结构 - 访问结构元素，不带 .和 -> [英] Structure - Access Structure element without . and ->

问题描述

我需要在不使用 . 和 ->

I am required to access some elements from nested structure without using . and ->

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.

I need to print out the values for keyValue and alternateKeyValue from the Test laptop definition without using the . or -> operators to directly reference the qwerty struct or its members.

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);
}

这是可行的，但是有没有办法在不使用."的情况下访问 KeyValue 和 alternateKeyValue?

This works, but is there a way to access KeyValue and alternateKeyValue without using '.'?

推荐答案

可能他们希望你使用这样的东西:

Probably they want you to use something like this:

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;
// ...

然后用一个 void * 到 keyboard_S :

and then with a void * to 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);
}

类型CHAR 应为小写，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.与上述 offset_of_ 变量定义中的 char 相同.

The type CHAR should be written in lowercase and is the type of the element keyValue. The other char must be char for every type, whatever it is. Same for the chars above in the offset_of_ variable definitions.

所以，我想，剩下的就是你的家庭作业了.

So, I guess, the rest is your homework.

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