c - C 中的结构扩展

Question

假设定义了 2 个结构，例如:

typedef struct {
   T x;
   T y;
} A;

typedef struct {
   A a;
   T z;
} B;

我可以将指向结构 B 的指针视为指向结构 A 的指针吗？

在实践中这是可靠的/标准的/可移植的/编译器不变的:

B b = {{1,2},3};
A * a = &b;

print(a->x);
print(a->y);

Answer 1

C17 6.7.2.1 声明了这一点(强调我的):

Within a structure object, the non-bit-field members and the units in which bit-fieldsreside have addresses that increase in the order in which they are declared. A pointer to astructure object, suitably converted, points to its initial member (or if that member is abit-field, then to the unit in which it resides), and vice versa.

这意味着您必须将 B b 对象的指针“适本地转换”为它的第一个成员的类型。这种转换不会隐式发生，您必须通过显式转换来完成:

A * a = (A*)&b;

根据上面引用的部分，这样做是明确且安全的。

同样，不允许编译器假设指向A 的指针和指向B 的指针不存在别名。有效类型规则 6.5.7(“严格别名”)给出了这种情况的异常(exception)情况:

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:
...

• an aggregate or union type that includes one of the aforementioned types among its members

例如，在优化期间调用函数 void func (B* b) 的编译器不允许假定外部连接变量 extern A a; 定义在一些其他的翻译单元没有被函数改变。