Hidden Features

wiki

Difference (from prior author revision) (major diff, minor diff)

Removed: 3,32d2

=== newproxy function ===

newproxy is an unsupported and undocumented function in the Lua base library. From Lua code, the setmetatable function may only be used on objects of table type. The newproxy function circumvents that limitation by creating a zero-size userdata and setting either a new, empty metatable on it or using the metatable of another newproxy instance. We are then free to modify the metatable from Lua. This is the only way to create a proxy object from Lua which honors certain metamethods, such as __len. Synopsis:


 do
   local a = newproxy(true)  -- create proxy object with new metatable
   assert(type(a) == 'userdata')
   getmetatable(a).__len = function() return 5 end
   assert(#a == 5)
   local b = newproxy(a)     -- create proxy object with same metatable as another proxy
   assert(b ~= a)
   assert(getmetatable(b) == getmetatable(a))
   assert(#b == 5)
   local c = newproxy(false) -- create proxy object with no metatable
   assert(not getmetatable(c))
 
   local is_collected = false
   local o = newproxy(true)
   getmetatable(o).__gc = function() is_collected = true end  -- finalizer
   o = nil; collectgarbage()  -- clear all references to object and ensure finalizer called
   assert(is_collected)
 end

See also [1][2] .

=== The frontier pattern %f ===

See FrontierPattern concerning %f in patterns.

Changed: 52c22

=== package.config ===

=== Old items ===

Changed: 54c24,26

LuaList:2006-07/msg00089.html (will be documentedin 5.2)

* lua_pushliteral is now documented in 5.1.3: [1][3][4]

* The FrontierPattern was added to the reference manual in Lua 5.2. [2]

Changed: 56c28

=== Old items ===

* The undocumented newproxy function was removed in Lua 5.2, since it was made redundant by other features added in that version.

Changed: 58c30

lua_pushliteral is now documented in 5.1.3: [1][2]

* Documentation for package.config was added in Lua 5.2. [1]

The Lua C API and standard libraries may contain some undocumented, unsupported, and/or experimental features. They are likely unsupported for a good reason, and may disappear or change at any time--use them at your own risk.

Closing a standard file

As of 5.1.3, it is not permitted to close a standard file (e.g. stdin, stdout, and stderr) due to a bug this can cause[1][5]:

> assert(io.stdout:close())
stdin:1: cannot close standard file

Here's a hack around that, which should be used with caution:

local f = assert(io.open '/dev/null') -- or possibly NUL on Windows
debug.setfenv(io.stdout, debug.getfenv(f))
f:close()
assert(io.stdout:close()) -- ok

Old items

lua_pushliteral is now documented in 5.1.3: [2][6][7]

The FrontierPattern was added to the reference manual in Lua 5.2. [3]

The undocumented newproxy function was removed in Lua 5.2, since it was made redundant by other features added in that version.

Documentation for package.config was added in Lua 5.2. [4]