c ++ 11联合包含具有虚函数的数据成员 [英] c++11 union contains data member with virtual function

问题描述

#include <iostream>
class derive1{
  public:
    derive1() = default;
    ~derive1() = default;
    virtual void func() { std::cout << "derive 1" << std::endl; }
};

class derive2 {
  public:
    derive2() = default;
    ~derive2() = default;
    virtual void func() { std::cout << "derice 2" << std::endl; }
};

union classUnion {
  classUnion() {};
  ~classUnion() {};
  derive1 obj1;
  derive2 obj2;
};

int main() {
  classUnion u1;
  u1.obj1.func();  // <-- OK print 'derive 1'
  derive1 &dev1 = u1.obj1;
  dev1.func(); // <-- OK print 'derive 1'
  derive1 *ptr = &(u1.obj1);
  ptr->func(); // <-- core dump/seg fault
  return 0;
}

我认为C ++ 11允许使用非平凡的构造函数（带有虚函数） 。
我看不到这里有什么问题。
我使用
g ++ -std = c + 11 test.cpp进行编译（gcc 4.8和gcc 5.0）。

I thought C++11 allow non-trivial constructor (with virtual function). I can't see what's the problem here. I use "g++ -std=c+11 test.cpp" to compile it (gcc 4.8 and gcc 5.0).

推荐答案

问题是您永远不会在联合体内初始化对象。至少，最简单的方法就是进行以下一些小的调整：

The problem is that you never initialize the object inside the union. At least, the easiest way to make it work is the following little tweak:

union classUnion {
  classUnion() {};
  ~classUnion() {};
  derive1 obj1={}; // unions can have one inline initializer
  derive2 obj2;
};

但是，如果您改为这样做：

However, if you instead do this:

int main() {
  classUnion u1;
  u1.obj1 = derive1{};
  ...
}

它仍然会崩溃。原因是因为您要分配给未初始化的对象，特别是您具有用户定义的析构函数（即虚拟析构函数）。

It still will crash. The reason why is because you are assigning into an uninitialized object, and in particular you have a user defined destructor (i.e. a virtual one).

请考虑以下内容：
（ http://zh.cppreference.com/w/cpp/language/union 。）

如果是成员联合的一部分是具有用户定义的构造函数和
析构函数的类，通常需要切换活动成员，显式析构函数和
放置新值：

If members of a union are classes with user-defined constructors and destructors, to switch the active member, explicit destructor and placement new are generally needed:

因此，要实际使用带有虚函数的类（通常需要虚拟析构函数），您将需要使用放置新的和手动销毁调用，例如：

So to realistically use classes with virtual functions (which typically need virtual destructors), you're going to need to use placement new and manual destruction calls, like so:

int main() {
  classUnion u1;
  new (&u1.obj1) derive1{};
  ... // use obj1
  u1.obj1.~derive1();
  new (&u1.obj2) derive2{};
  ... // use obj2
  u1.obj2.~derive2();
}

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