I've just started programming in Qt framework. Following is a very simple program:
#include <QtCore/QCoreApplication>
#include <QDebug>
class MyClass : public QObject
{
Q_OBJECT
public:
MyClass() {}
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
MyClass *c = new MyClass();
return a.exec();
}
But I receive following error when I try to compile & run it:
In function MyClass:
undefined reference to vtable for MyClass
But when I remove the QObject macro everything works fine. Please note that the class is defined in the same file as the main function.
I'm using Qt version 4.7, running on Win 7.
What is causing this issue?
Update: I get the same error when I define my class in a separate header file. mytimer.h:
#ifndef MYTIMER_H
#define MYTIMER_H
#include <QtCore>
class MyTimer : public QObject
{
Q_OBJECT
public:
QTimer *timer;
MyTimer();
public slots:
void DisplayMessage();
};
#endif // MYTIMER_H
mytimer.cpp:
#include "mytimer.h"
#include <QtCore>
MyTimer::MyTimer()
{
timer = new QTimer();
connect(timer,SIGNAL(timeout()),this,SLOT(DisplayMessage()));
timer->start(1000);
}
void MyTimer::DisplayMessage()
{
qDebug() << "timed out";
}
And this is the main.cpp:
#include <QtCore/QCoreApplication>
#include <QDebug>
#include "mytimer.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
MyTimer *mt = new MyTimer();
return a.exec();
}
You need to compile it using qmake, which is going to create mock methods for your custom QObject class. See here about more on generating moc files.
Since your example doesn't contain header files, it is not parsed, and no moc files are generated. You need to declare MyClass in a separate header file, and run moc generation tool.
When you are using QT Creator you should cleanup up your project and execute qmake, in the build menu.
Whenever u apply some changes first clean your project, then run qmake, and the finally build your project...
Related
Update:
If I delete Q_OBJECT, and I do not use SLOT SINGAL, just use connect() like this:
connect(this, &QWidget::destroyed, this, &QWidget::myslot),
my code will run well without any warnings and errors.
I want to write a little code to instruct some classes, so I try to simplify my code. But I encountered some strange things. I cannot write a simple widget in my main.cpp. If I write the widget in mywidget.cpp and mywidget.h, the program runs well. If I want to write the widget in the main.cpp, what should I do?
This is my code.
#include <QApplication>
#include <QWidget>
class Widget : public QWidget
{
Q_OBJECT
public:
Widget(QWidget *parent = 0);
~Widget();
};
Widget::Widget(QWidget *parent)
: QWidget(parent)
{
}
Widget::~Widget()
{
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
And the error information is:
Add following line to the end of main.cpp and rerun qmake:
#include "main.moc"
That will invoke moc tool for your main.cpp. It generates meta-object function definitions for your Widget class resolving your linker errors when you rebuild.
As stated in documentation:
Whenever qmake is run, it parses the project's header files and
generates make rules to invoke moc for those files that contain a
Q_OBJECT macro.
Thus, put the class declaration in a header file (e.g. widget.h) and the class definition in a source file with the same name (e.g. widget.cpp).
If you still want to make it work with signals you can remove the Q_OBJECT macro and use the QObject::connect()
for example:
#include <QApplication>
#include <QWidget>
#include <iostream>
class Widget : public QWidget
{
//Q_OBJECT
public:
Widget(QWidget *parent = 0);
~Widget();
};
Widget::Widget(QWidget *parent) : QWidget(parent)
{
// say bey
QObject::connect(this,&Widget::destroyed,
[](){std::cout<<"bye"<<std::endl;});
}
Widget::~Widget()
{
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
I test it on Qt5.9.4
i just started learning Qt few days ago, and i have a problem that i can't solve.
First there is the files :
main.cpp
#include <QApplication>
#include "test.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
test w;
w.show();
return a.exec();
}
test.h
#ifndef TEST_H
#define TEST_H
#include <QWidget>
#include <QTabWidget>
#include <QTextEdit>
#include <QPushButton>
class test : public QWidget
{
Q_OBJECT
public:
test();
~test();
private slots:
void addT();
private :
QTabWidget *tab;
QPushButton *b,*c;
};
#endif // TEST_H
and test.cpp
#include "test.h"
test::test()
{
QTabWidget *tab = new QTabWidget(this);
QPushButton *b = new QPushButton("Add",this);
tab->addTab(b,"test");
QObject::connect(b,SIGNAL(clicked()),this,SLOT(addT()));
}
test::~test()
{
}
void test::addT()
{
QPushButton *c= new QPushButton("Add",this);
tab->addTab(c,"test");
}
the program starts normally but when i push the button to add a new Tab it crashes
Please Help me!
In your constructor you are not assigning to the QTabWidget and QPushButton instanced declared in your header, but are creating two new instances (with the same name) that will be gone at the end of the scope. The tab instance is still a nullptr and when trying to derefence it in addT, your program will crash. You need to assign to the variables declared in test.h like this:
test::test() : tab(new QTabWidget(this), b(new QPushButton("Add", this) {
...
}
This is a tutorial code for Qt:
Header file:
#include <QMainWindow>
namespace Ui {
class Notepad;
}
class Notepad : public QMainWindow
{
Q_OBJECT
public:
explicit Notepad(QWidget *parent = 0);
~Notepad();
private:
Ui::Notepad *ui;
};
Source file:
#include "notepad.h"
#include "ui_notepad.h"
Notepad::Notepad(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::Notepad)
{
ui->setupUi(this);
}
Notepad::~Notepad()
{
delete ui;
}
And in main,
#include "notepad.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Notepad w;
w.show();
return a.exec();
}
So when we do Notepad w, an object is already created on the stack, and why does the code still create another object on the heap using new and assign to a member?
