4.7 引入NULL对象

【1】引入NULL对象范例

Book.h

 #ifndef _BOOK_H
 #define _BOOK_H

 #include <string>
 using namespace std;

 class INULLOperator
 {
 public:
     virtual bool isNull() = ;
 };

 class Book : public INULLOperator
 {
 public:
     Book();
     Book(int id, string name, double dprice);

     bool isNull();
     string getName();
     double getPrice();
     static Book* createNullBook();

 private:
     int m_nID;
     string m_strName;
     double m_dPrice;
 };

 class NullBook : public Book
 {
 public:
     bool isNull();
 };

Book.cpp

 #include "Book.h"

 Book::Book()
 {}

 Book::Book(int id, string name, double dprice)
     : m_nID(id)
     , m_strName(name)
     , m_dPrice(dprice)
 {}

 bool Book::isNull()
 {
     return false;
 }

 string Book::getName()
 {
     return m_strName;
 }

 double Book::getPrice()
 {
     return m_dPrice;
 }

 Book* Book::createNullBook()
 {
     return (new NullBook());
 }

 bool NullBook::isNull()
 {
     return true;
 }

main.cpp

 #include "Book.h"
 #include <iostream>
 using namespace std;

 class BookService
 {
 public:
     Book* getBook(int nId)
     {
         if (nId < )
         {
             return Book::createNullBook();
         }
         return (new Book(nId, "Design Pattern", ));
     }
 };

 void main()
 {
     BookService *pBService = new BookService();
     Book *pBook = pBService->getBook(-);
     if (pBook->isNull())
     {
         cout << "not found. " << endl;
     }
     else
     {
         cout << "name :: " << pBook->getName() << endl;
         cout << "price :: " << pBook->getPrice() << endl;
     }
 }

【2】总结

当你需要再三检查某个对象是否为NULL。将NULL值替换为NULL对象。

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