I am quite new to Qt framework. I try to code a class that contains a 2D array whose data is supposed to be displayed in the main window (using QTableWidget, for example). The class has the following declaration:
// gridgame.h
template <int gsize_r,int gsize_c>
class GridGame : public QObject
{
Q_OBJECT
private:
char field[gsize_r][gsize_c];
public:
GridGame();
void setupConnect(QMainWindow & win);
signals:
void onPrintStatus(char[gsize_r][gsize_c]);
};
The definition of the class is of course given in cpp file as follows:
//gridgame.cpp
#include "gridgame.h"
template <int gsize_r,int gsize_c>
GridGame<gsize_r,gsize_c>::GridGame()
{
int i,j;
for (i=0;i<gsize_r;++i)
for (j=0;j<gsize_c;++j)
field[i][j] = '-';
}
template <int gsize_r,int gsize_c>
void GridGame<gsize_r,gsize_c>::setupConnect(QMainWindow &win)
{
connect(this,SIGNAL(onPrintStatus()),&win,SLOT(onRefresh()));
}
template <int gsize_r,int gsize_c>
void GridGame<gsize_r,gsize_c>::onPrintStatus(char[gsize_r][gsize_c]){
emit field;
}
The class is used in main as follows:
#include <QtCore>
#include <QApplication>
#include <QObject>
#include "mainwindow.h"
#include "gridgame.cpp"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
const int sizer = 5, sizec = 5;
GridGame<sizer,sizec> gg;
w.show();
gg.printStatus();
return a.exec();
}
Question: The code seems to compile ok, but I get the linker errors even if I do not call any function of the class (just have a declaration of a variable)
-1: error: undefined reference to `GridGame<5, 5>::metaObject() const'
-1: error: undefined reference to `GridGame<5, 5>::qt_metacast(char const*)'
-1: error: undefined reference to `GridGame<5, 5>::qt_metacall(QMetaObject::Call, int, void**)'
-1: error: error: ld returned 1 exit status
What might be the problem? I am using Qt Creator 3.6.1 and Qt library 5.6.0.
Thanks in advance for any help.
Related
I wrote a little program with a my own class within the main.cpp. Here the code:
#include <QApplication>
#include <QPushButton>
#include <QLabel>
class MyWidget : public QWidget {
//Q_OBJECT
public:
MyWidget(QWidget* parent = 0);
QLabel* label;
QString string;
signals:
public slots:
void setTextLabel();
};
void MyWidget::setTextLabel() {
label->setText("Test");
}
MyWidget::MyWidget(QWidget* parent)
: QWidget(parent) {
}
int main(int argc, char** argv) {
QApplication app(argc, argv);
MyWidget widget;
widget.show();
return app.exec();
}
it seems work but not "completely". My slot doens't work. I suppose i have to put Q_OBJECT. BUT, doing so, I got a list of errors, like this:
undefined reference to `vtable for MyWidget'
........................................
collect2: error: ld returned 1 exit status
make: *** [mywidget] Error 1
I can I manage that? Where the problem?
Signals and slots in Qt are managed through the moc: meta object compiler. Basically, the moc generates additional C++ code for each class containing the Q_OBJECT macro in order to implement effectively the signals and slots mechanisms. The additional code is then linked to the original class declaration.
The problem here is that your class is declared in main.cpp: this conflicts with how the moc is working with your code. You should declare your class in a separate header.
More about the moc
Edit: as hyde pointed, an alternative is to include in your cpp the file generated by the moc: Why is important to include “.moc” file at end of a Qt Source code file?
just append the line #include"main.moc" to your cpp source file should be enough.
More information:
Why is important to include ".moc" file at end of a Qt Source code file?
Question: Is there a way to check at compile-time if qRegisterMetaType<T>() was called for a custom type T?
The custom type T needs to be registered in Qt meta-type system in order to be used in e.g. queued connections.
If such a connection is made, and a signal triggered, the runtime warning will be shown:
QObject::connect: Cannot queue arguments of type 'T'
(Make sure 'T' is registered using qRegisterMetaType().)
This is hard to track, so I would prefer to check this at compile-time. Is that in any way possible?
