FPC and Qt

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Introduction

There are a number of Qt bindings available:

Qt3

A QtC based binding by Theo

Another QtC based binding by Andreas

The first one aims on linux/unix users while the second one is for win32.

Qt/Embedded

The FPC arm port and this Qt/E binding allow graphical programming on devices such as the Zaurus

Qt4

More recently a small Qt4 Binding is available. It does not depend on the obsolete QtC library. The Qt4 binding page will provide more information on how the binding works and how to use it.

How it works

FPC still can't use C++ classes natively so these interface units use the same scheme as e.g. the official C interface to Qt does: there are wrapper procedures written which export an procedural interface. This procedural interface is wrapped by an object pascal interface again to make usage easier.

This might sound like an huge overhead but these wrapper are mainly necessary to hide name mangling differences so there shouldn't be a noticable speed decrease in real world applications.

Example

This piece of code is taken from the second package mentioned above.

 var
   app: QApplicationH;
   btn: QPushButtonH;
 begin
   // create static ( interfaced handled ) QApplicationH
   app := NewQApplicationH(ArgCount, ArgValues).get;
   // due to a bug in fpc 1.9.5 the WideString helper methods with default parameter are disabled
   //btn := NewQPushButtonH('Quit', nil).get;
   btn := NewQPushButtonH(qs('Quit').get, nil, nil).get;
   btn.setGeometry(100, 100, 300, 300);
   btn.show;
   // override the virtual eventFilter method of btn
   btn.OverrideHook.eventFilter := @TTest.MyEventFilter;
   // and install the btn as it's own eventFilter
   btn.installEventFilter(btn);
   // connect Qt signal to Qt slot
   QObjectH.connect(btn, SIGNAL('clicked()'), app, SLOT('quit()'));
   ...

If you know how Qt is used in C++, you can see that there is not much difference.