Whenever your class uses a type-field, as in the following case:
class Internl { //internationalization aid Lang lg; //type field FontResource fonts public: enum Lang {English, Hebrew, Danish} Internl(Lang lang) lg(lang) {}; Loadfonts(Lang lang); };Every modification to the Internl class affects all of its users, even when they should not be affected. For instance, when adding support to a new language, the users of the already-supported languages have to recompile (or worse: download) the new version. Moreover, as time goes by and support for new languages is added, the class becomes bigger, and its code harder to maintain. It takes longer to compile and contains more bugs. A bug introduced due to an addition of a new language will affect all users. Obviously, a much better choice is to use derivation:
class Internl { //now a base class FontResource fonts public: Internl(); virtual int Loadfonts(); virtual void SetDirectionality(); }; class English : public Internl { public: English(); Loadfonts() { fonts = TimesNewRoman; } SetDirectionality(){}//do nothing; default: left to right }; class Hebrew : public Internl { public: Hebrew(); Loadfonts() { fonts = David; } SetDirectionality() { directionality = right_to_left;} }; The class hierarchy advantages over a type-field are:
