3.2.4. Derived classes

C++ provides the hierarchical class mechanism, where derived classes inherit (some) of the functions and variables of their base class. This feature is heavily used within SystemC—for example all module classes are derived classes of the SystemC base class, sc_module.

The SystemC models in each section of this application note are built using derived classes of the models from previous sections.

Those functions and variables which other classes will use are declared as public. For SystemC modules this usually means the constructor and any SystemC ports or sockets. Occasionally there are some utility functions which are also made public (see for example Or1ksimExt::isLittleEndian in Section 6.3)^[2].

Variables and functions in classes that are not for use by other classes, but are required in derived classes are declared as protected (i.e. visible to derived classes).

The remaining functions and variables, which are for use only by the current class, are declared private (visible only to this class). This avoids any unplanned reuse by derived classes.

Some of the functions will be reimplemented in later derived classes. Such functions are also declared virtual.

In summary public functions and variables may be used by any other class, protected functions and variables may be used only by this class and any derived classes and private functions and variables may be used only by this class. virtual functions may be reimplemented in derived classes.

^[2] Object oriented purists prefer to expose only class functions as the public interface, so hiding all state implementation from external view. There is considerable merit in this, but the common SystemC convention is to expose actual ports or sockets, rather than accessor functions for those objects. This application note follows this practice.