OO Impelementation in C++

From the last series of GObject library, we know the approach of OOP using C. Now, I just want to have a comparison of OO implementation in all leading programming languages: C, C++, Java and C#. I will use C++/Qt in this article. Apart from basic features like encapsulation, inheritance, polymorphism, I will demonstrate how to use advanced features including properties, meta info and event-driven programming.

Now, let’s start. Since C++ supports inheritance and polymorphism in language level. They are not the problem. When encounter encapsulation, it does not do well. We CAN declare member variables as private to prohibit their directly access. But the internal implementation is still exposed. When adding/removing private member variables, the class structure is changed. This can cause binary compatible issues. According to the guide of Qt, we define a private class to hold all private member variables, and add the pointer of it to our public class. The size of pointer is constant in all platforms, so this will not break the binary compatibility. Here’s the code:

NOTE: To use Qt’s object mechanism, your class should inherit QObject class and include the Q_OBJECT macro.

Just note the forward declaration of QBasePrivate private class. It is define in *.c file, and cannot be used by client applications. We defined a d_ptr protected member variable of this type to hold all private data values. Qt library provideds a series of easy-to-use macros to support this scheme to implementation:

Qt library supports properties and meta info. properties are defined with Q_PROPERTYmacro, while class meta info are defined with Q_CLASSINFO. Both of them can be inherited by derived classes. Last is Qt’s event-driven mechanism: signals/slots. Since they are also based on QObject, we had to define a test class to include all slots:

Test code:

All source code is available in my skydrive: http://cid-481cbe104492a3af.office.live.com/browse.aspx/share/dev/TestOO. In the TestQObject-{date}.zip file.

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