Question with "container_of(ptr, type, member)"

[peter]

I have a question about the macro " container_of(ptr, type, member) "
I can write it as this,
 #define container_of(ptr, type, member) ({ \
	(type *) ((char *) ptr - offset_of(type, member)); \
	})
It can act the same as 
#define container_of(ptr, type, member) ({            \
	 const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
	 (type *)( (char *)__mptr - offsetof(type,member) );})
So why we don't use the first one ?
Thanks for your answer.
(I am a kernel newbie ,and sorry for my poor english~)


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