派生奇怪的循环模板和协方差

假设我有一个克隆派生类的基类：

class Base
{
    public:
        virtual Base * clone()
        {
            return new Base();
        }

        // ...
};

我有一组派生类，它们使用奇怪的循环模板模式实现：

template <class T>
class CRTP : public Base
{
    public:
        virtual T * clone()
        {
            return new T();
        }

        // ...
};

我试图从这样的进一步得出：

class Derived : public CRTP<Derived>
{
    public:
        // ...
};

编译错误的影响是：

error C2555: 'CRTP<T>::clone': overriding virtual function return type differs and is not covariant from 'Base::clone'

我意识到这可能是编译器在实例化CRTP时不完全知道Derived的继承树的结果。 此外，用（Base *）替换返回类型（T *）也会被编译。 但是，我想知道是否存在保留上述语义的工作。

---

一个不太漂亮的解决方法。

class Base
{
    protected:
        virtual Base * clone_p()
        {
            return new Base();
        }
};


template <class T>
class CRTP : public Base
{
    protected:
        virtual CRTP* clone_p()
        {
            return new T;
        }
    public:
        T* clone()
        {
            CRTP* res = clone_p();
            return static_cast<T*>(res);
        }
};


class Derived : public CRTP<Derived>
{
    public:
};

如果您觉得它更安全，请使用dynamic_cast<>而不是static 。

---

如果您不得不使用不同的语法来指定完整类型，则可以执行以下操作（警告：未经测试的代码）：

我们首先从机器开始：

// this gives the complete type which needs to be used to create objects
// and provides the implementation of clone()
template<typename T> class Cloneable:
  public T
{
public:
  template<typename... U> Cloneable(U&&... u): T(std::forward<U>(u) ...) {}
  T* clone() { return new Cloneable(*this); }
private:
  // this makes the class complete
  // Note: T:: to make it type dependent, so it can be found despite not yet defined
  typename T::CloneableBase::CloneableKey unlock() {}
};

// this provides the clone function prototype and also makes sure that only
// Cloneable<T> can be instantiated
class CloneableBase
{
  template<typename T> friend class Cloneable;

  // this type is only accessible to Clonerable instances
  struct CloneableKey {};

  // this has to be implemented to complete the class; only Cloneable instances can do that
  virtual CloneableKey unlock() = 0;
public:
  virtual CloneableBase* clone() = 0;
  virtual ~CloneableBase() {}
};

好的，现在是实际的课程层次结构。 那个很标准; 没有CRTP中间体或其他并发症。 但是没有类实现clone函数，但都从CloneableBase继承了声明（直接或间接）。

// Base inherits clone() from CloneableBase
class Base:
  public CloneableBase
{
  // ...
};

// Derived can inherit normally from Base, nothing special here
class Derived:
  public Base
{
  // ...
};

以下是您如何创建对象的方法：

// However, to create new instances, we actually need to use Cloneable<Derived>
Cloneable<Derived> someObject;
Derived* ptr = new Cloneable<Derived>(whatever);

// Now we clone the objects
Derived* clone1 = someObject.clone();
Derived* clone2 = ptr->clone();

// we can get rid og the objects the usual way:
delete ptr;
delete clone1;
delete clone2;

请注意， Cloneable<Derived>是Derived （它是一个子类），因此您只需要使用Cloneable来构建，否则就可以假装使用Derived对象（好吧， tyepinfo也会将它识别为Cloneable<Derived> ）。

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