goes away if I comment out Q_OBJECT from the declation of MSFSPlugin::MSFSPluginImpl, but then I lose the signal functionality.
Yup. Q_OBJECT is needed for declaring signals, slots, invokables, qobject_cast, translations, properties, enums and methods introspection, and so on.
The #1 golden rule comes from the fact that without Q_OBJECT you can't use stuff like qobject_cast on your class; if you use (even indirectly) introspection facilities, for instance to debug an object hierarchy or to dump all active connections to an object, then objects of your class will have the real class name shown (and not the one of the superclass); etc.
Q_OBJECT does two things:
it tells builsystems to add calls to moc, whose job is to generate some extra code for the class. This code will provide all the facilities listed above;
since it's a normal C++ macro, when compiled it expands to a few declarations, including a couple of virtuals: qt_metacall() and metaObject(). moc will generate the implementation for these virtuals.
The error you get is the typical symptom of having declared the virtuals (because the macro was expanded in your code) but moc wasn't run, you had some unimplemented virtuals which will make the linking fail.
With gcc and GNU ld, you get an even more cryptic error, about an undefined reference to vtable for ClassName. Of course, googling such errors will immediately tell you how to solve the issue.
EDIT 2: I see moc operates on header files only. Could this be related to the fact that my QObject derived class is declared & defined in a .cpp file?
So, the question is: why wasn't moc run on a file containing a definition of a class with the Q_OBJECT macro?
When we use qmake to generate your Makefiles, then qmake will scan all the header files listed in the HEADERS variable. When it finds that a header contains a class definition with the Q_OBJECT macro, it will also emit instructions (in the Makefile) to run moc over that header, compile moc's output and link the resulting object in the final target.
And we have rules #2, #3, #4 right here.
#2 tells us to put Q_OBJECT classes in headers; and that's because HEADERS lists headers, not sources.
#3 tells us to indeed put all headers into the HEADERS list. That obviously because if a header containing a Q_OBJECT is not in that list, then qmake won't find it and emit the rules. (While not strictly necessary for headers not containing QObject subclasses, it's good practice to put every header in there in order to forget none.)
#4 tells us to re-run qmake every time we add Q_OBJECT or modify the .pro file. The reason for the first part of the rule is that if qmake already scanned a header and found no Q_OBJECT, then it didn't emit any rules in the Makefile. But adding Q_OBJECT also needs those rules; hence we need qmake to re-scan the headers, and that's precisely what re-running qmake does.
The same reason applies when the .pro is modified (for instance, when adding more headers -- maybe with Q_OBJECT under HEADERS).
Note that if you're using a GNU-like make, then qmake will emit a special rule that tells make to re-run qmake and then restart the Makefile, if the .pro gets modified after the Makefile. That's why usually on UNIX you don't need to manually re-run qmake when you modify your .pro -- just running make will also run qmake again. But this doesn't work everywhere.
So, is it impossible to have classes definition containing Q_OBJECT in a .cpp file?
No, it's perfectly possible, but it requires using a somehow undocumented qmake feature. The trick is adding a line like:
#include "foobar.moc"
at the end of the foobar.cpp file, file that contains one of more class definitions with Q_OBJECT.
qmake will find this special inclusion and generate a rule for moc to create foobar.moc, which will then be included by the .cpp and thus compiled together with it. (Hence, there will be no extra rules for compiling foobar.moc nor to link the result.)
EDIT 1: The error goes away if I comment out Q_OBJECT from the declation of MSFSPlugin::MSFSPluginImpl, but then I lose the signal functionality.
Yes, the Q_OBJECT macro is necessary for signals, slots, properties and so on.
EDIT 2: I see moc operates on header files only. Could this be related to the fact that my QObject derived class is declared & defined in a .cpp file?
Yes and no. I will explain..
Usually, it is solved by separation as you write, but it is also possible to include the moc file after your class definition to get it work, but you need to remember not to put more than one in there to avoid strange consequences.
Therefore, in your case, you could establish a MSFSPlugin_p.h or MSFSPluginImpl.h file for the implementation header.
By the way, it is a bad idea to make pimpl protected. The pimpl idiom means private implementation, not protected.
I am learning GUI coding with Qt and hope to clear up some confusion on my part. When I create a dialog with Qt Creator it creates code for me like this...
#ifndef LISTDIALOG_H
#define LISTDIALOG_H
#include <QDialog>
#include "ui_listdialog.h" //Q1:Why was this auto paced in cpp file instead of h file?
//Q2: This is what I'm really confused about.
//Is Ui namespace wrapping ui_listdialog class or the ListDialog class?
namespace Ui {
class ListDialog;
}
class ListDialog : public QDialog
{
Q_OBJECT
public:
explicit ListDialog(QWidget *parent = 0); //Q3: Why is this constructor explicit?
~ListDialog();
//CUSTOM FUNCTIONALITY NOT ADDED BY CREATOR (IGNORE FOR MY POST)
private slots:
void addItem();
void editItem();
void deleteItem();
//END CUSTOM FUNCTIONALITY
private:
Ui::ListDialog *ui; //Placed on heap instead of stack.
