qmake missing object file - qt

Question

Alright... So I am very new to Qt. I just installed it on my Ubuntu distribution.
I created a new test application to see if it worked with the following file (helloworld.cpp) in my ../helloworld/ directory:
/* helloworld.cpp */
#include <QApplication>
#include <QWidget>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QWidget window;
window.resize(250, 150);
window.setWindowTitle("Simple example");
window.show();
return app.exec();
}
and I ran qmake -project in order to generate the helloworld.pro file.
Next I ran qmake in order to genereate the Makefile
Next I ran make in order to compile and got the following output/error:
g++ -c -m64 -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG
-DQT_GUI_LIB -DQT_CORE_LIB -I/usr/share/qt4/mkspecs/linux-g++-64 -I.
-I/usr/include/qt4/QtCore -I/usr/include/qt4/QtGui -I/usr/include/qt4
-I. -I. -o helloworld.o helloworld.cpp
helloworld.cpp:1:24: fatal error: QApplication: No such file or directory
compilation terminated.
make: *** [helloworld.o] Error 1
meaning the helloworld.o file is missing... How do I fix this? I tried adding QT += webkit as another discussion said... and that didn't work either.
Any code monkeys out there have any ideas how to fix this??? I would greatly appreciate it!

Answer 1

In your .pro file, add the below line
QT += core widgets gui
Don't forget to invoke qmake again in command line so that qmake recreates Makefile. This should fix the issue for you.

Answer 2

QApplication should be inside /usr/include/qt4/QtGui, provided you have installed the libqt4-dev package.
If that package is not installed, that's the reason of the error: just install it:
sudo apt-get install libqt4-dev

Answer 3

cpp:1:24: fatal error: QApplication: No such file or directory
compilation terminated.
The actual error is a missing header file (QApplication). That caused your code (helloworld.cpp) not to compile generating a secondary error
make: *** [helloworld.o] Error 1
Thus, the solution is to make sure QApplication can be found. That usually is handled by the Qt += statement in your .pro file.
Qt += webkit tells QMake that you are wanting to make a webkit (web browser) application. That should not be needed in this case.
What you are trying to build is a GUI application, so you need to make sure your .pro file specifies that. That can be done by specifying Qt += gui.
More over, I would suggest that you have Qt += gui core at the top of your file. This tells QMake that you are using both the core and gui components of Qt.