TITLE: Use virtual keyword explicitly in derived classes


RESPONSE: jamshid (Jamshid Afshar), 2 Jun 94

It's a good idea to keep the keyword "virtual" on member functions of
a derived class.  Although the compiler doesn't need to be told that a
member function defined in a derived class is virtual, humans often
do:

	// in design/tedgepath.h
	class TEdgePath {
	   //...
	public:
	   virtual TString buildPathName( boolean useNickNames = TRUE );
	};

	// in obDesign/tobpath.h
	class TOBPath : public TEdgePath {
	   //...
	public:
	   TString buildPathName( boolean useNickNames = TRUE );
	   // okay: compiler knows that this is a virtual function, but
	   // programmer might not
	};

Someone editing tobpath.h might want to add another parameter to
TOBPath::buildPathName().  Unless they've been looking in tedgepath.h,
they wouldn't realize that by adding the parameter they are no longer
overriding the base class's member function:

	// in obDesign/tobpath.h
	class TOBPath : public TEdgePath {
	   //...
	public:
	   TString buildPathName( boolean useNickNames = TRUE,
	                          boolean listPageKind = FALSE );
	   // woops: now TOBPath::buildPathName() hides, not overrides,
	   // TEdgePath::buildPathName()
	};

By always using the virtual keyword when overriding a member function
in a derived class, you remind future programmer that they shouldn't
mess with the parameters:

	// in obDesign/tobpath.h
	class TOBPath : public TEdgePath {
	   //...
	public:
	   // overrides base function; calls buildPathName(TRUE,FALSE)
	   virtual TString buildPathName( boolean useNickNames = TRUE );
	   // new function with extra option
	   virtual TString buildPathName( boolean useNickNames,
	                                  boolean listPageKind );
	};

Fortunately, most compilers now warn whenever you define a function in
a derived class that has the same name as a base virtual function but
different parameters: "warning: TOBPath::buildPathName() hides virtual
TEdgePath::buildPathName()".