通过基础 class 引用访问派生 class 成员

Question

考虑一个简单的 class stDeriv，它继承自 stBase。我很惊讶地发现我们无法通过 stBase 引用访问 stDeriv class 成员。

下面用基本的例子来说明我的观点:

#include <fstream>  // std::ifstream
#include <iostream> // std::cout

using namespace std;

typedef struct stBase
{
public:
  virtual ~stBase() { ; }
  stBase(int iB) { iB_ = iB; }

  // Copy operator
  stBase &operator=(const stBase &src)
  {
    iB_ = src.iB_;
    return *this;
  }

  virtual void Hello() { cout << " Hello from stBase" << endl; }

private:

  int iB_ = 0;

} stBase;

typedef struct stDeriv : public stBase
{
public:

  int iD_ = 0;

  stDeriv(int iB) : stBase(iB), iD_(0) { ; }
  virtual void Hello() { cout << " Hello from stDeriv" << endl; }

  // Copy operator
  stDeriv &operator=(const stDeriv &src)
  {
    iD_ = src.iD_;
    return *this;
  }
} stDeriv;

int main(int, char *[])
{
  int iErr = 0;
  stBase aBase(0);
  stDeriv aDeriv(1);
  stDeriv &rDeriv = aDeriv;    
  stBase &rBase = aBase;

  rBase.Hello();  // OK result : "Hello from stBase"
  rDeriv.Hello(); // OK result : "Hello from stDeriv"

  rBase = rDeriv; // KO!!! Cannot access to aDeriv.iD_ through rBase !!!
  rBase.Hello();  // KO!!!  result : "Hello from stBase" !!!

  return iErr;
}

为什么在“rBase = rDeriv;”之后无法通过 rBase 访问 stDeriv::iD_ ？

Answer 1

您不能像现在这样重新绑定引用。 rBase 已经有一个值，不能再次赋值。 why doesn't C++ allow rebinding a reference?

所以只做一个新的参考：

int main(int, char* [])
{
    int iErr = 0;
    stBase aBase(0);
    stDeriv aDeriv(1);
    stDeriv& rDeriv = aDeriv;
    stBase& rBase = aBase;

    rBase.Hello();  // OK result : "Hello from stBase"
    rDeriv.Hello(); // OK result : "Hello from stDeriv"

    // Make a new reference and all is fine
    stBase& rBase2 = rDeriv;  
    rBase2.Hello(); // OK result : "Hello from stDeriv"

    return iErr;
}

通过基础 class 引用访问派生 class 成员

Access to derived class members through a base class reference

c++

inheritance