The K-Zone: My first Qtopia application -- how to get started with Qtopia development -- part 3

A simple graphical program

If you've got this far, it's time to try `Hello, World!' with a Qtopia interface. Here is the source code, which I will assume is saved in a file called hello2.cpp.

#include <qmainwindow.h>
#include <qpe/qpeapplication.h>
                                                                                
int main(int argc, char** argv)
  {
  QPEApplication app(argc, argv);
  QMainWindow* mainWindow = new QMainWindow();
  mainWindow->setCaption("Hello, World!");
  app.showMainWidget(mainWindow);
  return app.exec();
  }

The first line creates an object of class QPEApplication. This class models a complete application, and contains the logic for event handling, and starting up and shutting down, among other things. We then create a main window for the application. The class QMainWindow models a main window, that is, one that can have a menu bar, tool bars, and a frame with a caption. The method call setCaption() applied to the main window... well, if you've followed it this far, you can probably guess what it does. Then we set the newly-created window to be the main `widget' (user interface element) for the application. Finally, we use the exec() method to run the application. The exec() method does not return until the application is shut down, either because the code calls a method to shut it down, or the user takes an action to shut it down (such as clicking the close icon in the frame).
Here's how to compile and link this simple application. First, to compile:

/opt/Embedix/tools/bin/arm-linux-g++ \
-I /opt/Qtopia/sharp/include/ \
-DQWS -fno-rtti -o hello2.o -c hello2.cpp

The backslash characters indicate continuation on the following line -- you can type this command in a single line, but I can't write it here in a single line as it won't fit on the page.
The -I switch tells the compiler to look for header files in /opt/Qtopia/sharp/include. `sharp' here refers to the Sharp Zaurus, but the Zaurus Linux distribution is sufficiently similar to the PMA's that we can use the same headers (lacking a specific set of headers for the PMA). This -I switch is necessary for the compiler to be able to find qpeapplication.h, for example. -DQWS indicates to the Qt headers that we're compiling for Qtopia. -fno-rtti tells the compiler not to include run-time type information in the compiled classes. Since the Qtopia classes are compiled without RTTI, including RTTI in our code will confuse the linker later.

To link the application, we need:

/opt/Embedix/tools/bin/arm-linux-g++ \
-L /opt/Qtopia/sharp/lib/ -o hello2  hello2.o \
-lqte -lqpe -ljpeg -luuid

The -L switch tells the linker where to look for the libraries which are specified later. These libraries are:

-lqte: contains Qt-embedded classes, such as QMainWwindow
-lqpe: contains Qtopia application classes, such as QPEApplication
-ljpeg: contains code for decoding JPEG images
-luuid: contains code for generating unique ID numbers

Although we haven't used any code from the JPEG or UUID libraries, the classes defined in the Qtopia and Qt-embedded libraries are capable of calling this code, so you won't get a proper link if these libraries are not included. By default, when you specify a library using `-l', you get a dynamic link, so specifying these libraries (or even every library on the device) does not increase the size of the application significantly.

If the link is successful, you should get an executable hello2, which you can strip, copy to the PMA, and test as described in the previous section. When you run this executable, you should get a blank window on the screen, which you can close by clicking the close icon in the frame. Good, isn't it? Go to part 4...
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