C++primer plus第六版课后编程题答案12.3

#include <iostream>
using namespace std;
class Stock{
private:
char *company;
int shares;
double share_val;
double total_val;
void set_tot(){total_val=shares*share_val;};
public:
Stock(){
company=new char[8];
strcpy(company,"no name");
shares=0;
share_val=0.0;
total_val=0.0;
}
Stock(const char *co,long n=0,double pr=0)
{
int len=strlen(co);
company=new char[len+1];
strcpy(company,co);
if(n<0)
{
cout<<"Number of shares can't be negative;"
<<company<<" shares set to 0"<<endl;
shares=0;
}
else
shares=n;
share_val=pr;
set_tot();
}
~Stock()
{
delete []company;
}
void buy(long num,double price)
{
if(num<0)
{
cout<<"Number of shares purchase can't be negative."
<<" Transaction is aborted."<<endl;
}
else
{
shares+=num;
share_val=price;
set_tot();
}
}
void sell(long num,double price)
{
if(num<0)
{
cout<<"Number of shares sold can't be negative."
<<"Transaction  is  aborted."<<endl;
}
else if(num>shares)
{
cout<<"You can't sell more than you have!"
<<"Transaction  is  aborted."<<endl;
}
else
{
shares-=num;
share_val=price;
set_tot();
}
}
void update(double price)
{
share_val=price;
set_tot();
}
const Stock &topval(const Stock &s)const
{
if(s.total_val>total_val)
return s;
else
return *this;

}

friend ostream &operator<<(ostream &os,const Stock &s)
{
//ios_base::fmtflags orig=os.setf(ios_base::floatfield);//格式化语句，看不太懂
ios_base::fmtflags orig=os.setf(ios_base::fixed,ios_base::floatfield);
streamsize prec=os.precision(3);
os<<"Company:"<<s.company
<<"  Shares:"<<s.shares<<endl;
os<<" Share Price:$"<<s.share_val;
os.precision(2);
os<<"    Total Worth:&"<<s.total_val<<endl;
os.setf(orig,ios_base::floatfield);
os.precision(prec);
return os;
}
};//倘若要被包含，这里必须要有；？？？
//是的，必须要，否则VS会报错，说是发现意外的文件尾

#include <iostream>
#include "Stock.cpp"
using namespace std;
const int STKS=4;
void main123()
{
Stock stocks[STKS]={
Stock("NanoSmart",12,20.0),
Stock("Boffo Object",200,2.0),
Stock("Monolithic obelisk",130,3.25),
Stock("Fleep Enterprises",60,6.5),
};
cout<<"Stock holdings"<<endl;
int st;
for(st=0;st<STKS;st++)
cout<<stocks[st];
const Stock *top=&stocks[0];
for(st=1;st<STKS;st++)
//top=&(top->topval(stocks[st]));//为何只能用&而不能用*
top=&top->topval(stocks[st]);//top是一个指向对象的指针，
//哦，我知道了，top的确是一个指针，因此才要&，你仔细想想，为什么呢？看我上一条语句的括号
cout<<*top;       //cout<<top;top是一个指针，应该使用*top
//&top->update(3.0);//怎么这一句就不行了呢？
//return 0;
}