c++ - Union 内部的奇怪行为类对象

Question

您好，我想知道以下代码的原因

void main()
{
  class test
  {
    public:
      test(){}
      int k;
  };

  class test1
  {
    public:
      test1(){}
      int k;
  };

  union Test
  {
    test t1;
    test1 t2;   
  };
}

对于上面的代码，它给出了错误“error C2620: union 'Test' : member 't1' has user-defined constructor or non-trivial default constructor”

class test
{
  public:
  //test(){}
  int k;
};

class test1
{
  public:
  //test()1{}; 
  int k;
};

union Test
{
  test t1;
  test1 t2; 
};

对于上述，没有错误。

我想知道原因。

在此先感谢您。 :)

Answer 1

根据 C++ 标准(§9.5.1，在其他答案中也有引用):

A union can have member functions (including constructors and destructors), but not virtual functions. A union shall not have base classes. A union shall not be used as a base class. An object of a class with a non-trivial constructor, a non-trivial copy-constructor, a non-trivial destructor, or a non-trivial copy assignment operator cannot be a member of a union, nor can an array of such objects. If a union contains a static data member, or a member of a reference type, the program is ill-formed.

我首先链接到 Wikipedia article about POD types其中指出:

A POD type in C++ is defined as either a scalar type or a POD class. POD class has no user-defined copy assignment operator, no user-defined destructor, and no non-static data members that are not themselves PODs. Moreover, POD class must be an aggregate, meaning it has no user-declared constructors, no private nor protected non-static data, no bases and no virtual functions. The standard includes statements about how PODs must behave in C++.

和

In certain contexts, C++ allows only POD types to be used. For example, a union in C++ cannot contain a class that has virtual functions, or nontrivial constructors or destructors. This restriction is imposed because the compiler cannot know which constructor or destructor should be called for a union.

第二段的第一句话可能会让您认为 C++ 只允许 POD 类型成为 union 的一部分。事实并非如此，因为它允许具有私有(private)成员的类成为 union 的一部分:

#include <iostream>
using namespace std;

class test1
{
  int i;
};

class test2
{
  int i;
};

union test
{
  test1 t1;
  test2 t2;
};

int main()
{
  cout << __is_pod(test1) << endl;
  cout << __is_pod(test2) << endl;
  cout << __is_pod(test) << endl;

  return 0;
}

上面用MSVC++编译的程序打印出来:

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