怎么搞类的对象做参数？？求指点

class BookSale{
public:
double calculate();
void show();
BookSale(string n,double p,int quantity);
private:
string name;
double price;
int amount; };
double BookSale::calculate()
{
if(price>=0)
return price*amount;
}
BookSale::BookSale(string n,double p,int quantity):name(n),price(p),amount(quantity){}
void BookSale::show(){cout<<"The book name is :"<<name<<endl<<"The price is:"<<price<<endl<<"The amount are:"<<amount<<endl<<"The total price is:"<<BookSale::calculate();}
编写一个信息输出函数，该函数必须以BookSale 类的对象作参数（注意属性或者方法的可访问性）
怎么修改....