/**/#ifdef Implementation1/**/class HelpHandler {public:    HelpHandler(HelpHandler* s) : _successor(s) { }    virtual void HandleHelp();private:    HelpHandler* _successor;};/**/void HelpHandler::HandleHelp () {    if (_successor) {        _successor->HandleHelp();    }}/**/class Request {  public:    int GetKind();};class HelpRequest : public Request {};class PrintRequest : public Request {};class Handler {  public:    virtual void HandleRequest(Request* theRequest);    void HandleHelp(HelpRequest*);    void HandlePrint(PrintRequest*);};const int Help = 1;const int Print = 2;const int Preview = 2;/**/void Handler::HandleRequest (Request* theRequest) {    switch (theRequest->GetKind()) {    case Help:        // cast argument to appropriate type        HandleHelp((HelpRequest*) theRequest);        break;/**/    case Print:        HandlePrint((PrintRequest*) theRequest);        // ...        break;/**/    default:        // ...        break;    }}/**/class ExtendedHandler : public Handler {public:    virtual void HandleRequest(Request* theRequest);    // ...};/**/void ExtendedHandler::HandleRequest (Request* theRequest) {    switch (theRequest->GetKind()) {    case Preview:        // handle the Preview request        break;/**/    default:        // let Handler handle other requests        Handler::HandleRequest(theRequest);    }}/**/#endif/**/#ifdef SampleCode#include"defs.H"class Application;class Dialog;class HelpHandler;/**/typedef int Topic;const Topic NO_HELP_TOPIC = -1;class HelpHandler {public:    HelpHandler(HelpHandler* = 0, Topic = NO_HELP_TOPIC);    virtual bool HasHelp();    virtual void SetHandler(HelpHandler*, Topic);    virtual void HandleHelp();private:    HelpHandler* _successor;    Topic _topic;};/**/HelpHandler::HelpHandler (    HelpHandler* h, Topic t) : _successor(h), _topic(t) { }/**/bool HelpHandler::HasHelp () {    return _topic != NO_HELP_TOPIC;}/**/void HelpHandler::HandleHelp () {    if (_successor != 0) {        _successor->HandleHelp();    }}/**/class Widget : public HelpHandler {protected:    Widget(Widget* parent, Topic t = NO_HELP_TOPIC);private:    Widget* _parent;};/**/Widget::Widget (Widget* w, Topic t) : HelpHandler(w, t) {    _parent = w;}/**/class Button : public Widget {public:    Button(Widget* d, Topic t = NO_HELP_TOPIC);    virtual void HandleHelp();    // Widget operations that Button overrides...};/**/Button::Button (Widget* h, Topic t) : Widget(h, t) { }void Button::HandleHelp () {    if (HasHelp()) {        // offer help on the button    } else {        HelpHandler::HandleHelp();    }}/**/class Dialog : public Widget {public:    Dialog(HelpHandler* h, Topic t = NO_HELP_TOPIC);    virtual void HandleHelp();    // Widget operations that Dialog overrides...    // ...};/**/Dialog::Dialog (HelpHandler* h,  Topic t) : Widget(0) {    SetHandler(h, t);}/**/void Dialog::HandleHelp () {    if (HasHelp()) {        // offer help on the dialog    } else {        HelpHandler::HandleHelp();    }}/**/class Application : public HelpHandler {public:    Application(Topic t) : HelpHandler(0, t) { }    virtual void HandleHelp();    // application-specific operations...};/**/void Application::HandleHelp () {    // show a list of help topics}/**/void dummy () {/**/const Topic PRINT_TOPIC = 1;const Topic PAPER_ORIENTATION_TOPIC = 2;const Topic APPLICATION_TOPIC = 3;Application* application = new Application(APPLICATION_TOPIC);Dialog* dialog = new Dialog(application, PRINT_TOPIC);Button* button = new Button(dialog, PAPER_ORIENTATION_TOPIC);/**/button->HandleHelp();/**/}#endif/**/