定义内联函数的三种方式

class Sales_item
{
public:
double Sales_item::avg_price() const
{
if(units_sold)
return revenue/units_sold;
else
return 0;
}
//
private:
string isbn;
unsigned units_sold;
double revenue;
}

class Sales_item
{
public:
inline double Sales_item::avg_price() const;
private:
//私有成员变量
}

class Sales_item
{
public:
double Sales_item::avg_price() const;
private:
//私有成员变量
}

inline double Sales_item::avg_price() const
{
//
}