C++中復制(copy) 和 虛復制(virtual copy) 的 區別

在繼承過程中, 需要區分復制(copy)和虛復制(virtual copy);

在派生類轉換為基類時, 復制(copy)有可能切掉(sliced down)派生對象的派生部分, 只保留基類部分, 使派生類的虛函數無法使用;

為了避免此情況, 如果傳入是對象, 則可以定義虛函數clone, 使派生類繼承此虛函數, 再傳入容器, 可以重載方法;

另外, 如果傳入是實參, 使用shared_ptr<Base>配合make_shared<Derived>添加容器, 也可以進行動態綁定;

再傳入容器中, 容器會自動調用派生類的重載方法, 實現動態綁定;

注意: 引用限定符(reference qualifier) GCC 4.8.1 才能支持;

代碼:

/* 
 * CppPrimer.cpp 
 * 
 *  Created on: 2013.11.12 
 *      Author: Caroline 
 */
      
/*eclipse cdt*/
      
#include <iostream>  
#include <string>  
#include <vector>  
#include <set>  
      
#include <utility>  
#include <memory>  
#include <cstddef>  
      
using namespace std;  
      
class Quote;  
      
class Quote {  
public:  
    Quote() = default;  
    Quote (const std::string& book, double sales_price) :  
        bookNo (book), price (sales_price) {}  
    void add_item (const Quote& sale);  
    std::string isbn() const { return bookNo; }  
    virtual double net_price (std::size_t n) const { return n* price; } //虛函數  
    virtual Quote* clone() const & {return new Quote(*this);}  
    virtual Quote* clone() && {return new Quote(std::move(*this));}  
    virtual ~Quote() = default; //動態綁定析構器  
private:  
    std::string bookNo;  
protected: //受保護類型  
    double price = 0.0;  
};  
      
class Disc_quote : public Quote { //抽象基類  
public:  
    Disc_quote() = default;  
    Disc_quote (const std::string& book, double price, std::size_t qty, double disc) :  
        Quote(book, price), quantity (qty), discount (disc) {}  
    double net_price (std::size_t) const = 0; //純虛函數  
protected:  
        std::size_t quantity = 0;  
        double discount = 0.0;  
};  
      
class Bulk_quote final : public Disc_quote { //final限定詞, 無法被繼承  
public:  
    Bulk_quote() = default;  
    Bulk_quote(const std::string& book, double p, std::size_t qty, double disc) :  
        Disc_quote(book, p, qty, disc) {} //使用基類的構造器  
    double net_price(std::size_t cnt) const override;  
    virtual Bulk_quote* clone() const & {return new Bulk_quote(*this);}  
    virtual Bulk_quote* clone() && {return new Bulk_quote(std::move(*this));}  
};  
      
double Bulk_quote::net_price(std::size_t cnt) const
{  
    if (cnt >= quantity)  
        return cnt * (1-discount) * price;  
    else
        return cnt * price;  
}  
      
double print_total(std::ostream &os, const Quote& item, std::size_t n)  
{  
    double ret = item.net_price(n);  
    os << "ISBN: " << item.isbn() << " # sold: " << n << " total due: " << ret << std::endl;  
    return ret;  
}  
      
class Basket {  
public:  
    void add_item (const std::shared_ptr<Quote> &sale) { items.insert(sale); }  
    void add_item (const Quote& sale)  
    {  
        //items.insert(std::shared_ptr<Quote>(new Quote(sale))); //不會動態綁定  
        items.insert(std::shared_ptr<Quote>(sale.clone()));  
    }  
    void add_item (Quote&& sale)  
    {  
        //items.insert(std::shared_ptr<Quote>(new Quote(std::move(sale)))); //不會動態綁定  
        items.insert(std::shared_ptr<Quote>(std::move(sale).clone()));  
    }  
    double total_reciept (std::ostream&) const;  
private:  
    static bool compare (const std::shared_ptr<Quote> &lhs,  
            const std::shared_ptr<Quote> &rhs)  
    { return lhs->isbn() < rhs->isbn(); };  
    std::multiset<std::shared_ptr<Quote>, decltype(compare)*> items{compare};  
};  
      
double Basket::total_reciept(std::ostream &os) const
{  
    double sum = 0.0;  
    for(auto iter = items.cbegin(); iter != items.cend();  
            iter = items.upper_bound(*iter)) { //跳過同名書, 直接計算count  
        sum += print_total(os, **iter, items.count(*iter)); //*it是shared_ptr; **it是object  
    }  

    os << "Total Sale: " << sum << std::endl;  
    return sum;  
}  
      
int main (void) {  
      
    Basket bsk;  
    /*bsk.add_item(std::make_shared<Quote>("CppPrimer", 45));  
    bsk.add_item(std::make_shared<Bulk_quote>("EffectiveCpp", 50, 2, 0.15));  
    bsk.add_item(std::make_shared<Bulk_quote>("EffectiveCpp", 50, 2, 0.15));  
    bsk.add_item(std::make_shared<Bulk_quote>("EffectiveCpp", 50, 2, 0.15));*/  
      
    bsk.add_item(Quote("CppPrimer", 45));  
    bsk.add_item(Bulk_quote("EffectiveCpp", 50, 2, 0.15));  
    bsk.add_item(Bulk_quote("EffectiveCpp", 50, 2, 0.15));  
    bsk.add_item(Bulk_quote("EffectiveCpp", 50, 2, 0.15));  
      
    bsk.total_reciept(std::cout);  
      
    return 0;  
      
}

輸出:

ISBN: CppPrimer # sold: 1 total due: 45  
ISBN: EffectiveCpp # sold: 3 total due: 127.5  
Total Sale: 172.5

作者：csdn博客 Spike_King