[笔记] 《c++ primer》书店程序 Chapter7

Sales_data.h

#ifndef SALES_DATA_H
#define SALES_DATA_H

#include "Version_test.h"

#include <string>
#include <iostream>

class Sales_data {
friend Sales_data add(const Sales_data&, const Sales_data&);
friend std::ostream &print(std::ostream&, const Sales_data&);
friend std::istream &read(std::istream&, Sales_data&);
public:
    // constructors
// using the synthesized version is safe only
// if we can also use in-class initializers
#if defined(IN_CLASS_INITS) && defined(DEFAULT_FCNS)
    Sales_data() = default;
#else
    Sales_data(): units_sold(0), revenue(0.0) { }
#endif
#ifdef IN_CLASS_INITS
    Sales_data(const std::string &s): bookNo(s) { }
#else
    Sales_data(const std::string &s): 
               bookNo(s), units_sold(0), revenue(0.0) { }
#endif
    Sales_data(const std::string &s, unsigned n, double p):
               bookNo(s), units_sold(n), revenue(p*n) { }
    Sales_data(std::istream &);

    // operations on Sales_data objects
    std::string isbn() const { return bookNo; }
    Sales_data& combine(const Sales_data&);
    double avg_price() const;
private:
    std::string bookNo;
#ifdef IN_CLASS_INITS   // using the synthesized version is safe only
    unsigned units_sold = 0;
    double revenue = 0.0;
#else
    unsigned units_sold;
    double revenue;
#endif
};


// nonmember Sales_data interface functions
Sales_data add(const Sales_data&, const Sales_data&);
std::ostream &print(std::ostream&, const Sales_data&);
std::istream &read(std::istream&, Sales_data&);

// used in future chapters
inline 
bool compareIsbn(const Sales_data &lhs, const Sales_data &rhs)
{
    return lhs.isbn() < rhs.isbn();
}
#endif

• 成员函数通过隐式this指针访问调用它的对象
• 常量成员函数不可修改调用对象的内容
• 通过非成员函数访问类的私有成员，需在类内将此函数声明为友元（友元声明不是函数声明，函数声明在类外）
• 经常使用的短函数声明为内联函数并在.h文件中定义，类内定义的函数是隐式内联的
• 构造函数没有返回值，不能被声明为const

Sales_data.cpp

#include <iostream>
using std::istream; using std::ostream;

#include "Sales_data.h"
Sales_data::Sales_data(std::istream &is) 
{
    // read will read a transaction from is into this object
    read(is, *this);
}

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

// add the value of the given Sales_data into this object
Sales_data& 
Sales_data::combine(const Sales_data &rhs)
{
    units_sold += rhs.units_sold; // add the members of rhs into 
    revenue += rhs.revenue;       // the members of ``this'' object
    return *this; // return the object on which the function was called
}

Sales_data 
add(const Sales_data &lhs, const Sales_data &rhs)
{
    Sales_data sum = lhs;  // copy data members from lhs into sum
    sum.combine(rhs);      // add data members from rhs into sum
    return sum;
}

// transactions contain ISBN, number of copies sold, and sales price
istream&
read(istream &is, Sales_data &item)
{
    double price = 0;
    is >> item.bookNo >> item.units_sold >> price;
    item.revenue = price * item.units_sold;
    return is;
}

ostream&
print(ostream &os, const Sales_data &item)
{
    os << item.isbn() << " " << item.units_sold << " " 
       << item.revenue << " " << item.avg_price();
    return os;
}

avg_price.cpp

#include <iostream>
using std::cerr; using std::cin; using std::cout; using std::endl;

#include "Sales_data.h"

int main()
{
    Sales_data total;         // variable to hold the running sum
    if (read(cin, total))  {  // read the first transaction
        Sales_data trans;     // variable to hold data for the next transaction
        while(read(cin, trans)) {      // read the remaining transactions
            if (total.isbn() == trans.isbn())   // check the isbns
                total.combine(trans);  // update the running total
            else {
                print(cout, total) << endl;  // print the results
                total = trans;               // process the next book
            }
        }
        print(cout, total) << endl;          // print the last transaction
    } else {                                 // there was no input
        cerr << "No data?!" << endl;         // notify the user
    }

    return 0;
}