读书笔记c 对话系列Visitor模式,文化模式读书笔记

需要在类体系中加入一个新的虚函数但不允许改动。
Visitor模式核心是将操作作为数据对象传递给类体系预留的Accept函数。

class Personnel{public:  virtual void Pay ()  0;  virtual void Promote()  0;  virtual void Accept(Visitor &)  0;};class Officer : public Personnel { /* 改写虚拟函数 */ };class Captain : public Officer { /* 改写虚拟函数 */ };class First : public Officer { /* 改写虚拟函数 */ };

Visitor模式行为型设计模式。允许为对象结构的元素添加进一步的操作而不必修改这些元素的类。以下为示例

class Element {public:    virtual ~Element() {}    virtual void accept(class Visitor &v)  0;};class ConcreteElementA : public Element {public:    void accept(Visitor &v) override{v.visit(*this);}    void operationA() {        std::cout << Element A operation. << std::endl;    }};class ConcreteElementB : public Element {public:    void accept(Visitor &v) override{v.visit(*this);}    void operationB() {        std::cout << Element B operation. << std::endl;    }};class Visitor {public:    virtual ~Visitor() {}    virtual void visit(ConcreteElementA &element)  0;    virtual void visit(ConcreteElementB &element)  0;};class ConcreteVisitor : public Visitor {public:    void visit(ConcreteElementA &element) override {        element.operationA();        std::cout << Visited by ConcreteVisitor. << std::endl;    }    void visit(ConcreteElementB &element) override {        element.operationB();        std::cout << Visited by ConcreteVisitor. << std::endl;    }};class NewOperationVisitor : public Visitor { //这个就是新增的操作范例public:    void visit(ConcreteElementA &element) override {        std::cout << New operation visited Element A. << std::endl;    }    void visit(ConcreteElementB &element) override {        std::cout << New operation visited Element B. << std::endl;    }};int main() {    ConcreteElementA elementA;    ConcreteElementB elementB;    ConcreteVisitor visitor;    //原操作    elementA.accept(visitor);    elementB.accept(visitor);    NewOperationVisitor newOperationVisitor;//这个就是新增的操作范例    elementA.accept(newOperationVisitor);    elementB.accept(newOperationVisitor);    return 0;}

以下为改写前面的代码

class Visitor {public:  virtual void Visit( Personnel& )  0;  virtual void Visit( Officer& )  0;  virtual void Visit( Captain& )  0;  virtual void Visit( First& )  0;};void Personnel::Accept( Visitor & v ) { v.Visit( *this ); } void Officer::Accept ( Visitor & v ) { v.Visit( *this ); } void Captain::Accept ( Visitor & v ) { v.Visit( *this ); } void First::Accept ( Visitor & v ) { v.Visit( *this ); } class DoSomething : public Visitor {public:  virtual void Visit( Personnel& );  virtual void Visit( Officer& );  virtual void Visit( Captain& );  virtual void Visit( First& );};void DoSomething::Visit( Captain& c ){  if( femaleGuestStarIsPresent )    c.TurnOnCharm();  else    c.StartFight();}void DoSomething::Visit( First& f ){  f.RaiseEyebrowAtCaptainsBehavior();}void f( Personnel& p ){  p.Accept( DoSomething() );}int main(){  Captain k;  First s;  f( k );  f( s );}