Lua源码剖析之Nil类型与nil值的实现

乙振斐 posted @ 2012年9月07日 18:27 in Lua , 3558 阅读

首先，要搞明白Nil类型和nil值的实现原理，就要搞明白Lua里变量、值、类型三者之间的关系和实现方式，搞明白了以后，Nil不过是其中一种例子而已。

Lua is a dynamically typed language. This means that variables do not have types; only values do. There are no type definitions in the language. All values carry their own type.

All values in Lua are first-class values. This means that all values can be stored in variables, passed as arguments to other functions, and returned as results.

–Lua Reference Manual

也即，Lua里变量没有类型，值才有类型，并且每一个值都携带了他的类型信息。同时，Lua中的所有值都是第一类值，意味着所有值都可以被存储在变量中、作为参数传递给其他函数以及作为返回值。

落实到代码上，具体实现如下：

typedef struct lua_TValue TValue;

struct lua_TValue {
TValuefields;
};

#define TValuefields    Value value_; int tt_

“值”在实现中用TValue类型的变量来存储。把以上语句手工展开一下，如下：

struct TValue {
    Value value;
    int tt_
};

这里value是具体的值（或对具体值的引用），tt_是该变量的类型。也即实现了上文所说的“值才有类型，并且每一个值都携带了他的类型信息。”

通过追踪若干个宏以后，可以得知tt_中存储的是例如LUA_TNIL、LUA_TBOOLEAN 之类的值，为Lua中的8种类型，此外还有一个NUMTAGS，具体作用未知。

其实到这里已经知道Nil类型是如何实现的了，nil值如何实现还有待发掘。不过既然已经挖到了这一层，不妨继续把Value类型也深挖一下：

union Value {
GCObject *gc;    /* collectable objects */
void *p;         /* light userdata */
int b;           /* booleans */
lua_CFunction f; /* light C functions */
numfield         /* numbers */
};

为了节省空间，Value是定义成一个联合体的，由于可以被回收的对象，需要包含更多的信息，所以这里用一个*gc指针引用。其他的四种类型就用C基本类型定义了，其中值得注意的是numfield，他是数字类型的实现，以前提到过Lua里数字是用双精度浮点数实现的，如果需要修改的话应该就是在这里下手，numfield定义如下：

#define numfield    lua_Number n;    /* numbers */

typedef LUA_NUMBER lua_Number;

#define LUA_NUMBER  double

其中第一个宏定义的作用我可以理解，但我还不太明白为什么中间要加上第二个宏定义，也许要等以后通读代码后才能找到答案。

GCObject的定义这里就不分析了，等到以后研究垃圾回收机制的时候再深入研究。

扯远了，回到正题，Nil类型如何实现已经找到，应该就是通过给_tt赋值LUA_TNIL来实现，那么nil值如何实现呢，答案在table的getint函数（通过数字索引返回表项的函数）中：

/*
** search function for integers
*/

const TValue *luaH_getint (Table *t, int key) {
   /* (1 <= key && key <= t->sizearray) */
   if (cast(unsigned int, key-1) < cast(unsigned int, t->sizearray))
     return &t->array[key-1];
   else {
     lua_Number nk = cast_num(key);
     Node *n = hashnum(t, nk);
     do {  /* check whether `key' is somewhere in the chain */
       if (ttisnumber(gkey(n)) && luai_numeq(nvalue(gkey(n)), nk))
         return gval(n);  /* that's it */
       else n = gnext(n);
     } while (n);
     return luaO_nilobject;
   }
}

该函数的流程是：首先在table的array部分查找，若不在，则在hash表部分查找，若还不在，则返回luaO_nilobject对象，毫无疑问，这就是nil值。定义如下：

/* 
** (address of) a fixed nil value
*/
#define luaO_nilobject      (&luaO_nilobject_)

通过注释可以知道，在Lua中，所有nil值实际上都是同一个(fixed)对象——luaO_nilobject_

该对象定义如下：

LUAI_DDEF const TValue luaO_nilobject_ = {NILCONSTANT};

#define NILCONSTANT {NULL}, LUA_TNIL

展开则得：

LUAI_DDEF const TValue luaO_nilobject_ = {
    {NULL},
    LUA_TNIL
};

也即，一个value为{NULL}，tt_为LUA_TNIL的TValue，这也印证了之前关于Nil类型实现方式的猜测。

至此，Nil类型和nil值的实现方法都已经明晰：

Nil类型：tt_ = LUA_TNIL

nil值：为同一个Nil类型的对象luaO_nilobject_

table类型对不存在的表项进行索引时返回nil值的机制也已经明了：

array部分和hash表部分都不存在该索引时则返回&luaO_nilobject_

转载请保证文章完整性，并请注明来自yizhenfei.is-programmer.com

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Lua源码剖析之Nil类型与nil值的实现

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