lua中metatable源码分析总结

云风的lua源码赏析对元表部分一带而过，估计是觉得太low。幸好还有 hoterran网友在做分析，以下很多文字参考他的。

先看一个例子：

person = {}
person.mp = 1

print(person.hp)

但是如果添加元表就不一样了：

person = {}
person.mp = 1

base = {}
base.hp = 10
setmetatable(base, {__index = function()
	return ("hey guy u call a unexisted table member!!")
	end})

setmetatable(person, {__index = base})

print(person.hp)
print(person.wp)

>lua -e "io.stdout:setvbuf ‘no‘" "tt.lua" 
10
hey guy u call a unexisted table member!!
>Exit code: 0

这里将base表作为person的元表，如果在person里找不到的key就会向元表里找，hp就找到了，wp找不到就打印那段提示啦，哈哈。

那lua内部是如何实现设置元表并如何在table.key/fun时查询相关元表呢？

1、设置元表

static int luaB_setmetatable (lua_State *L) {
  int t = lua_type(L, 2);
  luaL_checktype(L, 1, LUA_TTABLE);
  luaL_argcheck(L, t == LUA_TNIL || t == LUA_TTABLE, 2,
                    "nil or table expected");
  if (luaL_getmetafield(L, 1, "__metatable"))
    return luaL_error(L, "cannot change a protected metatable");
  lua_settop(L, 2);
  lua_setmetatable(L, 1);
  return 1;
}

就table来说，它有metatable字段，调用此函数

2、如何如果设置了__index，那是调用元表？

在

void luaV_execute (lua_State *L) 里会解析到opcode进行事件分发：

  for (;;) {
    Instruction i = *(ci->u.l.savedpc++);
    StkId ra;
    if ((L->hookmask & (LUA_MASKLINE | LUA_MASKCOUNT)) &&
        (--L->hookcount == 0 || L->hookmask & LUA_MASKLINE)) {
      Protect(traceexec(L));
    }
    /* WARNING: several calls may realloc the stack and invalidate `ra‘ */
    ra = RA(i);
    lua_assert(base == ci->u.l.base);
    lua_assert(base <= L->top && L->top < L->stack + L->stacksize);
    vmdispatch (GET_OPCODE(i)) {
     vmcase(OP_GETTABLE,
        Protect(luaV_gettable(L, RB(i), RKC(i), ra));
      )

void luaV_gettable (lua_State *L, const TValue *t, TValue *key, StkId val) {
  int loop;
  for (loop = 0; loop < MAXTAGLOOP; loop++) {
    const TValue *tm;
    if (ttistable(t)) {  /* `t‘ is a table? */
      Table *h = hvalue(t);
      const TValue *res = luaH_get(h, key); /* do a primitive get */
      if (!ttisnil(res) ||  /* result is not nil? */
          (tm = fasttm(L, h->metatable, TM_INDEX)) == NULL) { /* or no TM? */
        setobj2s(L, val, res);
        return;
      }
      /* else will try the tag method */
    }
    else if (ttisnil(tm = luaT_gettmbyobj(L, t, TM_INDEX)))
      luaG_typeerror(L, t, "index");
    if (ttisfunction(tm)) {
      callTM(L, tm, t, key, val, 1);
      return;
    }
    t = tm;  /* else repeat with ‘tm‘ */
  }
  luaG_runerror(L, "loop in gettable");
}

/*
** function to be used with macro "fasttm": optimized for absence of
** tag methods
*/
const TValue *luaT_gettm (Table *events, TMS event, TString *ename) {
  const TValue *tm = luaH_getstr(events, ename);
  lua_assert(event <= TM_EQ);
  if (ttisnil(tm)) {  /* no tag method? */
    events->flags |= cast_byte(1u<<event);  /* cache this fact */
    return NULL;
  }
  else return tm;
}

这样看下来，文章最前面写的lua例子就容易理解了。

lua中metatable源码分析总结,布布扣,bubuko.com

lua中metatable源码分析总结

原文：http://blog.csdn.net/boyxiaolong/article/details/24144407