结构 - 没有 . 的访问结构元素。和->答案

【问题标题】：Structure - Access Structure element without . and ->结构 - 没有 . 的访问结构元素。和->
【发布时间】：2016-11-01 02:46:00
【问题描述】：

我需要在不使用. 和-> 的情况下访问嵌套结构中的一些元素

我需要从测试笔记本电脑定义中打印出keyValue 和alternateKeyValue 的值，而不使用. 或-> 运算符直接引用qwerty 结构或其成员。

这是结构。

typedef struct
{
    bool leftButton;
    bool rightButton;
    bool middleButton;
    bool mouseOn;
    mouse_direction_E direction;
} mouse_S;

typedef struct
{
    char keyValue;
    char alternateKeyValue;
} keyboard_S;

typedef struct
{    
    mouse_S simpleMouse;
    keyboard_S qwerty;
} laptop_S;

laptop_S test=
{

    .simpleMouse =
    {
        .leftButton = false,
        .rightButton = false,
        .middleButton = false,
        .mouseOn = false,
        .direction = MOUSE_NONE,
    },
    .qwerty =
    {
        .keyValue = '5',
        .alternateKeyValue = '%'
    },
};

int main()
{

    char c = tesla.qwerty.keyValue;
    char d = tesla.qwerty.alternateKeyValue;
    printf("KeyValue = %c\n", c);
    printf("alternateKeyValue = %c\n", d);
}

这可行，但有没有办法在不使用“.”的情况下访问 KeyValue 和 alternateKeyValue？

【问题讨论】：

下次请用CTRL+K缩进（选中代码块再CTRL+K）
您可以使用offsetof() 和类型转换。但你为什么要这样做呢？

标签： c pointers struct

【解决方案1】：

可能他们希望你使用这样的东西：

union {
    mouse_S    mouse;
    keyboard_S keyboard;
    laptop_S   laptop;
} * oh; // oh = offset helper
size_t offset_of_mouse_leftButton = (char*)&oh->mouse->leftButton - (char*)&oh->mouse; // this should be 0
size_t offset_of_mouse_rightButton = (char*)&oh->mouse->rightButton - (char*)&oh->mouse; // but this one can be anything
size_t offset_of_mouse_middleButton = (char*)&oh->mouse->middleButton - (char*)&oh->mouse; // this too
// ...
size_t offset_of_keyboard_alternateKeyValue = (char*)&oh->keyboard->alternateKeyValue - (char*)&oh->keyboard;
// ...

然后用void * 到keyboard_S：

int get_keyValue(void * _keyboard) {
    // usual way:
    // keyboard_S * keyboard = _keyboard;
    // return keyboard->keyValue;
    // requested way:
    return *(CHAR*)((char*)_keyboard + offset_of_keyboard_keyValue);
}

CHAR 类型应该小写，是元素keyValue 的类型。对于每种类型，其他char 必须是char，无论它是什么。 offset_of_ 变量定义中的上述chars 相同。

所以，我猜，剩下的就是你的功课了。

【讨论】：

【解决方案2】：

更直接和更简单的 3 行代码可能是...

//Get the address of the tesla      
  char *addressoftesla  = (laptop_S*)(&tesla);

//structure of tesla = simpleMouse + qwerty + 2 byte paddings

/* Get the value at the starting address of qwerty i.e keyValue*/
  printf("keyValue of tesla keyboard is %c\n",*(addressoftesla+sizeof(mouse_S)));

/* Get the value at the starting address of qwerty i.e alternatekeyValue */
  printf("alternatekeyValue of tesla keyboard is %c\n",*(addressoftesla+sizeof(mouse_S)+1));

  /*
  * More info for further understanding: 
  * A simple check to see all the values in the each field of structure
  * tesla - you can also use sizeof to get the each structure and entire 
  * structure byte sizes and the same can be done using offsetof as in 
  * other solution
  */
  for(int i = 0; i< 12; i++)
  {
      printf("value at tesla[i] is %d \n",*(addressoftesla+i));
  }

【讨论】：