C工会-请解释

Question

据我所知，C中的联合一次只能保存一个值，而且我也不真正理解C中的这段代码为什么有意义，因为event.window不能与event.type同时填充

while(SDL_PollEvent(&event)) {
switch(event.type)
{
case SDL_WINDOWEVENT:
    switch(event.window.event)

该事件被定义为：

typedef union SDL_Event
{
    Uint32 type;                    /**< Event type, shared with all events */
    SDL_CommonEvent common;         /**< Common event data */
    SDL_WindowEvent window;         /**< Window event data */
    SDL_KeyboardEvent key;          /**< Keyboard event data */
    SDL_TextEditingEvent edit;      /**< Text editing event data */
    SDL_TextInputEvent text;        /**< Text input event data */
    SDL_MouseMotionEvent motion;    /**< Mouse motion event data */
    SDL_MouseButtonEvent button;    /**< Mouse button event data */
    SDL_MouseWheelEvent wheel;      /**< Mouse wheel event data */
    SDL_JoyAxisEvent jaxis;         /**< Joystick axis event data */
    SDL_JoyBallEvent jball;         /**< Joystick ball event data */
    SDL_JoyHatEvent jhat;           /**< Joystick hat event data */
    SDL_JoyButtonEvent jbutton;     /**< Joystick button event data */
    SDL_JoyDeviceEvent jdevice;     /**< Joystick device change event data */
    SDL_ControllerAxisEvent caxis;      /**< Game Controller axis event data */
    SDL_ControllerButtonEvent cbutton;  /**< Game Controller button event data */
    SDL_ControllerDeviceEvent cdevice;  /**< Game Controller device event data */
    SDL_QuitEvent quit;             /**< Quit request event data */
    SDL_UserEvent user;             /**< Custom event data */
    SDL_SysWMEvent syswm;           /**< System dependent window event data */
    SDL_TouchFingerEvent tfinger;   /**< Touch finger event data */
    SDL_MultiGestureEvent mgesture; /**< Gesture event data */
    SDL_DollarGestureEvent dgesture; /**< Gesture event data */
    SDL_DropEvent drop;             /**< Drag and drop event data */

    /* This is necessary for ABI compatibility between Visual C++ and GCC
       Visual C++ will respect the push pack pragma and use 52 bytes for
       this structure, and GCC will use the alignment of the largest datatype
       within the union, which is 8 bytes.

       So... we'll add padding to force the size to be 56 bytes for both.
    */
    Uint8 padding[56];
} SDL_Event;

Answer 1

每个聚合(structs可能)成员SDL_CommonEvent common;，SDL_WindowEvent window;，SDL_KeyboardEvent key;等.union SDL_Event的开头是一些Uint32字段，给出了type，而该公共type字段在每个联合成员中具有相同的地址和大小。

因此，虽然一个联合在内存中同时只包含一个字段(换句话说，所有的联合成员都有相同的地址)，但每个成员都以type开头，而event.type是有意义的；它获取的是type。

在C语言中，这种成语是实现有标记的工会的常用方法。

Answer 2

SDL_Event联盟的每个成员都以相同的两个成员( Uint32 type和Uint32 timestamp )开始。C标准明确指出，如果一个联合当前持有一个struct类型的值，但将其读取为另一个结构类型，其第一个成员与另一个struct类型相匹配，则可以读取那些匹配的成员。

Answer 3

所有其他SDL_X类型都以32位的类型开头。实际上，它们在开始时似乎都包括SDL_CommonEvent中的字段。这是为了便于加入所有子结构的共同要素。然后，通过event.common.x，您可以访问所有公共元素，而不必区分事件的确切类型。