Signal and SLOT C++ with QT

Question

I am developing a software in c++ that captures images from Webcam, before I used Opencv but received an error

Undefined Symbols for Architecture x86_64

Until I opened a topic here and nobody knew how to answer me the error. Quit using Opencv and went to use QT Qcamera. So far so good, the problem is that now I wanted to update with a Thread a Qlabel but when I do the Emit command I get the same error as when I used Opencv. See the code I’m using:

thread. h

#ifndef THREAD_H
#define THREAD_H

#include <QtCore>


class Thread : public QThread {
            private:
                void run();
            signals:
                void MySignal( void );
        };

#endif // THREAD_H

thread cpp.

#include "thread.h"
#include <iostream>
#include <thread>
#include <QtCore>

using namespace std;


void Thread::run(){
    qDebug()<<"From worker thread: "<<currentThreadId();
    emit MySignal();
}

Button that fires the Thread:

void MainWindow::on_pushButton_clicked() {
    qDebug()<<"From main thread: "<<QThread::currentThreadId();
    Thread t;
    QObject::connect( &t, SIGNAL( MySignal() ), this, SLOT( MySlot() ) );
    t.start();
}

Error output:

12:11:55: Running steps for project CameraControl...
12:11:55: Configuration unchanged, skipping qmake step.
12:11:55: Starting: "/usr/bin/make" 
Undefined symbols for architecture x86_64:
"Thread::MySignal()", referenced from:
  Thread::run() in thread.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [CameraControl.app/Contents/MacOS/CameraControl] Error 1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -headerpad_max_install_names -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -mmacosx-version-min=10.10.3 -Wl,-rpath,/Applications/QTFiles/5.4/clang_64/lib -o CameraControl.app/Contents/MacOS/CameraControl main.o mainwindow.o thread.o moc_mainwindow.o   -F/Applications/QTFiles/5.4/clang_64/lib -framework QtMultimediaWidgets -framework QtMultimedia -framework QtNetwork -framework QtCore -framework DiskArbitration -framework IOKit -framework QtGui -framework QtWidgets -framework OpenGL -framework AGL 
12:11:55: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project CameraControl (kit: Desktop Qt 5.4.2 clang 64bit)
When executing step "Make"
12:11:55: Elapsed time: 00:00.

File . pro

#-------------------------------------------------
#
# Project created by QtCreator 2015-06-27T21:51:05
#
#-------------------------------------------------

QT += core gui

QT += multimedia multimediawidgets

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = CameraControl
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp \
    thread.cpp

HEADERS  += mainwindow.h \
    thread.h

FORMS    += mainwindow.ui

target.path = $$[QT_INSTALL_EXAMPLES]/multimediawidgets/camera
INSTALLS += target

mainwindow.cpp

void MySlot( void ){
    qDebug() << "slot called";
}

mainwindow. h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <qcamera.h>
#include <QCameraImageCapture>
#include <QMediaRecorder>
#include <qcamerainfo.h>

#include <QMainWindow>

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void setCamera(const QCameraInfo &cameraInfo);
    void updateCaptureMode();
    void processSavedImage(int requestId, QString str);
    void task();

    void on_actionSobre_triggered();

    void on_pushButton_clicked();

signals:
    void signal();

public:
    Ui::MainWindow *ui;
private:


    QCamera *camera;
    QCameraImageCapture *imageCapture;
    QMediaRecorder* mediaRecorder;

    QImageEncoderSettings imageSettings;
    QAudioEncoderSettings audioSettings;
    QVideoEncoderSettings videoSettings;
    QString videoContainerFormat;
    bool isCapturingImage;
    bool applicationExiting;
};

#endif // MAINWINDOW_H

Answer 1

You must include the macro Q_OBJECT in the Thread class.

Of documentation:

The Meta-object Compiler, moc, is the program that Handles Qt’s C++ Extensions. The moc tool reads a C++ header file. If it finds one or more class declarations that contain the Q_OBJECT macro, it produces a C++ file containing the meta-object code for those classes. Among other Things, meta-object code is required for the Signals and slots Mechanism, the run-time type information, and the Dynamic Property system.

Your Thread class would look like this:

#ifndef THREAD_H
#define THREAD_H

#include <QtCore>

class Thread : public QThread 
{
   Q_OBJECT

signals:
   void MySignal( void );

private:
   void run();
};

#endif