在C ++中复制多态对象 [英] Copying a Polymorphic object in C++

问题描述

我有基类基本从中派生 Derived1 ， Derived2 和 Derived3 。

我已经为一个派生类构造了一个实例存储为 Base * a 。我现在需要做一个对象的深层副本，我将存储为 Base * b 。

据我所知，复制类的正常方法是使用复制构造函数并重载 operator = 。但是，由于我不知道 a 是 Derived1 ， Derived2 或 Derived3 ，我不能想到使用复制构造函数或 operator = 的方法。我可以想到干净地使这项工作的唯一方法是实现像：

  class Base 
 {
 public：
 virtual Base * Clone（）= 0; 
 
}; 
  

和实现 Clone 类如：

  class Derivedn：public Base 
 {
 public：
 Base * Clone（）
 {
 Derived1 * ret = new Derived1; 
复制所有的数据成员
} 
}; 
  

Java倾向于使用 Clone 有没有更多的C ++方式这样做？

解决方案

这仍然是我们做的东西在C + +多态类，如果您为对象创建一个复制构造函数（可能是隐式或私有），则不需要对成员进行显式复制。

  class Base 
 {
 public：
 virtual Base * Clone（）= 0; 
}; 
 
 class Derivedn：public Base 
 {
 public：
 //这是OK，它的调用协变量返回类型。 
 Derivedn * Clone（）
 {
 return new Derivedn（* this）; 
} 
 private：
 Derivedn（const Derivedn）：... {} 
}; 
  

I have base-class Base from which is derived Derived1, Derived2 and Derived3.

I have constructed an instance for one of the the derived classes which I store as Base* a. I now need to make a deep copy of the object which I will store as Base* b.

As far as I know, the normal way of copying a class is to use copy constructors and to overload operator=. However since I don't know whether a is of type Derived1, Derived2 or Derived3, I cannot think of a way of using either the copy constructor or operator=. The only way I can think of to cleanly make this work is to implement something like:

class Base
{
public:
  virtual Base* Clone() = 0;

};

and the implement Clone in in the derived class as in:

class Derivedn : public Base
{
public:
  Base* Clone() 
  {
    Derived1* ret = new Derived1;
    copy all the data members
  }
};

Java tends to use Clone quite a bit is there more of a C++ way of doing this?

解决方案

This is still how we do stuff in C++ for polymorphic classes, but you don't need to do the explicit copy of members if you create a copy constructor (possibly implicit or private) for your objects.

class Base
{
public:
  virtual Base* Clone() = 0;
};

class Derivedn : public Base
{
public:
  //This is OK, its called covariant return type.
  Derivedn* Clone() 
  {
    return new Derivedn(*this);
  }
private:
  Derivedn(const Derivedn) : ... {}
};

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