(I understand that if it was possible, it would probably already be a part of Qt Framework itself, but maybe...?)
Note: I know I can check if a type was declared as metatype (Check if type is declared as a meta type system (for SFINAE)), but this doesn't solve my problem.
The code example would be:
#include <QCoreApplication>
#include <QDebug>
#include <QMetaMethod>
#include <QObject>
#include <QThread>
#include <QTimer>
struct Payload {
Payload() = default;
};
// Type is declared as metatype
Q_DECLARE_METATYPE(Payload)
class ObjectOne : public QObject {
Q_OBJECT
public:
using QObject::QObject;
void emitPayloadChanged() { Payload p; emit payloadChanged(p); }
signals:
void payloadChanged(const Payload& p);
};
class ObjectTwo : public QObject {
Q_OBJECT
public:
using QObject::QObject;
void handlePayload(const Payload& p) { qDebug() << "handling payload"; }
};
int main(int argc, char* argv[]) {
QCoreApplication app(argc, argv);
// Uncommenting the following line fixes the runtime warning
// qRegisterMetaType<Payload>();
QThread t1, t2;
ObjectOne o1;
o1.moveToThread(&t1);
ObjectTwo o2;
o2.moveToThread(&t2);
t1.start();
t2.start();
QObject::connect(&o1, &ObjectOne::payloadChanged, &o2, &ObjectTwo::handlePayload);
QTimer::singleShot(0, &o1, [&] { QMetaObject::invokeMethod(&o1, &ObjectOne::emitPayloadChanged); });
return app.exec();
}
#include "main.moc"
My child widget does not get keyPressEvents, while if I put the same widget as top level window, it does. I try to set it get focus, but it has no effect on this. Code is below, showing what I try to get to work.
#include <QApplication>
#include <QKeyEvent>
#include <QLCDNumber>
#include <QLabel>
#include <QVBoxLayout>
class DigitSummer: public QLCDNumber {
Q_OBJECT
public:
DigitSummer(QWidget *parent = nullptr) : QLCDNumber(parent) {
}
protected:
void keyPressEvent(QKeyEvent *event) override {
display(intValue() + event->text().toInt());
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
#if 1 // this version does not work, number does not increase
QWidget widget;
widget.setLayout(new QVBoxLayout());
widget.layout()->addWidget(new QLabel("Press digits!"));
DigitSummer summer; // in stack: must be after widget to avoid child delete
widget.layout()->addWidget(&summer);
widget.setFocusProxy(&summer); // I notice no effect!
widget.show();
#else // this version works, number grows with keypresseas
DigitSummer summer;
summer.show();
#endif
return a.exec();
}
#include "main.moc"
And for completenes, .pro file for the same:
QT += core gui widgets
TARGET = QtMCVE
TEMPLATE = app
DEFINES += QT_DEPRECATED_WARNINGS
CONFIG += c++11
QMAKE_CXXFLAGS += -Wall -Wextra
SOURCES += main.cpp
How to fix the widget to receive key events?
This related question suggests installing event filter, but I don't want to do that, there must be a self-contained way to fix the widget itself.
I think you need to set the focus policy for the widget before it will accept keyboard input. In your ctor try...
setFocusPolicy(Qt::StrongFocus);
Having said that, I'm really not sure why the behaviour would differ for top-level and non-top-level widgets.
