An Introduction to Design Patterns, part II

In this article I extended the TFileObject class into 2 subclasses; TVCLStringList and TSTLStringList. I talk about exceptions and consistency in behaviour across the sibling classes and set the framework for more advanced discussions into individual design patterns.

Last month, I presented an overview of design patterns and we created an abstract base class, TFileObject, to serve as part of the set of core classes for exploring various design-pattern themes. This month, I'll demonstrate how to use the Template Method design pattern; specifically, we’ll create two concrete TFileObject descendant classes.

//---------------------------------------------------------------------------
class TFileObject //: public TObject
{
private:
  const unsigned int     MAX_ITEMS;
 
public:
  virtual int        __fastcall Count() = 0;
  virtual AnsiString __fastcall GetItem(unsigned int index = 0) = 0;
  virtual void       __fastcall SetItem(AnsiString item) = 0;
  virtual void       __fastcall Clear() = 0;
 
  __property unsigned int MaxItems = { read=MAX_ITEMS } ;
 
                     __fastcall TFileObject(unsigned int ItemCount = 0);
  virtual            __fastcall ~TFileObject();
};
 
//---------------------------------------------------------------------------
class TVCLStringList : public TFileObject
{
private:
  TStringList   *MenuList;
public:
  int           __fastcall Count();
  AnsiString    __fastcall GetItem(unsigned int index = 0);
  void          __fastcall SetItem(AnsiString item);
  void          __fastcall Clear();
 
                __fastcall TVCLStringList(unsigned int ItemCount = 0);
  virtual       __fastcall ~TVCLStringList() {delete MenuList;};
};
 
 
//---------------------------------------------------------------------------
class TSTLList : public TFileObject
{
private:
//Example only today, until I get the vector documentation...
typedef std::list<AnsiString> flistStrings;
flistStrings MenuList;
public:
  int        __fastcall Count();
  AnsiString __fastcall GetItem(unsigned int index = 0);
  void       __fastcall SetItem(AnsiString item);
  void       __fastcall Clear();
 
             __fastcall TSTLList(unsigned int ItemCount = 0);
  virtual    __fastcall ~TSTLList();
};

Publication: C++ Builder Developer's Journal
Issue: Volume 8, Number 9, September 2004

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