Programming in Lua, 2Nd Edition - Chapter 14: The Environment

 

 

 

Chapter 14: The Environment

 

Lua 把所有全局变量都存在一个普通的表environment 里面。（更准确的说，是多个环境）这样做的好外是简化了lua 的内部实现。environment 本身又存在_G 里面。

 

打印当前已定义的所有全局变量

 

for n in pairs(_G) do print(n) end

 

 

14.1 Global Variables with Dynamic Names

 

function getfield (f)

       local v = _G -- start with the table of globals

       for w in string.gmatch(f, "[%w_]+") do

              v = v[w]

       end

       return v

end

 

function setfield (f, v)

       local t = _G -- start with the table of globals

       for w, d in string.gmatch(f, "([%w_]+)(.?)") do

              if d == "." then -- not last field?

                     t[w] = t[w] or {} -- create table if absent

                     t = t[w] -- get the table

              else -- last field

                     t[w] = v -- do the assignment

              end

       end

end

 

setfield("t.x.y", 10)   -- creates a global table t, another table t.x, and assigns 10 to t.x.y

print(t.x.y) --> 10

print(getfield("t.x.y")) --> 10

 

 

14.2 Global-Variable Declarations

 

全局变量无需声明。虽然这对小程序提供了便利，但大程序中易引起难以找出的错误。可以改变这一行为，如果我们愿意。因为Lua 将全局变量存在一个普通的表里面，当访问全局变量的时侯，我们可以使用元表改变它的行为。

 

检测以不存在的key访问全局表

 

setmetatable(_G, {

       __newindex = function (_, n)

              error("attempt to write to undeclared variable " .. n, 2)

         end,

       __index = function (_, n)

              error("attempt to read undeclared variable " .. n, 2)

         end,

})

 

print(a) -- stdin:1: attempt to read undeclared variable a

 

但是，我们如何声明新变量？一种方法是使用rawset ，它会忽略元方法：

 

不允许未经声明就使用全局变量

 

function declare (name, initval)

       rawset(_G, name, initval or false)

end

 

setmetatable(_G, {

       __newindex = function (_, n)

              error("attempt to write to undeclared variable " .. n, 2)

         end,

       __index = function (_, n)

              error("attempt to read undeclared variable " .. n, 2)

         end,

})

 

a=1  -- stdin:1: attempt to read undeclared variable a

declare "a"

a=1  -- OK

print(a)

 

 

只允许在main chunk 中定义全局变量

 

function declare (name, initval)

       rawset(_G, name, initval or false)

end

 

setmetatable(_G, {

       __newindex = function (t, n, v)

              local w = debug.getinfo(2, "S").what

              if w ~= "main" and w ~= "C" then

                     error("attempt to write to undeclared variable " .. n, 2)

              end

              rawset(t, n, v)

         end,

       __index = function (_, n)

              error("attempt to read undeclared variable " .. n, 2)

         end,

})

 

a=1 -- OK

print(a)

 

检测一个变量是否存在，我们不能简单的与nil 比较，而是使用rawget：

 

if rawget(_G, var) == nil then

-- 'var' is undeclared

...

end

 

 

14.3 Non-Global Environments

 

Lua 允许每个函数有自已的环境。使用setfenv (set function environment) 改变函数环境。

 

a = 1 -- create a global variable

-- change current environment to a new empty table

setfenv(1, {})

print(a)

-------------------------------------------

a = 1 -- create a global variable

setfenv(1, {g = _G}) -- change current environment

g.print(a) --> nil

g.print(g.a) --> 1

--------------------

a = 1

local newgt = {} -- create new environment

setmetatable(newgt, {__index = _G})

setfenv(1, newgt) -- set it

print(a) --> 1

a = 10

print(a) --> 10

print(_G.a) --> 1

_G.a = 20

print(_G.a) --> 20

-------------------------

function factory ()

       return function ()

              return a -- "global" a

       end

end

 

a = 3

f1 = factory()

f2 = factory()

print(f1()) --> 3

print(f2()) --> 3

setfenv(f1, {a = 10})

print(f1()) --> 10

print(f2()) --> 3