具有虚拟赋值运算符的多态性

Question

我不明白为什么下面的代码调用了基类 (=A) 的实现

A::operator=(A&)

（由输出行 15. 和 17. 指示）只要作为参数传入的引用属于派生类类型。由于所有指针都指向派生类的对象，我希望所有调用都具有相同的行为（=从派生类 B 调用运算符）。

执行下面列出的代码时的输出：

1. A::Ctor objID=1280 text=BObject1
2. B::Ctor objID=1280 text=BObject1
3. A::Ctor objID=3279 text=BObject2
4. B::Ctor objID=3279 text=BObject2
5. --- ptrA->print():
6. B::print() objID=1280 text=BObject1
7. B::print() objID=3279 text=BObject2
8. --- *ptrAObj1 = *ptrAObj2:
9. B::operator=(A&) objID=1280 text=BObject1
10. --- ptrAObj1->operator=(*ptrAObj2):
11. B::operator=(A&) objID=1280 text=BObject2
12. --- *ptrBObj1 = *ptrAObj2:
13. B::operator=(A&) objID=1280 text=BObject2
14. --- *ptrBObj1 = *ptrBObj2:
15. A::operator=(A&) objID=1280 text=BObject2
16. --- ptrBObj1->operator=(*ptrBObj2):
17. A::operator=(A&) objID=1280 text=BObject2
18. --- ptrAObj1->operator=(*ptrBObj2):
19. B::operator=(A&) objID=1280 text=BObject2
20. ---- TearDown()
21. B::Dtor objID=1280 text=BObject2
22. A::Dtor objID=1280 text=BObject2
23. B::Dtor objID=3279 text=BObject2
24. A::Dtor objID=3279 text=BObject2

代码：

#include "gtest/gtest.h"
#include <stdio.h>
#include <string>
#include <ctime>

class A
{
public:
  A(const std::string& sLabel=""):
    m_sText(sLabel)
  {
    std::srand(std::time(nullptr)); // use current time as seed for random generator
    int random_no = (std::rand()+(m_iObjectCounter*1999)) % 9999;
    m_iObjectID = random_no;
    m_iObjectCounter++;

    std::cout << "A::Ctor objID=" << m_iObjectID << " text=" << m_sText << std::endl;
   }

   virtual ~A()
   {
     std::cout << "A::Dtor objID=" << m_iObjectID << " text=" << m_sText << std::endl;
   }

   virtual void print() const
   {
     std::cout << "A::print() objID=" << m_iObjectID << " text=" << m_sText << std::endl;
   }

   virtual int operator=(A& rIn)
   {
     std::cout << "A::operator=(A&) objID=" << m_iObjectID << " text=" << m_sText << std::endl;
     m_sText = rIn.m_sText;
     return EXIT_SUCCESS;
   }

  std::string m_sText;
  int m_iObjectID;

  static int m_iObjectCounter;
};

int A::m_iObjectCounter=0;



class B : public A
{
public:
  B(const std::string& sLabel = "") :
    A(sLabel)
  {
    std::cout << "B::Ctor objID=" << m_iObjectID << " text=" << m_sText << std::endl;
  }

  virtual ~B()
  {
    std::cout << "B::Dtor objID=" << m_iObjectID << " text=" << m_sText << std::endl;
  }

  virtual void print() const
  {
    std::cout << "B::print() objID=" << m_iObjectID << " text=" << m_sText << std::endl;
  }

  virtual int operator=(A& rIn)
  {
    std::cout << "B::operator=(A&) objID=" << m_iObjectID << " text=" << m_sText << std::endl;
    m_sText = rIn.m_sText;
    return EXIT_SUCCESS;
  }
};



class TestAssignmentOperator : public ::testing::Test {
public:
  TestAssignmentOperator() :
    ptrAtoBObject1(NULL),
    ptrAtoBObject2(NULL)
  {
    ptrAtoBObject1 = new B("BObject1");
    ptrAtoBObject2 = new B("BObject2");
  }

  void SetUp(){}

  void TearDown() {
    std::cout << " ---- TearDown()" << std::endl;
    delete ptrAtoBObject1;
    delete ptrAtoBObject2;
  }

  ~TestAssignmentOperator(){}

  A* ptrAtoBObject1;
  A* ptrAtoBObject2;

};



TEST_F(TestAssignmentOperator, Test1)
{

  std::cout << " --- ptrA->print():" << std::endl;

  ptrAtoBObject1->print();
  ptrAtoBObject2->print();


  int iError = EXIT_FAILURE;
  std::cout << " --- *ptrAObj1 = *ptrAObj2:" << std::endl;
  iError = *ptrAtoBObject1 = *ptrAtoBObject2;


  std::cout << " --- ptrAObj1->operator=(*ptrAObj2):" << std::endl;
  iError = ptrAtoBObject1->operator=(*ptrAtoBObject2);


  std::cout << " --- *ptrBObj1 = *ptrAObj2:" << std::endl;
  B* ptrBToObject1 = dynamic_cast<B*>(ptrAtoBObject1);
  iError = *ptrBToObject1 = *ptrAtoBObject2;


  std::cout << " --- *ptrBObj1 = *ptrBObj2:" << std::endl;
  B* ptrBtoBObject2 = dynamic_cast<B*>(ptrAtoBObject2);
  *ptrBToObject1 = *ptrBtoBObject2;


  std::cout << " --- ptrBObj1->operator=(*ptrBObj2):" << std::endl;
  ptrBToObject1->operator=(*ptrBtoBObject2);


  std::cout << " --- ptrAObj1->operator=(*ptrBObj2):" << std::endl;
  ptrAtoBObject1->operator=(*ptrBtoBObject2);

}


int main(int argc, char **argv)
{

  ::testing::InitGoogleTest(&argc, argv);
  return RUN_ALL_TESTS();
}

Answer 1

我相信这是因为 B 类中的 virtual int operator=(A& rIn) 不是真正的默认 operator=，所以编译器会为您创建一个 B& operator=(const B& other) 并使用它。

除此之外，仅覆盖您的虚拟 operator= 是不够的，您还必须定义常规 operator=。

当然，最好使用 C++11 功能，例如 override 关键字，以确保您确实编写了您想要的函数签名，而不是创建新的虚函数。

Answer 2

在这两种情况下，您都调用默认的B::operator=(const B&)。它的默认实现调用然后A::operator=(A&)