从另一个结构更改一个结构的值

Question

我刚刚读过Extending a struct in C而且我似乎不明白代码的某些部分是做什么的。
这是他当前的代码:

typedef struct A {
  int x;
}A;

typedef struct B {
  A a;
  int d;
}B;

void fn(){
  B *b;
  ((A*)b)->x = 10;
}

我想知道，((A*)b)->x = 10; 如何变得与 b->a.x = 10; 相同？

你如何解释这行代码？

编辑

(A*) 和 (A*)b 做什么？

Answer 1

根据标准，指向结构的指针必须等于指向该结构的初始成员的指针。换句话说，在专用于存储struct A 内容的区域前面，不得在struct B 中插入任何填充。这就是强制转换有效的原因:

Section 6.7.2.1.13

Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning