初学者购买哪种开发板，TQ2440和友善之臂mini2440哪个更适合初学者？

宏定义如下：
/**
* container_of - cast a member of a structure out to the containing structure
* @ptr: the pointer to the member.
* @type: the type of the container struct this is embedded in.
* @member: the name of the member within the struct.
*
*/
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})

为什么要定义一个临时变量__mptr，为什么不直接作如下定义？
#define container_of(ptr,type,member) (type *)( (char *)(ptr) - offsetof(type,member) )