Qt loads dll objects into separate thread, why? - c++

Question

I am having some issues with starting a timer-based object from a dll with the event loop. Therefore I made a minimal example and this is my output:
QObject: Cannot create children for a parent that is in a different thread.
(Parent is MainWindow(0x28fe08), parent's thread is QThread(0x1222aab0), current thread is QThread(0x121421c8)
QObject::startTimer: Timers can only be used with threads started with QThread
MainWindow thread: 0x1222aab0
DllThreadTest thread: 0x121421c8
I am using Windows 8 and Qt5.3.2.
So my questions are:
Why is the object that I create from the dll inside a different thread than the MainWindow? As you can see in the code, I am not using threads.
I can not pass this as a parameter when creating the object from my library as you can see in the first error message. See my example below.
I guess that when I solve the thread-issue, I also solve the timer-problem.
Example:
dllthreadtest.h
#ifndef DLLTHREADTEST_H
#define DLLTHREADTEST_H
#include <QObject>
#include <QTimer>
#if defined TEST_BUILD_DLLTHREADTEST
#define TEST_COMMON_DLLSPEC Q_DECL_EXPORT
#else
#define TEST_COMMON_DLLSPEC Q_DECL_IMPORT
#endif
class TEST_COMMON_DLLSPEC DllThreadTest : public QObject
{
Q_OBJECT
public:
explicit DllThreadTest(QObject *parent = 0);
signals:
public slots:
void onTimeOut();
private:
QTimer timer;
};
#endif // DLLTHREADTEST_H
dllthreadtest.cpp
#include "dllthreadtest.h"
#include <QDebug>
DllThreadTest::DllThreadTest(QObject *parent) :
QObject(parent)
{
timer.start(1000);
}
void DllThreadTest::onTimeOut()
{
qDebug()<<"time out";
}
DllThreadTest.pro
QT += core
QT -= gui
TARGET = DllThreadTest
CONFIG += console
CONFIG -= app_bundle
CONFIG += debug_and_release build_all
CONFIG(debug, debug|release) {
win32: TARGET = $$join(TARGET,,,d)
}
TEMPLATE = lib
DEFINES += TEST_BUILD_DLLTHREADTEST
SOURCES += main.cpp \
dllthreadtest.cpp
HEADERS += \
dllthreadtest.h
I am building it and then including the library in a newly created default project that just creates a QMainWindow. There I instantiate a DllThreadTest-object.
DllImportTest.pro
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = DllImportTest
TEMPLATE = app
INCLUDEPATH += \
$$PWD/libs/
win32{
LIBS += -L$$PWD/libs/ -ldllthreadtest
}
SOURCES += main.cpp\
mainwindow.cpp
HEADERS += mainwindow.h
FORMS += mainwindow.ui
maindwindow.cpp (only class of DllImportTest.pro)
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "dllthreadtest.h"
#include <QDebug>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
connect(ui->pushButton,SIGNAL(clicked()),
this,SLOT(onButtonClick()));
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::onButtonClick()
{
dllThreadTest_=new DllThreadTest;
qDebug()<<"MainWindow thread:"<<this->thread();
qDebug()<<"DllThreadTest thread:"<<dllThreadTest_->thread();
}
main.cpp (entry point of DllImportTest)
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}