使用METHOD宏的函数定义：
METHOD(message_t, get_message_id, uint32_t,
private_message_t *this)
{
return this->message_id;
}
METHOD宏的定义：
/**
* Method declaration/definition macro, providing private and public interface.
*
* Defines a method name with this as first parameter and a return value ret,
* and an alias for this method with a _ prefix, having the this argument
* safely casted to the public interface iface.
* _name is provided a function pointer, but will get optimized out by GCC.
*/
#define METHOD(iface, name, ret, this, ...) \
static ret name(union {iface *_public; this;} \
__attribute__((transparent_union)), ##__VA_ARGS__); \
static typeof(name) *_##name = (typeof(name)*)name; \
static ret name(this, ##__VA_ARGS__)
将METHOD宏展开后的定义：
static uint32_t get_message_id(union {message_t *_public; private_message_t *this;} __attribute__((transparent_union)));
static typeof(get_message_id)* _get_message_id = (typeof(get_message_id)*)get_message_id;
static uint32_t get_message_id(private_message_t *this)
关于transparent_union的定义：
http://nanjingabcdefg.is-programmer.com/categories/5966/posts