#include <QApplication>
#include <QPushButton>

 int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);

     QPushButton hello("Hello world!");
     hello.resize(100, 30);

     hello.show();
     return app.exec();
 }

**** Build of configuration Debug for project hello ****

make all 
Building file: ../src/hello.cpp
Invoking: GCC C++ Compiler
g++ -I/Library/Frameworks/QtGui.framework/Versions/4/Headers -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/hello.d" -MT"src/hello.d" -o"src/hello.o" "../src/hello.cpp"
Finished building: ../src/hello.cpp
 
Building target: hello
Invoking: MacOS X C++ Linker
g++  -o "hello"  ./src/hello.o   
Undefined symbols:
  "QString::free(QString::Data*)", referenced from:
      QString::~QString()in hello.o
  "QPushButton::QPushButton(QString const&, QWidget*)", referenced from:
      _main in hello.o
  "QWidget::resize(QSize const&)", referenced from:
      QWidget::resize(int, int)in hello.o
  "QString::fromAscii_helper(char const*, int)", referenced from:
      QString::QString(char const*)in hello.o
  "QPushButton::~QPushButton()", referenced from:
      _main in hello.o
      _main in hello.o
  "QApplication::QApplication(int&, char**, int)", referenced from:
      _main in hello.o
  "QApplication::exec()", referenced from:
      _main in hello.o
  "QApplication::~QApplication()", referenced from:
      _main in hello.o
      _main in hello.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
make: *** [hello] Error 1