如何在不邀请未来对象切片的情况下实现 ICloneable

Question

我的问题是关于如何实现经典的 ICloneable 接口，以便在未来的程序员没有密切注意时不会导致无意的对象切片。这是我想检测的编程错误类型的示例（最好是在编译时）：

#include <stdio.h>

class ICloneable
{
public:
   virtual ICloneable * clone() const = 0;
};

class A : public ICloneable
{
public:
   A() {}
   A(const A & rhs) {}

   virtual ICloneable * clone() const {return new A(*this);}
};

class B : public A
{
public:
   B() {}
   B(const B & rhs) {}

   // Problem, B's programmer forget to add a clone() method here!
};

int main(int, char**)
{
   B b;
   ICloneable * clone = b.clone();  // d'oh!  (clone) points to an A, not a B!
   return 0;
}

如果 B（或 B 的任何其他非抽象子类）没有定义自己的 [=14=，C++ 中是否有任何方法可以说服编译器发出错误] 方法？除此之外，是否有任何自动方法可以在运行时检测到此错误？

Answer 1

不久前，我在同样的情况下遇到了同样的问题，但没有找到令人满意的解决方案。

再次考虑这个问题，我发现了一些可能是解决方案（充其量）的方法：

#include <iostream>
#include <typeinfo>
#include <typeindex>

class Base { // abstract
  protected:
    Base() = default;
    Base(const Base&) = default;
    Base& operator=(const Base&) = default;
  public:
    virtual ~Base() = default;
    
    Base* clone() const
    {
      Base *pClone = this->onClone();
      const std::type_info &tiClone = typeid(*pClone);
      const std::type_info &tiThis = typeid(*this);
#if 0 // in production
      assert(std::type_index(tiClone) == type_index(tiThis)
        && "Missing overload of onClone()!");
#else // for demo
      if (std::type_index(tiClone) != std::type_index(tiThis)) {
        std::cout << "ERROR: Missing overload of onClone()!\n"
          << "  in " << tiThis.name() << '\n';
      }
#endif // 0
      return pClone;
    }
    
  protected:
    virtual Base* onClone() const = 0;
};

class Instanceable: public Base {
  public:
    Instanceable() = default;
    Instanceable(const Instanceable&) = default;
    Instanceable& operator=(const Instanceable&) = default;
    virtual ~Instanceable() = default;
    
  protected:
    virtual Base* onClone() const { return new Instanceable(*this); }
};

class Derived: public Instanceable {
  public:
    Derived() = default;
    Derived(const Derived&) = default;
    Derived& operator=(const Derived&) = default;
    virtual ~Derived() = default;
    
  protected:
    virtual Base* onClone() const override { return new Derived(*this); }
};

class WrongDerived: public Derived {
  public:
    WrongDerived() = default;
    WrongDerived(const WrongDerived&) = default;
    WrongDerived& operator=(const WrongDerived&) = default;
    virtual ~WrongDerived() = default;

  // override missing
};

class BadDerived: public Derived {
  public:
    BadDerived() = default;
    BadDerived(const BadDerived&) = default;
    BadDerived& operator=(const BadDerived&) = default;
    virtual ~BadDerived() = default;

  // copy/paste error
  protected:
    virtual Base* onClone() const override { return new Derived(*this); }
};

#define DEBUG(...) std::cout << #__VA_ARGS__ << ";\n"; __VA_ARGS__ 

int main()
{
  DEBUG(Instanceable obj1);
  DEBUG(Base *pObj1Clone = obj1.clone());
  DEBUG(std::cout << "-> " << typeid(*pObj1Clone).name() << "\n\n");
  DEBUG(Derived obj2);
  DEBUG(Base *pObj2Clone = obj2.clone());
  DEBUG(std::cout << "-> " << typeid(*pObj2Clone).name() << "\n\n");
  DEBUG(WrongDerived obj3);
  DEBUG(Base *pObj3Clone = obj3.clone());
  DEBUG(std::cout << "-> " << typeid(*pObj3Clone).name() << "\n\n");
  DEBUG(BadDerived obj4);
  DEBUG(Base *pObj4Clone = obj4.clone());
  DEBUG(std::cout << "-> " << typeid(*pObj4Clone).name() << "\n\n");
}

输出：

Instanceable obj1;
Base *pObj1Clone = obj1.clone();
std::cout << "-> " << typeid(*pObj1Clone).name() << '\n';
-> 12Instanceable

Derived obj2;
Base *pObj2Clone = obj2.clone();
std::cout << "-> " << typeid(*pObj2Clone).name() << '\n';
-> 7Derived

WrongDerived obj3;
Base *pObj3Clone = obj3.clone();
ERROR: Missing overload of onClone()!
  in 12WrongDerived
std::cout << "-> " << typeid(*pObj3Clone).name() << '\n';
-> 7Derived

BadDerived obj4;
Base *pObj4Clone = obj4.clone();
ERROR: Missing overload of onClone()!
  in 10BadDerived
std::cout << "-> " << typeid(*pObj4Clone).name() << '\n';
-> 7Derived

Live Demo on coliru

技巧其实很简单：

它不是覆盖 clone() 本身，而是用作 virtual onClone() 方法的蹦床。因此，clone() 可以在返回之前检查结果的正确性。

这不是编译时检查，而是运行时检查（我认为这是第二好的选择）。假设开发中的 class 库的每个 class 都应该至少在开发过程中被检查/调试，我发现这非常可靠。

的公认答案向我展示了一种方法，使它更能避免 copy/paste 错误：

不是在每个覆盖的 clone() 中键入 class 名称，而是通过 decltype():

获得 class 名称

class Instanceable: public Base {
  public:
    Instanceable() = default;
    Instanceable(const Instanceable&) = default;
    Instanceable& operator=(const Instanceable&) = default;
    virtual ~Instanceable() = default;
    
  protected:
    virtual Base* onClone() const
    {
      return new std::remove_const_t<std::remove_pointer_t<decltype(this)>>(*this);
    }
};

class Derived: public Instanceable {
  public:
    Derived() = default;
    Derived(const Derived&) = default;
    Derived& operator=(const Derived&) = default;
    virtual ~Derived() = default;
    
  protected:
    virtual Base* onClone() const override
    {
      return new std::remove_const_t<std::remove_pointer_t<decltype(this)>>(*this);
    }
};

Live Demo on coliru

Answer 2

不要让 A 和 B 继承自 IClonable。使用包装器 (BluePrint) 代替：

struct IClonable {
    virtual ~IClonable() = default;
    virtual IClonable * clone() const = 0;
};

template<typename T>
class BluePrint final : IClonable {
public:
    explicit BluePrint(T * element) : element(element) {
    }
    IClonable * clone() const override {
        T * copy = element->clone();
        return new BluePrint(copy);
    }
    T * get() const {
        return element;
    }

private:
    T * const element;
};

struct A {
    A * clone() const;
};

struct B : A {
    B * clone() const;
};

尽管如此，您将不得不稍微修改一下代码，因为此 returns 是包装器的克隆，而不是立即克隆要克隆的元素。话又说回来，我不知道你打算如何使用IClonable接口，所以我无法为你完成这个例子。

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