派生类(derived class) 的 构造(construct) 和 析构(destruct)顺序 详解

 

本文地址: http://blog.youkuaiyun.com/caroline_wendy/article/details/16115261

 

派生类因为要调用基类, 所以构造和析构都是按照一定顺序进行;

构造的顺序是: 基(base) -> 派生(derived);即先构造基类, 再构造派生类;

因为 基类 是独立于派生类的, 即不会调用派生类中的对象, 所以应该先被生成;

如果派生类先于基类生成, 则因为无法调用基类资源, 可能生成失败;

析构的顺序是: 派生(derived) -> 基(base); 即先释放派生类, 再释放基类;

因为 派生类 需要先释放调用的基类资源, 所以应该优先释放;

如果基类先析构, 则有可能某些资源被派生类占用, 可能导致析构失败;

派生类的构造和析构顺序正好相反;

 

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() {  		std::cout << "this is Quote constructor" << std::endl;  	}  	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  	virtual ~Quote() {  		std::cout << "this is Quote destructor" << std::endl;  	}  private:  	std::string bookNo;  protected: //受保护类型  	double price = 0.0;  };    class Disc_quote : public Quote { //抽象基类  public:  	//Disc_quote() = default;  	Disc_quote() {  		std::cout << "this is Disc_quote constructor" << std::endl;  	}  	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  	virtual ~Disc_quote() override{  		std::cout << "this is Disc_quote destructor" << std::endl;  	}  protected:  		std::size_t quantity = 0;  		double discount = 0.0;  };    class Bulk_quote final : public Disc_quote { //final限定词, 无法被继承  public:  	//Bulk_quote() = default;  	Bulk_quote() {  		std::cout << "this is Bulk_quote constructor" << std::endl;  	}  	Bulk_quote(const std::string& book, double p, std::size_t qty, double disc) :  		Disc_quote(book, p, qty, disc) {} //使用基类的构造器  	double net_price(std::size_t cnt) const override;  	virtual ~Bulk_quote() override{  		std::cout << "this is Bulk_quote destructor" << std::endl;  	}  };    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) {    	Bulk_quote* bq = new Bulk_quote;  	delete bq;    	return 0;    }


输出:

this is Quote constructor  this is Disc_quote constructor  this is Bulk_quote constructor  this is Bulk_quote destructor  this is Disc_quote destructor  this is Quote destructor