为什么这个联盟显然持有不止一种价值观

我是使用并集的新手，对如何通过这个测试感到困惑，SDL_Event是一个并集：

TEST(basic_check, test_eq) {
  Dot dot;
  SDL_Event event;            // this is a union, see below
  event.type = SDL_KEYDOWN;   // <= I use one member here
  SDL_Keysym keysym;          // this is a struct
  keysym.sym = SDLK_UP;
  event.key.keysym = keysym;  // <= I use another member here
  dot.handleEvent(event);     // <= but this function accesses value of the first member
  EXPECT_EQ(-Dot::DOT_VEL, dot.getVelY());
}

我的理解是，一个工会只能拥有一种价值观。

然而，在这个测试中，我在联合的一个成员event.type中设置了一个值；然后我更新了联盟的另一个成员event.key。更确切地说，event.key是一个结构，我更新它的成员，使其具有结构SDL_Keysym。

这里是函数的代码，然后被调用：

void Dot::handleEvent(SDL_Event& e) {
  if (e.type == SDL_KEYDOWN && e.key.repeat == 0) { //<== access two alternate members?
    switch (e.key.keysym.sym) {   
      case SDLK_UP: 
        velY -= DOT_VEL;
        break;
      case SDLK_DOWN:
        ...   // followed by a lot of other cases 
    }
  }
}

我很困惑，因为if条件访问联合的两个成员(请参阅上面的注释(。我以为它们会是独家的。

作为参考，SDL_Event和SDL_KeyboardEvent的定义如下：

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 */
    ...            // and a long list of other events 
    ...
} SDL_Event;

typedef struct SDL_KeyboardEvent
{
    Uint32 type;        /**< ::SDL_KEYDOWN or ::SDL_KEYUP */
    Uint32 timestamp;
    Uint32 windowID;    /**< The window with keyboard focus, if any */
    Uint8 state;        /**< ::SDL_PRESSED or ::SDL_RELEASED */
    Uint8 repeat;       /**< Non-zero if this is a key repeat */
    Uint8 padding2;
    Uint8 padding3;
    SDL_Keysym keysym;  /**< The key that was pressed or released */
} SDL_KeyboardEvent;