C++中如安在容器中利用担任和虚函数

容器不支持殽杂范例, 假如直接把派生类工具, 存入基类容器中, 则无法利用派生-基转换(derived-base conversion);

因为转换只能产生在指针和引用 进程中, 不能产生在 工具直接赋值, 假如是直接转换, 则会发生截断(sliced down);

即派生类部门被切除, 只留下基类部门; 所以存入容器中的派生类 输出为基类部门 的虚函数;

假如想在容器中, 举办担任, 则需要利用指针, 包罗智能指针(如:shared_ptr<>), 则会输出派生类的覆写(override)版本的虚函数;

代码:

/* 
 * CppPrimer.cpp 
 * 
 *  Created on: 2013.11.12 
 *      Author: Caroline 
 */
      
/*eclipse cdt*/
      
#include <iostream>  
#include <string>  
#include <vector>  
#include <memory>  
#include <cstddef>  
      
using namespace std;  
      
class Quote {  
public:  
    Quote() = default;  
    Quote (const std::string& book, double sales_price) :  
        bookNo (book), price (sales_price) {}  
    std::string isbn() const { return bookNo; }  
    virtual double net_price (std::size_t n) const { return n* price; } //虚函数  
    virtual ~Quote() = default; //动态绑定析构器  
private:  
    std::string bookNo;  
protected: //受掩护范例  
    double price = 0.0;  
};  
      
class Disc_quote : public Quote { //抽象基类  
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; //纯虚函数  
protected:  
        std::size_t quantity = 0;  
        double discount = 0.0;  
};  
      
class Bulk_quote final : public Disc_quote { //final限定词, 无法被担任  
public:  
    Bulk_quote() = default;  
    Bulk_quote(const std::string& book, double p, std::size_t qty, double disc) :  
        Disc_quote(book, p, qty, disc) {} //利用基类的结构器  
    double net_price(std::size_t cnt) const override;  
};  
      
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;  
}  
      
int main (void) {  
      
    //正常存放, 无法挪用派生类的虚函数  
    std::vector<Quote> basket;  
    basket.push_back(Quote("CppPrimer", 50));  
    basket.push_back(Bulk_quote("CppPrimer", 50, 10, .25));  
    std::cout << "Bulk_quote : " << basket.back().net_price(20) << std::endl;  
      
    //利用指针存放, 可以正确挪用  
    std::vector<std::shared_ptr<Quote> > sbasket;  
    sbasket.push_back(std::make_shared<Quote>("CppPrimer", 50));  
    sbasket.push_back(std::make_shared<Bulk_quote>("CppPrimer", 50, 10, .25));  
    std::cout << "Bulk_quote : " << sbasket.back()->net_price(20) << std::endl;  
      
      
    return 0;  
      
}

输出:

Bulk_quote : 1000  
Bulk_quote : 750

作者：csdn博客 Spike_King