// throws exception if both objects do not refer to the same isbn
Sales_item
operator+(const Sales_item& lhs, const Sales_item& rhs)
{
if (!lhs.same_isbn(rhs))
throw isbn_mismatch("isbn mismatch",
lhs.book(), rhs.book());
Sales_item ret(lhs); // copy lhs into a local object that we'll return
ret += rhs; // add in the contents of rhs
return ret; // return ret by value
}
bool compare ( const Sales_item& i, const Sales_item& j )
{
return i->book() < j->book();
}
//compare function of Sales_Item for multiset to use
inline bool compare(const Sales_item& rh1,const Sales_item& rh2)
{
return rh1->book() < rh2->book() ;
}