c++ - SFML 2.1 C++ 文档 - sf::Event 真的是一个 union 吗？

Question

在阅读 SFML 2.1 的文档时，我偶然发现了这个:

Before dealing with events, it is important to understand what the sf::Event type is, and how to correctly use it. sf::Event is a union, which means that only one of its member is valid at a time (remember your C++ lesson: all the members of a union share the same memory space).

然而，在检查 sf::Event 的声明后，我发现了这个:

    class Event
    {
    public :

        ////////////////////////////////////////////////////////////
        /// \brief Size events parameters (Resized)
        ///
        ////////////////////////////////////////////////////////////
        struct SizeEvent
        {
            unsigned int width;  ///< New width, in pixels
            unsigned int height; ///< New height, in pixels
        }; 
.....
 EventType type; ///< Type of the event
....

sf::Event 不能是 union 体，因为例如 sf::Event::EventType 不与 sf::Event 的任何其他成员共享内存区域，正如定义中明确定义的那样。

我看到的唯一 union 声明如下，再次取自类定义:

union
    {
        SizeEvent            size;            ///< Size event parameters (Event::Resized)
        KeyEvent             key;             ///< Key event parameters (Event::KeyPressed, Event::KeyReleased)
        TextEvent            text;            ///< Text event parameters (Event::TextEntered)
        MouseMoveEvent       mouseMove;       ///< Mouse move event parameters (Event::MouseMoved)
        MouseButtonEvent     mouseButton;     ///< Mouse button event parameters (Event::MouseButtonPressed, Event::MouseButtonReleased)
        MouseWheelEvent      mouseWheel;      ///< Mouse wheel event parameters (Event::MouseWheelMoved)
        JoystickMoveEvent    joystickMove;    ///< Joystick move event parameters (Event::JoystickMoved)
        JoystickButtonEvent  joystickButton;  ///< Joystick button event parameters (Event::JoystickButtonPressed, Event::JoystickButtonReleased)
        JoystickConnectEvent joystickConnect; ///< Joystick (dis)connect event parameters (Event::JoystickConnected, Event::JoystickDisconnected)
    };

那么我是不是遗漏了什么，或者这只是文档中的真正错误？

Answer 1

它不是 union ，并且由于枚举类型成员甚至不是类 union 类

9.5.8

A union-like class is a union or a class that has an anonymous union as a direct member. A union-like class X has a set of variant members.

你可以自己检查一下:

class IsUnion {
  public:

  struct struct1 {
      float tt;
  };

  struct struct2 {
      float tt;
  };

  enum EventType { no, yes };

  EventType type; 

  union
  {
      struct1 obj;
      struct2 obj2;
  };

};

int main() {

    cout << is_union<IsUnion>::value; // 0
}

Example

无论如何，该类的大部分由匿名 union 组成，我相信文档希望简化这一假设并使读者易于理解。