用 C 扩展写 Lua closure
这个是《programming in lua》中的有的一个例子，通过 closure（闭包）来实现一个 tuple（元组）。
元组是一种具有匿名字段的常量记录。可以用一个数字索引来检索某个字段，或者一次性检索所有字段。
这里的实现将元组表示为函数，元组的值存放在函数绑定的 upvalue 中。大概的 Lua 代码示意如下：
local tuple = {}
function tuple.new(...)
local t = {...}
return function(index)
if index then
if type(index) ~= "number" then error("invalid index type") end
if index < 0 then error("index is out of range") end
return t[index]
else
return unpack(t)
end
end
end
闭包工厂 tuple.new 中的 table t 就是用于存放元组内容的 upvalue，这个 tuple 的使用如下：
local x = tuple.new(10, "hi", {}, 4)
print(x(1))
print(x(2))
print(x())
当用一个数字参数来调用此函数时，它会返回指定的字段。当不用参数来调用此函数时，则返回所有的字段。输出为：
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
static int
t_tuple(lua_State *L)
{
int op = luaL_optint(L, 1, 0);
if (op == 0) { /* 无参数？ */
int i;
/* 将所有合法 upvalue 压入栈中 */
for (i = 1; !lua_isnone(L, lua_upvalueindex(i)); i++)
lua_pushvalue(L, lua_upvalueindex(i));
return i - 1; /* 栈中值的数量 */
}
else { /* 获取 'op' 字段 */
luaL_argcheck(L, 0 < op, 1, "index out of range");
if (lua_isnone(L, lua_upvalueindex(op))) { /* 无此字段 */
lua_pushnil(L); /* 返回 nil 表明不存在 */
return 1;
}
lua_pushvalue(L, lua_upvalueindex(op));
return 1;
}
}
static int
t_new(lua_State *L)
{
lua_pushcclosure(L, t_tuple, lua_gettop(L));
return 1;
}
static const struct luaL_Reg tuplelib[] = {
{"new", t_new},
{NULL, NULL}
};
int luaopen_tuple(lua_State *L)
{
luaL_register(L, "tuple", tuplelib);
return 1;
}
编译成“tuple.dll”(windows 下)然后仍然用上面的 tuple 使用代码测试，不过要先“require 'tuple'”来加载这个扩展。DLL 中的需要导出 lutopen_tuple 这个函数符号。
t_new 是创建元组的工厂函数。将需要作为 upvalue 的参数实现压入栈中，然后调用 lua_pushcclosure 来创建一个基于 t_tuple 的 closure 。lua_gettop 返回栈的大小，在这里也就等于已经压入栈中作为 upvalue 的数量。lua_upvalueindex 是一个用来索引 upvalue 的宏，当索引一个不存在的 upvalue ，结果会是一个类型为 LUA_TNONE 的伪值(pseudo-value)，当索引超出当前栈范围时候，也会得到这样的 LUA_TNONE 的伪值。不能以负数来调用 lua_upvalueindex ，所以做了范围检查，luaL_argcheck(L, 0 < op, 1, "index out of range") 第二个参数是个条件表达式，第三个是检查的参数数量，最后一个是检查失败传递的消息。
用于检测可选参数的函数是 luaL_optint ，它允许参数不存在，如果不存在，则返回一个指定的默认值，也就是最后一个参数。
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