C++:检查对象是否为非空以便使用 cout 索引每个输出

C++: checking if an object is non-empty in order to index each output with cout

我正在研究 1.20 out of C++ primer。我试图让用户输入他们认为合适的尽可能多的项目,然后输出将是 Item no. is item。对于每个条目,没有。只要 item 不为空,就会递增 1。

I am trying to get my program to check that user inputs a sales item in order to index each item.

我的代码如下:

/* http://www.informit.com/title/0321714113 contains a copy of Sales_item.h
 * in the chapter 1 code directory. Copy that file to your pwd. Use it to
 * wrtie a program that reads a set of book sales transactions, writing each
 * transaction to the standard output.
 */

#include <iostream>
#include <string>
#include "IncC++files/1/Sales_item.h"

int main() {
  Sales_item item;  // call the item argument in Sales_item.h
  // prompt the user for the desired information to enter
  // std::string is used for long strings that run to multiple lines
  std::string my_phrase = "Input ISBN, number of copies sold, and"
                          " price sold and when complete enter C-d";
  std::cout << my_phrase << std::endl;
  /* defined while to allow users to continually enter in items until stopped
   * by C-d
   */
  for (int i = 0; i >= 0; ++i) {  // index the item number for the count
    while (std::cin >> item) {
      if (item != NULL) {
        // if item is not empty add one for the next item number
        i += 1;
        std::cout << "Item " << i << " is " << item << std::endl;
      } 
    }
  }
  return 0;
}

我面临的问题是试图确定项目是否为空。

exercise1_20.cpp:24:16: error: no match for ‘operator!=’ (operand types are
‘Sales_item’ and ‘long int’)
       if (item != NULL) {

What operator will allow me to check if item is non-empty?


linux 系统 Sales_item.h 的书籍 C++ 代码:

/*
 * This file contains code from "C++ Primer, Fourth Edition", by Stanley B.
 * Lippman, Jose Lajoie, and Barbara E. Moo, and is covered under the
 * copyright and warranty notices given in that book:
 * 
 * "Copyright (c) 2005 by Objectwrite, Inc., Jose Lajoie, and Barbara E. Moo."
 * 
 * 
 * "The authors and publisher have taken care in the preparation of this book,
 * but make no expressed or implied warranty of any kind and assume no
 * responsibility for errors or omissions. No liability is assumed for
 * incidental or consequential damages in connection with or arising out of the
 * use of the information or programs contained herein."
 * 
 * Permission is granted for this code to be used for educational purposes in
 * association with the book, given proper citation if and when posted or
 * reproduced.Any commercial use of this code requires the explicit written
 * permission of the publisher, Addison-Wesley Professional, a division of
 * Pearson Education, Inc. Send your request for permission, stating clearly
 * what code you would like to use, and in what specific way, to the following
 * address: 
 * 
 *  Pearson Education, Inc.
 *  Rights and Contracts Department
 *  75 Arlington Street, Suite 300
 *  Boston, MA 02216
 *  Fax: (617) 848-7047
*/ 

#ifndef SALESITEM_H
#define SALESITEM_H

// Definition of Sales_item class and related functions goes here


#include <iostream>
#include <string>

class Sales_item {
friend bool operator==(const Sales_item&, const Sales_item&);
// other members as before
public:
    // added constructors to initialize from a string or an istream
    Sales_item(const std::string &book):
              isbn(book), units_sold(0), revenue(0.0) { }
    Sales_item(std::istream &is) { is >> *this; }
    friend std::istream& operator>>(std::istream&, Sales_item&);
    friend std::ostream& operator<<(std::ostream&, const Sales_item&);
public:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);
    // other members as before

public:
    // operations on Sales_item objects
    double avg_price() const;
    bool same_isbn(const Sales_item &rhs) const
        { return isbn == rhs.isbn; }
    // default constructor needed to initialize members of built-in type
    Sales_item(): units_sold(0), revenue(0.0) { }
// private members as before
private:
    std::string isbn;
    unsigned units_sold;
    double revenue;

};


// nonmember binary operator: must declare a parameter for each operand
Sales_item operator+(const Sales_item&, const Sales_item&);

inline bool 
operator==(const Sales_item &lhs, const Sales_item &rhs)
{
    // must be made a friend of Sales_item
    return lhs.units_sold == rhs.units_sold &&
           lhs.revenue == rhs.revenue &&
       lhs.same_isbn(rhs);
}

inline bool 
operator!=(const Sales_item &lhs, const Sales_item &rhs)
{
    return !(lhs == rhs); // != defined in terms of operator==
}

using std::istream; using std::ostream;

// assumes that both objects refer to the same isbn
inline
Sales_item& Sales_item::operator+=(const Sales_item& rhs) 
{
    units_sold += rhs.units_sold; 
    revenue += rhs.revenue; 
    return *this;
}

// assumes that both objects refer to the same isbn
inline
Sales_item 
operator+(const Sales_item& lhs, const Sales_item& rhs) 
{
    Sales_item ret(lhs);  // copy lhs into a local object that we'll return
    ret += rhs;           // add in the contents of rhs 
    return ret;           // return ret by value
}

inline
istream& 
operator>>(istream& in, Sales_item& s)
{
    double price;
    in >> s.isbn >> s.units_sold >> price;
    // check that the inputs succeeded
    if (in)
        s.revenue = s.units_sold * price;
    else 
        s = Sales_item();  // input failed: reset object to default state
    return in;
}

inline
ostream& 
operator<<(ostream& out, const Sales_item& s)
{
    out << s.isbn << "\t" << s.units_sold << "\t" 
        << s.revenue << "\t" <<  s.avg_price();
    return out;
}

inline
double Sales_item::avg_price() const
{
    if (units_sold) 
        return revenue/units_sold; 
    else 
        return 0;
}


#endif

NULL仅用于指针和指针。现在我们有了 C++11,你应该使用它更好的对应物 nullptr

除此之外,您的输入运算符使用默认构造函数将对象设置为空白对象,因此您应该检查您的 item 是否与默认 Sales_item.[=16 相同=]

if (item != NULL)

变成

if (item != Sales_item())

编辑

您还需要定义一个默认构造函数,因为您已经提供了一个用户定义的构造函数。

编辑2

您应该将构造函数分组在一起,而不是将它们放在 class 的对面。