实现层次结构中的冗余私有成员数据复制

Question

#include<iostream>

using std::cout;
using std::endl;

class MathOps {
public:
  virtual int add(int x) = 0;
  virtual int subtr(int x) = 0;
};

class MathOps_Impl : public virtual MathOps {
private:
  int m_y;
public:
  MathOps_Impl(int y) : m_y(y) {
    cout << "MathOps_Impl initialized with value: " << m_y << endl;
  }

  virtual int add(int x) { return x + m_y;}
  virtual int subtr (int x) { return m_y - x;}
};

class MathOps_Extn  : public virtual MathOps {
  // Extends MathOps by adding mult() and div()                                                                                                                                 
public:
  virtual int mult(int x) = 0;
  virtual int div(int x) = 0;
};

class MathOps_Extn_Impl : public    virtual MathOps_Extn, private MathOps_Impl {
private:
  int m_y; // Have to replicate member data m_y here.                                                                                                                           
public:
  MathOps_Extn_Impl(int y) : MathOps_Impl(y), m_y(y) {
    cout << "MathOps_Extn_Impl initialized with value: " << m_y << endl;
  }

  virtual int mult(int x) {
    return x * m_y;
  }
  virtual int div(int x) {
    int quotient = x == 0? 0 : m_y/x;
    return quotient;
  }
};

int main() {                                                                                                                                           
  MathOps_Extn* B =  new MathOps_Extn_Impl(10);
  cout << "add 20: " << B->add(20) << endl;
  cout << "subtr 20: " << B->subtr(20) << endl;
  cout << "mult 2: " << B->mult(2) << endl;
  cout << "div 5: " << B->div(5) << endl;

注意Mathops_extn_impl中的M_Y复制。有没有办法避免这种复制？

Answer 1

注意mathops_extn_impl中m_y的复制。有没有办法避免这种复制？

是的。授予Mathops_impl::m_yprotected访问权限，而不是private。

您明确地询问为什么派生类不能访问私有数据。那是故意的。