c - 结构包装 : how to add struct members at the beginning?

Question

我在 C89 中实现了一个二叉树，我试图通过组合在所有节点结构之间共享公共(public)属性。因此我有以下代码:

enum foo_type
{
    FOO_TYPE_A,
    FOO_TYPE_B
};

struct foo {
    enum foo_type type;
};

struct foo_type_a {
    struct foo base;
    struct foo * ptr;
};

struct foo_type_b {
    struct foo base;
    char * text;
};

我在所有结构定义中包含一个类型为 struct foo 的成员作为它们的初始成员，以便提供对 enum foo_type 持有的值的访问，而不管结构如何类型。为实现这一点，我希望指向结构对象的指针指向其初始成员，但我不确定这种假设在这种情况下是否成立。对于 C99，标准规定如下(参见 ISO/IEC 9899:1999 6.7.2.1 §13)

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.

尽管所有结构共享一个共同的 struct foo 对象作为它们的初始成员，填充开始发挥作用。 struct foo 只有一个int 大小的成员，struct foo_type_a 和struct foo_type_b 都包含指针成员，在某些情况下会增加对齐，从而增加填充。

因此，考虑到这种情况，C 编程语言(C89 或任何后续版本)是否确保通过指向对象的指针访问 struct foo::type 的值是安全的，无论该对象的类型为 struct foo 或包含类型为 struct foo 的对象作为其第一个成员，例如 struct foo_type_a 或 struct foo_type_b?

Answer 1

正如您自己引用的 C 标准，您所描述的内容受 C99 及更高版本的支持。

C89 似乎也支持它，因为您引用的语言自 1988 年以来就已经存在于 ANSI-C 文档中:

3.5.2.1 Structure and union specifiers

...

Within a structure object, the non-bit-field members and the unitsin which bit-fields reside have addresses that increase in the orderin which they are declared. A pointer to a structure object, suitablycast, 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 maytherefore be unnamed holes within a structure object, but not at itsbeginning, as necessary to achieve the appropriate alignment.