如果你宣告自己的copying函式，當你你的程式碼出錯時編譯器不會告訴你

void logCall(const string& funcName)
{
    cout << funcName << endl;
}

class Customer
{
public:
    Customer(const string& str) :name(str)
    {

    }
    Customer(const Customer& rhs):name(rhs.name)
    {
        logCall("Customer copy Customer"
 
);
    }
    Customer& operator=(const Customer& rhs)
    {
        logCall("Customer copy assignment operator");
        name = this->name;
        return *this;
    }
private:
    string name;
};
Customer c("Hello");
Customer a(c);
//上述程式碼看起來很好，直到另一個成員變數加入改變了格局


class Date
{

}
class Customer
{
public
 
:

private:
    string name;
    Date lastTransaction;
};
/*
上面的程式碼增加了成員變數lastTransaction，copying函式執行
的是區域性拷貝，複製了顧客的name但是沒有複製新新增的lastTransaction。
*/


class PriorityCustomer :public Customer
{
public:
    PriorityCustomer()
    {

    }
    PriorityCustomer(const PriorityCustomer& rhs):priority(rhs.priority)
    {
        logCall("PriorityCustomer copy constructor"
 
);
    }
    PriorityCustomer& operator=(const PriorityCustomer& rhs)
    {
        logCall("PriorityCustomer copy assignment constructor");
        priority = rhs.priority;
        return *this;
    }
private:
    int priority=1;
};
PriorityCustomer a;
PriorityCustomer b(a);
/*
上面的程式碼發生了繼承，PriorityCustomer的
copying函式自己申明的成員變數，但是每個PriorityCustomer
還包含所繼承Customer的成員變數，這些成員變數未複製。
*/


PriorityCustomer(const PriorityCustomer& rhs):priority(rhs.priority),Customer(rhs)
{
        logCall("PriorityCustomer copy constructor");
}
PriorityCustomer& operator=(const PriorityCustomer& rhs)
{
        logCall("PriorityCustomer copy assignment constructor");
        Customer::operator=(rhs);
        priority = rhs.priority;
        return *this;
}
/*
當你為derived class撰寫copying函式時，你也要承擔
複製base class成分的責任，
*/