Working version of the question code:
#include <QApplication>
#include <QKeyEvent>
#include <QLCDNumber>
#include <QLabel>
#include <QVBoxLayout>
class DigitSummer: public QLCDNumber {
Q_OBJECT
public:
DigitSummer(QWidget *parent = nullptr) : QLCDNumber(parent) {
setFocusPolicy(Qt::StrongFocus);
}
protected:
void keyPressEvent(QKeyEvent *event) override {
display(intValue() + event->text().toInt());
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QWidget widget;
widget.setLayout(new QVBoxLayout());
widget.layout()->addWidget(new QLabel("Press digits!"));
widget.layout()->addWidget(new DigitSummer);
widget.show();
return a.exec();
}
#include "main.moc"
I can't see what I'm doing wrong. I'm trying to create some properties in a class (as I did before) but this time I'm getting the error "class Foo has no member named MyProp"
The header is:
#ifndef P_H
#define P_H
#include <QObject>
class P : public QObject
{
Q_OBJECT
Q_PROPERTY(int Prop READ getProp WRITE setProp)
public:
explicit P(QObject *parent = 0);
int getProp() const;
void setProp(int nP);
private:
int m_p;
};
#endif // P_H
and the cpp file is:
#include "p.h"
P::P(QObject *parent) :
QObject(parent)
{
}
int P::getProp() const
{
return m_p;
}
void P::setProp(int nP)
{
m_p = nP;
}
But when I try to use foobar.P I got the error class P has no member named P. I've been reading Qt documentation and I can't see any difference. Does anyone see what I'm doing wrong?
I'm using Qt Creator 2.4.1 and Qt 4.8.
[... Edit ...]
Here is how I'm trying to use it:
#include "p.h"
int main(int argc, char *argv[])
{
P c;
c.Prop = 2;
return 0;
}
This is the simplest example I could think of and I got the same error.
Thanks in advance.
You need to use it like this:
P c;
c.setProperty("Prop", 42); // set the property
c.property("Prop"); // retrieve the property
I tried declaring a signal in a prototype and then connecting it is script funcition for some reason it does not work as I hoped. My code is as follows. Could some one help me in this.
What I expected was, once I called p.setText('New String') in the script code, since setText emits the textChanged signal it should invoke the slot which is catchSignal(text) already connected in the script code.
Prototype header
#ifndef SCRIPTACTION_H
#define SCRIPTACTION_H
#include <QObject>
#include <QtScript>
class ScriptAction : public QObject , public QScriptable
{
Q_OBJECT
public:
ScriptAction(QObject *parent = 0);
signals:
void textChanged(const QString changedString);
};
#endif // SCRIPTACTION_H
Class
#include "scriptaction.h"
#include <QAction>
Q_DECLARE_METATYPE(QAction*)
ScriptAction::ScriptAction(QObject *parent) : QObject(parent)
{
}
Main Class
#include <QApplication>
#include <QDebug>
#include <QAction>
#include "scriptaction.h"
#include <QPushButton>
Q_DECLARE_METATYPE(QAction*)
QScriptValue qAction_Constructor(QScriptContext *ctx, QScriptEngine *eng)
{
qDebug() << "QAction is called";
if(ctx->isCalledAsConstructor())
{
QObject *parent = ctx->argument(0).toQObject();
QAction *action = new QAction("Test",parent);
return eng->newQObject(action, QScriptEngine::ScriptOwnership);
} else {
return QString("invalid call. Use new Constructor");
}
}
int main(int argc, char *argv[])
{
QApplication app(argc,argv);
QScriptEngine engine;
//Evaluating a simaple expresssion
qDebug() << engine.evaluate("1+2").toNumber();
QPushButton button;
QScriptValue buttonScript= engine.newQObject(&button);
engine.globalObject().setProperty("button", buttonScript);
engine.evaluate("button.text ='Hello Text'; button.show()");
//QAction Prototype
ScriptAction qsAction ;
QScriptValue script_proto = engine.newQObject(&qsAction);
engine.setDefaultPrototype(qMetaTypeId<QAction*>(), script_proto);
QScriptValue ctor = engine.newFunction(qAction_Constructor , script_proto);
QScriptValue metaObject = engine.newQMetaObject(&QObject::staticMetaObject, ctor);
engine.globalObject().setProperty("QSAction" , metaObject);
engine.evaluate("var p = new QSAction(button);p.textChanged.connect(catchSignal);");
engine.evaluate("function catchSignal(text) { print ('PROTOTYPE SIGNAL IS CALLED ',text); } p.setText('New String'); " );
return app.exec();
}
I got rid of the issue, and now I see the signal is being triggered and slot is called properly.
All I did was moving the code to a separate script file and start using the QScriptDebugger to see its output. Then I figured there was an error and the code is edited to work.
Anyone who wants an example prototype class, this will hopefully be a good guideline.