effective c++ 条款12 copy all parts of an object

这经常发生在更改代码的时候，当有自己的copy 赋值函数或者copy 构造函数时，编译器就不会维护这两个函数。导致发生遗忘。

可能出现的场景

class Customer
{
  private:
    std::string name;
  public:
    Customer(const Customer& rhs):name(rhs.name){};
    Customer& operator=(const Customer& rhs)
    {
        name = rhs.name;
        return *this;
    }
}

这一切都很美好直到

case 1：在代码维护时很常见

class Customer
{
  private:
    std::string name;
    Date modifiedDate; // added
  public:
    Customer(const Customer& rhs):name(rhs.name){};
    Customer& operator=(const Customer& rhs)
    {
        name = rhs.name;
        return *this;
    }
}

这个时候增加了ModifiedDate 却忘记修改拷贝构造函数和赋值函数

case 2：发生在继承的时候

class AdvancedCustomer:Customer
{
  private:
    int creditAmount;
  public:
    AdvancedCustomer(const AdvancedCustomer& rhs):creditAmount(rhs.creditAmount){};
    AdvancedCustomer& operator=(const AdvancedCustomer& rhs)
    {
        creditAmount = rhs.creditAmount;
        return *this;
    }
}

这个时候只有继承类的成员被赋值，基类的成员却没有，解决方法是调用基类的相关函数

class AdvancedCustomer:Customer
{
  private:
    int creditAmount;
  public:
    AdvancedCustomer(const AdvancedCustomer& rhs): Customer(rhs) // added to initial member of base class
                                                  creditAmount(rhs.creditAmount){};
    AdvancedCustomer& operator=(const AdvancedCustomer& rhs)
    {
        Customer::operator=(rhs); // added to initial member of base class
        creditAmount = rhs.creditAmount;
        
        return *this;
    }
}