容器与继承

Exercise 15.28:

Define a vector to hold Quote objects but put Bulk_quote objects into that vector. Compute the total net_price of all the elements in the vector.

Exercise 15.29:

Repeat your program, but this time store shared_ptrs to objects of type Quote. Explain any discrepancy in the sum generated by the this version and the previous program.

由于前面练习中的vector包含对象，因此不存在多态性在调用虚函数net_price时发生。从本质上讲,对象被推回去的Bulk_quote对象的Quote主题，因此，调用的虚拟net_price函数是Quote::net_price。作为一个结果,没有任何折扣。结果是9090

本练习的对象是指向Quote对象的智能指针。在这个在这种情况下，多态性如期发生。被调用的实际虚函Bulk_quote::net_price确保折扣被应用。因此，结果是6363。可以发现，在价格的计算中采用了30%的折扣。如果没有矛盾，解释为什么没有矛盾。

#include <iostream>
#include <string>
#include <vector>
#include <memory>
#include "quote.h"
#include "bulk_quote.h"
#include "limit_quote.h"
#include "disc_quote.h"

int main()
{
    /**
     * @brief ex15.28   outcome == 9090
     */
    std::vector<Quote> v;
    for(unsigned i = 1; i != 10; ++i)
        v.push_back(Bulk_quote("sss", i * 10.1, 10, 0.3));

    double total = 0;
    for (const auto& b : v)
    {
        total += b.net_price(20);
    }
    std::cout << total << std::endl;

    std::cout << "======================\n\n";

	 /**
     * @brief ex15.29   outccome == 6363
     */
    std::vector<std::shared_ptr<Quote>> pv;

    for(unsigned i =1; i != 10; ++i)
        pv.push_back(std::make_shared<Bulk_quote>(Bulk_quote("sss", i * 10.1, 10, 0.3)));

    double total_p = 0;
    for (auto p : pv)
    {
        total_p +=  p->net_price(20);
    }
    std::cout << total_p << std::endl;

    return 0;
}

basket类

Exercise 15.30: 编写自己的Basket类版本，并使用它计算在前面练习中使用的相同交易的价格。

#ifndef BASKET_H
#define BASKET_H

#include "quote.h"
#include <set>
#include <memory>


// a basket of objects from Quote hierachy, using smart pointers.
class Basket
{
public:
    // copy verison
    void add_item(const Quote& sale)
    { items.insert(std::shared_ptr<Quote>(sale.clone())); }
    // move version
    void add_item(Quote&& sale)
    { items.insert(std::shared_ptr<Quote>(std::move(sale).clone())); }

    double total_receipt(std::ostream& os) const;

private:

    // function to compare needed by the multiset member
    static bool compare(const std::shared_ptr<Quote>& lhs,
                        const std::shared_ptr<Quote>& rhs)
    { return lhs->isbn() < rhs->isbn(); }

    // hold multiple quotes, ordered by the compare member
    std::multiset<std::shared_ptr<Quote>, decltype(compare)*>
                items{ compare };
};

#endif // BASKET_H

#include "basket.h"
double Basket::total_receipt(std::ostream &os) const
{
    double sum = 0.0;

    for(auto iter = items.cbegin(); iter != items.cend();
        iter = items.upper_bound(*iter))
        //  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
        // @note   this increment moves iter to the first element with key
        //         greater than  *iter.

    {
        sum += print_total(os, **iter, items.count(*iter));
    }                                   // ^^^^^^^^^^^^^ using count to fetch
                                        // the number of the same book.
    os << "Total Sale: " << sum << std::endl;
    return sum;
}

#include <iostream>
#include <string>
#include <vector>
#include <memory>
#include <fstream>
#include "quote.h"
#include "bulk_quote.h"
#include "limit_quote.h"
#include "disc_quote.h"
#include "basket.h"

int main()
{
    Basket basket;

    for (unsigned i = 0; i != 10; ++i)
        basket.add_item(Bulk_quote("Bible", 20.6, 20, 0.3));

    for (unsigned i = 0; i != 10; ++i)
        basket.add_item(Bulk_quote("C++Primer", 30.9, 5, 0.4));

    for (unsigned i = 0; i != 10; ++i)
        basket.add_item(Quote("CLRS", 40.1));

    std::ofstream log("log.txt", std::ios_base::app|std::ios_base::out);

    basket.total_receipt(log);
    return 0;
}