错误：从“底部**”到“lefta **”的无效转换

Question

我正在尝试编译并运行此程序。显然，它不起作用！我的问题是为什么它是从底部**到lefta **的无效转换，而底部*可以转换为lefta *？

#include<iostream>
using namespace std;
class top
{
private:
    int a;
public:
    top(int b):a(b){}
    virtual void output(){cout<<a<<endl;}
};
class lefta:virtual public top
{
private:
    int b;
public:
    lefta(int c,int d):top(c),b(d){}
    void output(){cout<<b<<endl;}
};
class righta:virtual public top
{
private:
    int c;
public:
    righta(int c,int d):top(c),c(d){}
    void output(){cout<<c<<endl;}
};
class bottom:public lefta,public righta
{
private:
    int d;
public:
    bottom(int e,int f,int g,int h):top(e),lefta(e,f),righta(e,g),d(h){}
    void output(){cout<<d<<endl;}
};
int main()
{
    bottom* bo=new bottom(1,2,3,4);
//  lefta* le=bo;
//  le->output();
    bottom** p_bo=&bo;//here
    lefta** p_le=p_bo;//here
    (*p_le)->output();
    return 0;
 }

Answer 1

class leftb : public lefta { /* blah */ };

bottom* bo = new bottom(1,2,3,4);
bottom** p_bo = &bo;
lefta** p_le = p_bo;// let's pretend it works
// now p_le points to the variable bo, which is of type bottom*
// so *p_le is a reference to the variable bo
*p_le = new leftb(1,2); // wait, did we just assign a leftb* to a bottom*?
// (and yeah, I'm leaking memory. Sue me)
// bo now points to a leftb, but it is a bottom*
// oops, we just broke the type system