};
#endif // LISTDIALOG_H
There are things in the above code that differ from my 3 Qt books (all 3 out of date and ignore Creator).
My main confusion comes from Q2. Is "Ui" wrapping "ui_listdialog.h" or the class I have posted here ( class ListDialog )? The syntax seems to imply to me that it is wrapping the latter but I feel it must be actually wrapping the ui_listdialog.h class instead. Very confused about this. Can someone explain this clearly?
I also don't understand why the constructor was made explicit by Creator. I have not seen that in any of the 3 books.
Q1. The #include is placed in the .cpp to avoid too many dependencies in the header file. This shortens compilation time, because if you change the dialog, the only thing you have to recompile is the .cpp and not everything that includes your header file. In general, if a forward declaration of a class is enough (i.e. you only have a pointer or a reference to it in your class), then it's better not to #include the class's definition.
Q2. Ui is a namespace that contains a different class called ListDialog. You can open the header file and see the definition of this other class. A bit confusing until you get used to it.
Q3. It's a good habit to use the explicit keyword with constructors that take a single parameter. Otherwise the constructor can also be used as an automatic conversion operator, and this can cause problems if you're not aware of it. For example, if you have a function that takes a ListDialog parameter, and you pass a QWidget * parameter, it may call the constructor when in fact you want the compiler to shout (invalid parameter).
The ui_listdialog.h contains implementation to generate your user interface based on Qt Designer. It isn't necessary when declaring the class -- that's why the file was #included in the .cpp (Q1). Without the ui_listdialog.h in the header, the class declaration is necessary (Q2).
As for Q3, it's probably there to make you use the constructor syntax. Else, you could write misleading statements like
ListDialog dialog = parentDialog ;
here's the code of my problem
class Base : QThread
{
Q_OBJECT
virtual void run() = 0;
signals:
void Ping(int);
};
class Derived : public Base
{
void run()
{
emit Ping(42);
}
}
the signal(42) won't reach/call to slots. what's wrong?
thanks in advance.
Did that 100 times, it does work. Are you sure your base class is properly MOC'ed ? (i.e. defined in a file contained in HEADERS section of .pro) Also when connecting your signal, check the return status of QObject::connect (it's a boolean). A good practice is something like that
bool r=false;
r=QObject::connect(pObj1,SIGNAL(signalStuff()),pObj2,SLOT(gotStuff()));
Q_ASSERT(r);
As Liz noticed, if something went wrong in your connect, you can check the traces to know what happened.
I can also note :
you don't have to redefine run in your base class, it's already defined by QThread
Common pitfall with QThread: Base class and Derived class belong to the thread which created them, not in the newly created thread
you don't connect your signal to any slot in your sample, so it won't trigger anything (I suppose it's done elsewhere)
your class Derived lacks the final ;
EDIT:
Edited to take into account liz' interesting comment.
I came upon the same problem but managed to find solution. The problem in my case wasnt the inheritance (even tho i did emit from derived class).
The problem was the code calling emit signal was execute BEFORE the code connecting signal with slot. Therefor both signal was emitted and signal-slot connection worked fine, but the code in the slot wasnt executed because emit happened before connecting the slot to signal.
Maybe that helps someone in the future.
I think you are trying to connect signal and slot from different threads.
Did you read this article?
For signal and slot of below type
signals:
void textChanged(const QString &);
public slots:
void setText(const QString & text)
the type of argument of textChanged and setText seems to work invarable of const and &. Does the constant and reference qualification make any difference compared to just using QString ?
QObject::connect(a,SIGNAL(textChanged(QString)),b,SLOT(setText(QString)));
QObject::connect(a,SIGNAL(textChanged(const QString &)),b,SLOT(setText(const QString &)));
EDIT:
I did not notice the output window showing error messages when there is incompatible type being used in SIGNAL or SLOT. I thought the signal slot mechanism is capable of detecting argument type error at compile time.
Qt checks a normalized signature, meaning
Normalization reduces whitespace to a
minimum, moves 'const' to the front
where appropriate, removes 'const'
from value types and replaces const
references with values.
Disclaimer: My qt is rather rusty, but the signal/slot mechanism is still just C++ function calls. If the signal/slot mechanism actually copies objects into internal storage, my apologies (you'll need to check the Qt pages, there's a big one on signals/slots afaik) - as the bits below will only be relevant in a C++ context, not in a C++ + Qt context.
If you leave out the reference, the string will be copied (and having the const would not matter, any changes made to it will remain in the function alone).
If you leave in the reference but take out the const, you allow the method to modify the string that you give it. They both work, but do different things to the object you pass (amount of copying/possibility of retaining changes).
I suggest you read the following resources:
(on const correctness) https://isocpp.org/wiki/faq/const-correctness
(on references) https://isocpp.org/wiki/faq/references
to understand exactly what passing a parameter is and how
void foo(const A&)/
void foo(const A)/
void foo(A&)/
void foo(A)
are all different.