lua的数据对象表示

#define CommonHeader	GCObject *next; lu_byte tt; lu_byte marked

struct GCObject {
  CommonHeader;
};

typedef union Value {
  GCObject *gc;    /* collectable objects */
  void *p;         /* light userdata */
  int b;           /* booleans */
  lua_CFunction f; /* light C functions */
  lua_Integer i;   /* integer numbers */
  lua_Number n;    /* float numbers */
} Value;

#define TValuefields	Value value_; int tt_

typedef struct lua_TValue {
  TValuefields;
} TValue;

#define LUA_TNIL			0
#define LUA_TBOOLEAN		1
#define LUA_TLIGHTUSERDATA	2
#define LUA_TNUMBER			3
#define LUA_TSTRING			4
#define LUA_TTABLE			5
#define LUA_TFUNCTION		6
#define LUA_TUSERDATA		7
#define LUA_TTHREAD			8

数据对象由lua_TValue表示，分为两部分。
int tt_表示对象的类型；Value value_为具体值。
tt_的最后4bit表示对象的主类型，倒数5、6位表示子类型，7位表示是否可回收，其它位表示对象的tag
获取对象的值的过程：根据类型tt_从联合体value_中取出相应的值。
例：tt_为LUA_TNUMBER时，获取值分两种情况
    当tt_等于LUA_TNUMINT取value_的i（整型）
	当tt_等于LUA_TNUMFLT取value的n（浮点型）的值。

以下分九种类型分别描述lua的对象	
0、nil
obj = {value_ = {NULL},tt_ = LUA_TNIL}
1、boolean
true = {value_ = {b=1},tt_ = LUA_TBOOLEAN}
false= {value_ = {b=0},tt_ = LUA_TBOOLEAN}
2、light userdata
pc表示C语言的指针
obj = {value_ = {p=pc},tt_ = LUA_TLIGHTUSERDATA}
3、整数和浮点数
#define LUA_TNUMFLT	(LUA_TNUMBER | (0 << 4))  /* float numbers */
#define LUA_TNUMINT	(LUA_TNUMBER | (1 << 4))  /* integer numbers */
int_value为整数的值
float_value为浮点数的值
int_obj = {value_ = {i=int_value},tt_=LUA_TNUMINT}
float_obj = {value_ = {n=float_value},tt_=LUA_TNUMFLT}
4、string
lua_TValue对象并不直接存放string的字符串，gc指向TString ts
str = {value_ = {gc=ts},tt_ = LUA_TSTRING}

string对象的真实结构
// Header for string value; string bytes follow the end of this structure
typedef struct TString {
  CommonHeader;
  lu_byte extra;  /* reserved words for short strings; "has hash" for longs */
  lu_byte shrlen;  /* length for short strings */
  unsigned int hash;
  union {
    size_t lnglen;  /* length for long strings */
    struct TString *hnext;  /* linked list for hash table */
  } u;
} TString;
5、table
typedef union TKey {
  struct {
    TValuefields;
    int next;  /* for chaining (offset for next node) */
  } nk;
  TValue tvk;
} TKey;

/* copy a value into a key without messing up field 'next' */
#define setnodekey(L,key,obj) \
	{ TKey *k_=(key); const TValue *io_=(obj); \
	  k_->nk.value_ = io_->value_; k_->nk.tt_ = io_->tt_; \
	  (void)L; checkliveness(L,io_); }

typedef struct Node {
  TValue i_val;
  TKey i_key;
} Node;

typedef struct Table {
  CommonHeader;
  lu_byte flags;  /* 1<