#include <iostream> using namespace std;                class Base { public:   virtual Base& operator=(const Base& rhs) //重載操作符可設為virtual   {     cout << "Base" << endl;     return *this;   } };                class Derived : public Base { public:   //與基類的operator=完全不同，不是重新定義，不會動態綁定。   //如果不定義該操作符，會自動合成一個，并自動調用基類的operator=，不會動態綁定   Derived& operator=(const Derived& rhs)    {     cout << "Derived_D" << endl;     return *this;   }                  //重新定義基類的operator=，會動態綁定   //virtual Base& operator=(const Base& rhs) //返回值兩種都可以   virtual Derived& operator=(const Base& rhs)    {     cout << "Derived_B" << endl;     return *this;   } };                class Derived2 : public Derived {   //此類需要3個operator=   //可定義private的copy函數，由3個operator=調用   //可使用dynamic_cast將基類引用參數轉為子類，并捕獲異常。   //如果未發生異常，則調用copy，發生異常則不需賦值 };                int main() {   Base b1, b2;   Derived d1, d2;                  Derived &rd = d1;                  Base &rb1 = b1; //動態類型為Base   Base &rb2 = d2; //動態類型為Derived                    rb1 = d1; //輸出"Base"   rb2 = d2; //輸出"Derived_B"                  rb1 = rb2; //輸出"Base"   rb2 = rb1; //輸出"Derived_B"                  rd = d1; //輸出"Derived_D"   rd = b1; //輸出"Derived_B"                    getchar();   return 0; }