析构函数有一点不理解

stock.c:
#include "stdafx.h"
#include "stock.h"
#include <iostream>

Stock::Stock()
{
    std::cout<<"Default constructor called\n";
    company="no name";
    shares=0;
    share_val=0.0;
    total_val=0.0;

}

Stock::Stock(const std::string &co,long n,double pr)
{
    std::cout <<"Constructor using"<<co<<"called\n";
    company=co;
    shares=n;
    share_val=pr;
    set_tot();

}

Stock::~Stock()
{
    std::cout<<"bye,"<<company<<"!\n";

}

void Stock::buy(long num,double price)
{
    shares+=num;
    share_val=price;
    set_tot();
}

void Stock::sell(long num,double price)
{
    shares-=num;
    share_val=price;
    set_tot();
}

void Stock::update(double price)
{
    share_val=price;
    set_tot();
}

void Stock::show()
{
    using std::cout;
    using std::ios_base;

    ios_base::fmtflags orig=cout.setf(ios_base::fixed,ios_base::floatfield);
    std::streamsize prec =cout.precision(3);
   
    cout<<"Company: "<<company
        <<" Shares: "<<shares<<'\n';
    cout<<" Share price:$"<<total_val<<'\n';
    cout.precision(2);
    cout<<" Total Worth:$"<<total_val<<'\n';
    cout.setf(orig,ios_base::floatfield);
    cout.precision(prec);
}

stock.h:
#ifndef STOCK_H
#define STOCK_H
#include <string>

class Stock
{
public:
    Stock();
    Stock(const std::string &co,long n=0,double pr=0.0);
    ~Stock();
    void buy(long num,double price);
    void sell(long num,double price);
    void update(double price);
    void show();

private:
    std::string company;
    long shares;
    double share_val;
    double total_val;
    void set_tot() {total_val=shares*share_val;}

};
#endif

main:
#include "stdafx.h"
#include <iostream>
#include "stock.h"
#include <stdio.h>

   
   
int main()
{
    {
        using std::cout;
        cout<<"Using constructors to create new objects\n";
        Stock stock1("Nanosmart",12,20.0);
        stock1.show();
        Stock stock2=Stock("Boffo Objects",2,2.0);
        stock2.show();

        cout<<"Assigning stock1 to stock2:\n";
        stock2=stock1;
        cout<<"Listing stock1 and stock2\n";
        stock1.show();
        stock2.show();

        cout<<"Using a constructor to reset an object\n";
        stock1=Stock("Nifty Foods",10,50.0);
        cout<<"Revised stock1\n";
        stock1.show();
        cout<<"Done\n";

    }
    return 0;
}

Using constructors to create new objects
Constructor usingNanosmartcalled
Company: Nanosmart Shares: 12

 Share price:$240.000

 Total Worth:$240.00
Constructor usingBoffo Objectscalled
Company: Boffo Objects Shares: 2

 Share price:$4.000

 Total Worth:$4.00
Assigning stock1 to stock2:
Listing stock1 and stock2
Company: Nanosmart Shares: 12

 Share price:$240.000

 Total Worth:$240.00
Company: Nanosmart Shares: 12

 Share price:$240.000

 Total Worth:$240.00
Using a constructor to reset an object
Constructor usingNifty Foodscalled
bye,Nifty Foods!
Revised stock1
Company: Nifty Foods Shares: 10

 Share price:$500.000

 Total Worth:$500.00
Done
bye,Nanosmart!
bye,Nifty Foods!
请按任意键继续. . .