Can't get a Qt Hello World to work - c++

Question

I've compiled Qt on OSX Lion using the instructions provided at this official guide. I've then tried compiling the following Hello World with gcc hello_world.cpp -o hello_world
#include <QApplication>
int main(int argc, char **argv)
{
QApplication app (argc, argv);
return app.exec();
}
I've got the following error:
hello_world.cpp:1:10: fatal error: 'QApplication' file not found
#include <QApplication>
^
1 error generated.

Answer 1

try instead #include <QtGui/QApplication>

Answer 2

Use -I option of gcc to give additional include locations.
gcc hello_world.cpp -I/path-to-qt/include -o hello_world
If you use it like that, you have to use your includes like this:
#include <QtGui/QApplication>
if you want your includes to be shorter like #include <QApplication>, you can give multiple include folders like this:
gcc hello_world.cpp -I/path-to-qt/include/QtCore -I/path-to-qt/include/QtGui -o hello_world
But that is not all. You also have to give library directories and which libraries to link to, which is done like this:
gcc hello_world.cpp -I/path-to-qt/include/QtCore -I/path-to-qt/include/QtGui -o hello_world -L/path-to-qt/lib -lQtCore -lQtGui
It is also better to use g++, since you are using C++.
g++ hello_world.cpp -I/path-to-qt/include/QtCore -I/path-to-qt/include/QtGui -o hello_world -L/path-to-qt/lib -lQtCore -lQtGui

Answer 3

Try with g++ -I<path_to_include_directory> -L<path_to_library_dir> -lQtCore.
For example, in my Debian I would do: g++ -I/usr/local/include/Qt4 -L/usr/local/lib -lQtCore -lQtGui whatever.cpp
EDIT: Thanks to #erelender to point out that QApplication is in the QtGui library and that it depends on QtCore.

Answer 4

What if you try to add additional include path for gcc with using -I flags? Something like:
gcc -I/usr/local/Qt-5.1.1/include hello_world.cpp -o hello_world

Answer 5

Not sure about the path in mac, but on Linux the class QApplication
is defined at following location (qt4)
/usr/include/qt4/QtGui/qwindowdefs.h
Do you have something similar on Mac?
If you are building from command line, including a header file with gcc can be done with following switch
-I<path to .h file>