结构成员变量偏移量的两个应用

最近在阅读linux kernel的代码，发现很多地方都使用了一个名container_of的宏，根据上下文可以看出，它的功能是得到包含某个结构成员的结构的指针，看了一下宏的定义，原来并没有用什么高深的技巧，只是C语言中常识而已，其实现如下：

(kernel.h)

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

#define container_of(ptr, type, member) ({                      \

        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \

        (type *)( (char *)__mptr - offsetof(type,member) );})

想起以前学习COM时，看过< COM本质论>，里面提到了类似的方法：

(COM本质论：inttable.h)

#define BASE_OFFSET(ClassName, BaseName) \

    (DWORD(static_cast<BaseName*>(reinterpret_cast<ClassName*>(0x10000000))) - 0x10000000)

#define COMPOSITE_OFFSET(ClassName, BaseName, MemberType, MemberName) \

    (DWORD(static_cast<BaseName*>(reinterpret_cast<MemberType*>(0x10000000 + offsetof(ClassName, MemberName)))) - 0x10000000)

#define BEGIN_INTERFACE_TABLE(ClassName) \

typedef ClassName _InterfaceTableClassName;\

static const INTERFACE_ENTRY *GetInterfaceTable(void) \

{\

    static const INTERFACE_ENTRY table[] = {\

#define INTERFACE_TABLE_ENTRY(piid, pfn, dwData) \

        { piid, pfn, dwData },

#define IMPLEMENTS_INTERFACE(ItfName) \

        { &IID_##ItfName, ENTRY_IS_OFFSET,  BASE_OFFSET(_InterfaceTableClassName, ItfName) },

#define IMPLEMENTS_INTERFACE_AS(RequestedItfName, BaseClassName) \

        { &IID_##RequestedItfName, ENTRY_IS_OFFSET ,  BASE_OFFSET(_InterfaceTableClassName, BaseClassName)},

#define IMPLEMENTS_INTERFACE_WITH_COMPOSITE(RequestedItfName, DataMemberType, DataMemberName) \

        { &IID_##RequestedItfName, ENTRY_IS_OFFSET ,  COMPOSITE_OFFSET(_InterfaceTableClassName, RequestedItfName, DataMemberType, DataMemberName) },

#define END_INTERFACE_TABLE() \

        { 0, 0, 0 }\

    };\

    return table;\

}