c++ - 预定义派生类

Question

我正在尝试按如下方式预定义派生类，

class Base{

}

class Derived;

class Unreleated{
   Base* ptr;
public:
  Unreleated& operator, (const Unreleated& m){
    static_cast<Derived*>(ptr); // pointer is a derived.

    return *this;
  }


}

class Derived : public Base{
 //this is a container holding a bunch of unrelated.
}

但是我得到了从 Base* 到 Derived* 的无效静态转换？

Answer 1

将Unreleated::operator,的定义移到Derived的定义之后。

class Base{

};

class Derived;

class Unreleated{
   Base* ptr;
public:
  Unreleated& operator, (const Unreleated& m);

};

class Derived : public Base{
 //this is a container holding a bunch of unrelated.
};

  Unreleated& Unreleated::operator, (const Unreleated& m){
    static_cast<Derived*>(ptr); // pointer is a derived.

    return *this;
  }