Question

我不明白为什么下面的代码会调用基类的（= A）实现

A::operator=(A&)

每当作为参数传入的引用属于派生类类型时，

（由输出行15和17表示）。由于所有指针都指向派生类的对象，因此我希望所有调用都具有相同的行为（=从派生类B中调用运算符）。

执行下面列出的代码时的输出：

A :: Ctor objID = 1280 text = BObject1

B :: Ctor objID = 1280 text = BObject1

A :: Ctor objID = 3279 text = BObject2

B :: Ctor objID = 3279 text = BObject2

--- ptrA-＆gt; print（）：

B :: print（）objID = 1280 text = BObject1

B :: print（）objID = 3279 text = BObject2

--- * ptrAObj1 = * ptrAObj2：

B :: operator =（A＆amp;）objID = 1280 text = BObject1

--- ptrAObj1-＆gt; operator =（* ptrAObj2）：

B :: operator =（A＆amp;）objID = 1280 text = BObject2

--- * ptrBObj1 = * ptrAObj2：

B :: operator =（A＆amp;）objID = 1280 text = BObject2

--- * ptrBObj1 = * ptrBObj2：

A :: operator =（A＆amp;）objID = 1280 text = BObject2

--- ptrBObj1-＆gt; operator =（* ptrBObj2）：

A :: operator =（A＆amp;）objID = 1280 text = BObject2

--- ptrAObj1-＆gt; operator =（* ptrBObj2）：

B :: operator =（A＆amp;）objID = 1280 text = BObject2

---- TearDown（）

B :: Dtor objID = 1280 text = BObject2

A :: Dtor objID = 1280 text = BObject2

B :: Dtor objID = 3279 text = BObject2

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

我认为这是因为B类中的virtual int operator=(A& rIn)不是真正的默认operator=所以编译器正在为您创建B& operator=(const B& other)并使用该版本。

要添加到此，仅覆盖虚拟运算符=是不够的，还必须定义常规运算符=。

当然最好使用像override关键字这样的C ++ 11功能来保证你实际编写了你想要的函数签名，而不是创建一个新的虚函数。

Answer 2

在这两种情况下，您都会调用默认B::operator=(const B&)。其默认实现调用A::operator=(A&)

虚拟赋值运算符

2 个答案: