Lua的数据对象表示

发表于2018-08-27
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Lua并非严格意义上的面向对象语言,在语言层面上并没有直接提供诸如class这样的关键字,也没有显式的继承语法和virtual函数,但Lua提供了一种创建这些面向对象要素的能力。

Lua中的table就是一种对象。

直接上代码:
#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<<p means tagmethod(p) is not present */
  lu_byte lsizenode;  /* log2 of size of 'node' array */
  unsigned int sizearray;  /* size of 'array' array */
  TValue *array;  /* array part */
  Node *node;
  Node *lastfree;  /* any free position is before this position */
  struct Table *metatable;
  GCObject *gclist;
} Table;
t指向Table的指针
lua_TValue obj = {value_ = {gc=t},tt_ = LUA_TTABLE}
6、function
#define LUA_TLCL	(LUA_TFUNCTION | (0 << 4))  /* Lua closure */
#define LUA_TLCF	(LUA_TFUNCTION | (1 << 4))  /* light C function */
#define LUA_TCCL	(LUA_TFUNCTION | (2 << 4))  /* C closure */
#define ClosureHeader \
	CommonHeader; lu_byte nupvalues; GCObject *gclist
typedef struct CClosure {
  ClosureHeader;
  lua_CFunction f;
  TValue upvalue[1];  /* list of upvalues */
} CClosure;
typedef struct LClosure {
  ClosureHeader;
  struct Proto *p;
  UpVal *upvals[1];  /* list of upvalues */
} LClosure;
typedef union Closure {
  CClosure c;
  LClosure l;
} Closure;
lua closure
obj = {value_ = {gc=x},tt_ = LUA_TLCL}
light C function
typedef int (*lua_CFunction) (lua_State *L);
obj = {value_ = {f=x},tt_ = LUA_TLCF}
C closure
obj = {value_ = {gc=x},tt_ = LUA_TCCL}
7、user data
typedef struct Udata {
  CommonHeader;
  lu_byte ttuv_;  /* user value's tag */
  struct Table *metatable;
  size_t len;  /* number of bytes */
  union Value user_;  /* user value */
} Udata;
obj = {value_ = {gc=x},tt_ = LUA_TUSERDATA}
8、thread
x表示lua_State指针
obj = {value_ = {gc=x},tt_ = LUA_TTHREAD}

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