C++ 继承 : defining a pointer to either derived or base class

Question

如果我有一个来自基类的派生类，并且我想根据标志为它们中的任何一个的对象分配一个指针，那么指向所选对象的指针的定义应该是什么。

例子:

void main(int argc, char* argv[])
{
   class Base B;
   class Derived D;
   class Base *P;  /* My question is output this definition */
   int flag;   /* This will be an input */
   if(flag)
      P = &B;   /* Should refer to the base class B */
   else
      P = &D;   /* Should refer to the derived class D
   /* Then I'd use P in the rest of my code */ 
} 

我的问题是，类指针 P 如何定义为能够基于标志“flag”引用 B(基类)或 D(派生类)？

Answer 1

假设您的代码是基本形式:

class Base
{
public:
   virtual ~Base() {}
};

class Derived : public Base
{};

然后 Derived 根据语言规则是 Base，因此指向基类的指针可以包含 Derived 或一个 Base。

Base* p = nullptr;
Base b;
Derived d;

if(flag == UseBase)
{    
   // Works
   p = &b;
}
else if(flag == UseDerived)
{
   // also works
   p = &d;
}

就是说，Base 不是 Derived，因此反向 (Derived* pd = &b) 不会像上面写的那样工作.