派生出奇怪的重复模板和协方差 [英] Derived curiously recurring templates and covariance

问题描述

假设我有一个克隆派生类的基类：

Suppose I have a base class which cloning of derived classes:

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

        // ...
};

我有一组派生类，使用奇怪的重复模板模式实现：

I have a set of derived classes which are implemented using a curiously recurring template pattern:

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

        // ...
};

我试图从这样得出：

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

我遇到的编译错误：

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 *）也会编译。但是，我想知道是否有一个保留上述语义的工作。

I realize this is probably a result of the compiler not fully knowing the inheritance tree for Derived when instantiating CRTP. Furthermore, replacing the return type (T*) with (Base*) also compiles. However, I would like to know if there is a work around which retains the above semantics.

推荐答案

不那么漂亮解决方法。

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:
};

使用 dynamic_cast<> code> static 如果你觉得它更安全。

Use dynamic_cast<> instead of static if you feel it's safer.

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