参考Lisp_Object定义,在Emacs源代码的lisp.h中,Lisp_Object是一个既可以作为指针使用又可以作为整数使用的类型具体代码见下,注意Lisp_Int为0,使得非负整数可以直接转换为对应的List_Object类型:

#define VALBITS 29
#define GCTYPEBITS 3
enum Lisp_Type{Lisp_Int = 0,/*Symbol.  XSYMBOL (object) points to a struct Lisp_Symbol.*/Lisp_Symbol = 2,/*Miscellaneous.  XMISC (object) points to a union Lisp_Misc,
whose first member indicates the subtype.*/Lisp_Misc = 3,/*String.  XSTRING (object) points to a struct Lisp_String.
The length of the string, and its contents, are stored therein.*/Lisp_String = LISP_STRING_TAG,/*Vector of Lisp objects, or something resembling it.
XVECTOR (object) points to a struct Lisp_Vector, which contains
the size and contents.  The size field also contains the type
information, if it's not a real vector object.*/Lisp_Vectorlike = 5,/*Cons.  XCONS (object) points to a struct Lisp_Cons.*/Lisp_Cons = 6,Lisp_Float = 7,};typedef union Lisp_Object{/*Used for comparing two Lisp_Objects;
also, positive integers can be accessed fast this way.*/EMACS_UINT i;struct{EMACS_INT val  : VALBITS;enum Lisp_Type type : GCTYPEBITS;} s;struct{EMACS_UINT val : VALBITS;enum Lisp_Type type : GCTYPEBITS;} u;}
Lisp_OLisp_Stringbject;

具体细节参考这里的讨论.

tagged pointer的细节请参考上文的C代码,高地址的3bit用作判别数据类型,低地址的29bit用作指针或者整数(取决于数据类型)

• Emacs教我什么是软件自由(freedom for software)
• Emacs教我如何读代码
• Emacs让我认识到Lisp的威力
• Emacs教我如何实现语言核心
• Emacs教我如何进行内存自动回收(Garbage Collector)
• Emacs有助于我写代码和调试
• 我用Emacs写文档邮件
• Emacs让我成为高效程序员
• Emacs让我成为黑客
• Emacs永远改变了我的人生