エラー： 'bottom **'から 'lefta **'への変換が無効

Question

私はこのプログラムをコンパイルして実行しようとしていますが、明らかに動作しません！ leftaに** bottomは* leftaに変換できますか？彼らは間接のそのレベルで関係していないので

#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