C++ Primer第五版學習筆記七 類簡介（編譯用到C++11特性，編譯命令需要加-std=c++11參數，break結束while）

定義一個類：Sales_item

用來表示一本書的總銷售額，售出冊數，平均售價

Sales_item類有以下功能：

● 調用isbn函數，獲取一本書的ISBN書號

● 使用 >> 和 << 操作符可讀寫Sales_item類型對象

● 使用 = 賦值運算符可以將一個Sales_item對象的值賦給另一個Sales_item對象

● 使用 + 運算符將兩個書號相同的Sales_item對象的總銷售額和售出冊數相加，生成一個新的Sales_item對象

● 使用 += 運算符將一個Sales_item對象加到另一個對象上（書號相同的對象）

下面的sales_item.h文件內容是從該書的網站上（http://www.informit.com/title/0321714113）拷貝的，先不在意其內容含義，主要體會如何使用一個自定義的類

sales_item.h

/*
 * This file contains code from "C++ Primer, Fifth Edition", by Stanley B.
 * Lippman, Josee Lajoie, and Barbara E. Moo, and is covered under the
 * copyright and warranty notices given in that book:
 * 
 * "Copyright (c) 2013 by Objectwrite, Inc., Josee 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 Permissions Department
 *     One Lake Street
 *     Upper Saddle River, NJ  07458
 *     Fax: [phone]
*/ 

/* This file defines the Sales_item class used in chapter 1.
 * The code used in this file will be explained in
 * Chapter 7 (Classes) and Chapter 14 (Overloaded Operators)
 * Readers shouldn't try to understand the code in this file
 * until they have read those chapters.
*/

#ifndef SALESITEM_H
// we're here only if SALESITEM_H has not yet been defined 
#define SALESITEM_H

// Definition of Sales_item class and related functions goes here
#include <iostream>
#include <string>

class Sales_item {
// these declarations are explained section 7.2.1, p. 270 
// and in chapter 14, pages 557, 558, 561
friend std::istream& operator>>(std::istream&, Sales_item&);
friend std::ostream& operator<<(std::ostream&, const Sales_item&);
friend bool operator<(const Sales_item&, const Sales_item&);
friend bool 
operator==(const Sales_item&, const Sales_item&);
public:
    // constructors are explained in section 7.1.4, pages 262 - 265
    // default constructor needed to initialize members of built-in type
    Sales_item() = default;
    Sales_item(const std::string &book): bookNo(book) { }
    Sales_item(std::istream &is) { is >> *this; }
public:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);
    
    // operations on Sales_item objects
    std::string isbn() const { return bookNo; }
    double avg_price() const;
// private members as before
private:
    std::string bookNo;      // implicitly initialized to the empty string
    unsigned units_sold = 0; // explicitly initialized
    double revenue = 0.0;
};

// used in chapter 10
inline
bool compareIsbn(const Sales_item &lhs, const Sales_item &rhs) 
{ return lhs.isbn() == rhs.isbn(); }

// 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.isbn() == rhs.isbn();
}

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

// assumes that both objects refer to the same ISBN
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
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
}

std::istream& 
operator>>(std::istream& in, Sales_item& s)
{
    double price;
    in >> s.bookNo >> 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;
}

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

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

 Sales_item類的使用(編譯需要用到C++11特性，編譯命令：g++ main.cpp -std=c++11  -o test)

讀寫Sales_item

#include <iostream>
#include "sales_item.h" // 使用""來引入非標准庫頭文件

int main() {

    Sales_item book;

    std::cout << "請輸入：書號 冊數 單價：" << std::endl;
    std::cin >> book;

    std::cout << book << std::endl;

    return 0;
}

 Sales_item對象的加法

#include <iostream>
#include "sales_item.h"

int main() {

    Sales_item item1,item2;

    std::cout << "請輸入：書號 冊數 單價：" << std::endl;
    std::cin >> item1 >> item2;

    std::cout << item1 + item2 << std::endl;

    return 0;
}

練習：讀取兩個書號相同Sales_item對象，輸出它們的和

#include <iostream>
#include "sales_item.h"

int main() {

    Sales_item item1,item2;

    std::cout << "請輸入：書號 冊數 單價：" << std::endl;
    std::cin >> item1 >> item2;

    if(item1.isbn() == item2.isbn()){
        std::cout << item1 + item2 << std::endl;
    }
    else{
        std::cout << "書號不同，無法相加" << std::endl;
    }

    return 0;
}

練習：讀取多個具有相同書號的銷售記錄，輸出所有記錄的和

#include <iostream>
#include "sales_item.h"

int main() {

    Sales_item countItem,item;

    std::cout << "請輸入：書號 冊數 單價：" << std::endl;
    std::cin >> countItem;
    while(std::cin >> item){
        if(countItem.isbn() == item.isbn()){
            countItem += item;
        }
        else{
            std::cout << "書號不同，結束相加" << std::endl;
            break;
        }
    }

    std::cout << countItem << std::endl;

    return 0;
}

 類的成員函數也叫類的方法，類對象使用.運算符來進行調用，點運算符只能用於類類型的對象

練習：讀取多條銷售記錄，統計相同書號的銷售記錄數（與筆記六類似，沒有將間隔書號相同的銷售記錄累計）

#include <iostream>
#include "sales_item.h"

int main() {

    Sales_item currentItem,item;
    int count;

    std::cout << "請輸入：書號 冊數 單價：" << std::endl;
    std::cin >> currentItem;
    count = 1;
    while(std::cin >> item){
        if(currentItem.isbn() == item.isbn()){
            ++count;
        }
        else{
            std::cout << "書號：" << currentItem.isbn() << "的銷售記錄條數是" << count << std::endl;
            currentItem = item;
            count = 1;
        }
    }

    std::cout << "書號：" << currentItem.isbn() << "的銷售記錄條數是" << count << std::endl;

    return 0;
}