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  1. //Simple demo of visitor pattern
  2. //Uses raw poitner instead of smartpointers for demo purpose only
  3.  
  4. #include <iostream>
  5. #include <string>
  6. #include <map>
  7. using namespace std;
  8.  
  9. class Visitor; 
  10. struct Item        // items in inventory
  11. {
  12. 	virtual Item* clone() const=0;  // duplicate an item for sotring in inventory
  13. 	virtual int code() const=0;     // code for the map
  14. 	virtual void acceptVisitor(Visitor &v)=0;   // allow for visitors
  15. 	virtual ~Item() {}              
  16. };
  17.  
  18. class Book : public Item   // a concrete item
  19. {
  20. 	int isbn;              // with its specific data (a dvd has no isbn)
  21. 	string name;
  22. public: 
  23.     Book() = default; 
  24.     Book(const Book& b) = default; 
  25.     Book(int isbn, const string &name);
  26.     int code() const override ;    // implement item members
  27.     Item* clone() const override ;
  28.   	void acceptVisitor(Visitor &v) override;
  29.   	int getIsbn() const ;          // provide getters to ensure encapsulation
  30.   	const string& getTitle() const ;
  31. };
  32.  
  33. class Inventory 
  34. {
  35. private:
  36.     map<int, Item *> items; 
  37. public:
  38.     void addItem(const Item& i);     
  39.     void acceptVisitor(Visitor &v);  
  40. };
  41.  
  42. class Visitor {                   // generic visitor.  Can be used for report, for saving data, etc..
  43. public:
  44.     virtual void visitInventory(Inventory *iv) = 0;
  45.     virtual void visitBook(Book*) = 0; 
  46.     //... if you have new type of items, just add a function here    
  47.     virtual void visitEnd() {}; 
  48. };
  49.  
  50. class Report : public Visitor   // FIrst report:  just tell what to do fpor every class you use  
  51. {
  52. private:
  53.     Inventory* i;
  54.     int count=0; 
  55. public:
  56.     Report(Inventory& i);
  57.     void visitInventory(Inventory *iv); 
  58.     void visitBook(Book*b);
  59.     void visitEnd();   // to end the report
  60. };
  61.  
  62. Book::Book(int isbn, const string &name) : isbn(isbn), name(name) {}
  63. int Book::code() const  { return isbn; }
  64. Item* Book::clone() const  { return new Book(*this); }
  65. void Book::acceptVisitor(Visitor &v) { v.visitBook(this); }
  66. int Book::getIsbn() const { return(isbn); }
  67. const string& Book::getTitle() const { return (name); }
  68.  
  69. void Inventory::addItem(const Item& i)
  70. {
  71.     items[i.code()] =i.clone();
  72. }
  73. void Inventory::acceptVisitor(Visitor &v) { 
  74.     	v.visitInventory(this); 
  75.     	for (auto &x : items)
  76.     	    x.second->acceptVisitor(v);
  77.     	v.visitEnd();
  78. }
  79.  
  80. Report::Report(Inventory& i) : i(&i),count(0)
  81. { i.acceptVisitor(*this); }
  82.  
  83. void Report::visitInventory(Inventory *iv) {
  84. 	cout << "Nice report:"<<endl; 
  85. 	cout << "============"<<endl; 
  86. }
  87. void Report::visitBook(Book*b) {
  88. 	cout << "Book: "<< b->getTitle() << " (ISBN "<<b->getIsbn()<<" )"<<endl;
  89. 	count++; 
  90. }
  91. void Report::visitEnd() {
  92. 	cout << "Total of "<<count<<" items"<<endl;
  93. }
  94.  
  95.  
  96. int main()   // CORRECT: never main(void) !!!!
  97. {
  98.     Inventory i;
  99.  
  100.     i.addItem(Book(1234, "Foo Bar I"));
  101.     i.addItem(Book(4567, "Foo Bar II"));
  102.  
  103.     Report r(i);
  104.  
  105.     return 0;
  106. }
  107.  
Success #stdin #stdout 0s 3464KB
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stdin
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Standard input is empty
stdout
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Nice report:
============
Book: Foo Bar I (ISBN 1234 )
Book: Foo Bar II (ISBN 4567 )
Total of 2 items
https://ideone.com/U9O8ib
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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