The one on the stack is of type Notepad, and it's the application's main window. The dynamic one is of type Ui::Notepad. That's a class automatically generated by Qt's uic tool; it contains the widgets created in UI creator as data members.
In a way, you could say that Notepad is concerned with the logic and uses an instance of Ui::Notepad to provide the GUI for it.
Okay I'm using Qt Designer to build a GUI. I've managed to figure out how to make the menuBar and I've added some actions to the bar, but now I need to connect the actions to make them do something.
Specifically, on my file menu, I have the simple open action. I want this action to run a function that calls my QFileDialog and so on, but I don't know how to do this.
So, how do I connect my actionOpen to my static function?
I am using the latest Qt, 5.0.2
I'm a little frustrated here. This is obviously one of the most basic things someone might need to do, yet I cannot find any real solution to this anywhere on the web. From the lacking Qt wiki, to other people's questions, nobody really seems to have a clear answer. There are answers for older versions of Qt, yet in those old versions apparently signals couldn't connect to static functions, so those are irrelevant. And nobody seems to know how to do this through the Qt Designer. Also, nobody ever clarifies where to put what.
I have this line in my main.cpp file:
QObject::connect(actionOpen, &actionOpen::triggered, fileOpen)
I have an object called 'actionOpen' made in Qt Designer, there is a signal called triggered, and I have a function defined just below my main inside main.cpp called 'fileOpen'. This seems to follow the proper syntax, yet it throws many errors.
Also, I can repeatedly click build in Qt Creator and every single time it comes up with a different number of errors, disappearing and reappearing, without me even touching the code. I'm starting to think this IDE is sort of a POS.
EDIT:
Here are my files. Maybe this will help somewhat.
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
public slots:
void fileOpen();
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
main.cpp
#include "mainwindow.h"
#include <QApplication>
#include <iostream>
#include <fstream>
using namespace std;
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <iostream>
using namespace std;
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
QObject::connect(ui->actionOpen, &QAction::triggered, &MainWindow::fileOpen);
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete ui;
}
void fileOpen()
{
/*
QString fileName = QFileDialog::getOpenFileName(this, tr("Open File"), QString(),
tr("Text Files (*.txt);;C++ Files (*.cpp *.h)"));
if (!fileName.isEmpty()) {
QFile file(fileName);
if (!file.open(QIODevice::ReadOnly)) {
QMessageBox::critical(this, tr("Error"), tr("Could not open file"));
return;
}
QTextStream in(&file);
ui->textEdit->setText(in.readAll());
file.close();
}
*/
cout << "Hello!";
}
The second argument is incorrect. You should specify the class name, not object name. So it should be:
QObject::connect(actionOpen, &QAction::triggered, fileOpen);
Complete working example (tested):
void fileOpen() {
qDebug() << "ok";
}
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
QMenu menu;
QAction* actionOpen = menu.addAction("test");
QObject::connect(actionOpen, &QAction::triggered, fileOpen);
menu.show();
return a.exec();
}
1.) Create regular slot and call the static function.
OR
2.) I suppose you could create a singleton Q_OBJECT class and connect to it - of course like option 1 you'd then make the call to whatever static/global function.
/**
* Pseudo-code!, the singleton should be in its own header file
* not in the global main.cpp file.
*/
class Singleton : public QObject
{
Q_OBJECT
public:
Singleton() : QObject() {}
static Singleton* getInstance() {
if(!_instance) _instance = new Singleton();
return _instance;
}
public slots:
void mySlot() { qDebug() << __FILE__ << " " << __FUNCTION__ << __LINE__;
}
private:
static Singleton* _instance;
};
Singleton* Singleton::_instance = NULL;
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
Singleton* instance = Singleton::getInstance();
QObject::connect(&w, SIGNAL(destroyed()), instance, SLOT(mySlot()));
return a.exec();
}
I'm just trying my best to answer the question, but like many of the comments above - this isn't something that I've needed ever needed to do. I can say that QtCreator/Designer is a really amazing tool and as you overcome some of the learning curves it'll be less frustrating.
I'm trying to write an application using QNetworkManager. I have simplified the code down to the problem. The following code hangs, and I have no idea why:
main.cpp:
#include <QApplication>
#include "post.h"
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
post("http://google.com/search", "q=test");
return app.exec();
}
post.h:
#ifndef _H_POST
#define _H_POST
#include <QNetworkAccessManager>
#include <QNetworkRequest>
class post : public QObject {
Q_OBJECT
public:
post(QString URL, QString data);
public slots:
void postFinished(QNetworkReply* reply);
protected:
QNetworkAccessManager *connection;
};
#endif
post.cpp:
#include <QApplication>
#include <QUrl>
#include "post.h"
post::post(QString URL, QString data) {
connection = new QNetworkAccessManager(this);
connect(connection, SIGNAL(finished(QNetworkReply*)), this, SLOT(postFinished(QNetworkReply*)));
connection->post(QNetworkRequest(QUrl(URL)), data.toAscii());
}
void post::postFinished(QNetworkReply*) {
qApp->exit(0);
}
Some Googling shows it may be because I have everything on one thread, but I have no idea how to change that in Qt... none of the network examples show this.
I just tried it with the same results. The problem is that you are creating the post object by only calling the constructor. Since you are not specifying an object it is getting destroyed right away (to check this create a destructor and see when it gets called.)
try:
post p("http://google.com/search","q=test");
Then your slot gets called.