模板代替虚函数实现多态

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虚函数和继承的实现：

class File {
   public:
      virtual void Open() = 0;
      virtual void Save() = 0;
      virtual void SaveAS(String&) = 0;
      virtual void Close() = 0;
};
class TextFile:public File{
   public:
      void Open();
      void Save();
      void SaveAS(String&);
      void Close();
};
class ImageFile:public File{
   public:
      void Open();
      void Save();
      void SaveAS(String&);
      void Close();
};

// user menu selection to open the file
void menu_open_file(File f_ptr){
   f_ptr->Open();
   ….
   …
}
By using the above code, you would have the following:
File file_ptr = new TextFile();
menu_open_file(file_ptr);//open the text file
.
.
file_ptr = new ImageFile();
menu_open_file(file_ptr);//open the image file

模板的实现：

class TextFile{
public:
   void Open();
};

class ImageFile{
public:
void Open();
};

//user menu selection to open the file

template<typename T> void menu_open_file(T file){
   file.Open();
}
Using the code:
TextFile txt_file;
ImageFile img_file;
menu_open_file(txt_file);    //open the text file
menu_open_file(img_file);    //open the image file