Re: uservalue from Lua

[Gaspard Bucher <gaspard@...>]

I know my use case very easy to grok but I think it is a pretty common pattern (Lua sub-classes of C++ classes) and it would really be nice to have some feedback before I start updating lots of C++ and Lua code..

;-)

Gaspard

On Wed, Jun 29, 2011 at 10:55 PM, Gaspard Bucher <gaspard@teti.ch> wrote:

I am considering some changes to the way I handle "self" with regards to userdata. Currently, whenever I need "self" (a Lua table) and a user data, I fake some inheritance mechanism and I use upvalues to call original functions. From the other side, every callback (like "paint") is a lua thread stored in the C++ object.

-- == create a new instance

function SpecialWidget()

  local self = {

    super = mimas.Widget() -- userdata

  }

  -- wrap calls from C

  function super.paint(p, w, h)

    self:paint(p, w, h)

  end

  return self

end

--- === etc, you get the idea

I have a new idea that would use the "uservalue" from Lua 5.2

-- ============ Create new class (code with colors: https://gist.github.com/1054915)

SpecialEdit = {}

setmetatable(SpecialEdit, mimas.LineEdit_mt)

-- calling SpecialEdit() should do the following things

-- 1. create userdata

-- 2. set new table as uservale                 ---> self

-- 3. create new lua thread and install "self" on the stack

-- 4. set access to userdata from self          ---> self.super = userdata

-- 5. set "SpecialEdit" as metatable for "self" ---> setmetatable(self, SpecialEdit)

-- 6. return "self"

-- This is a callback called from the C side. The call goes as follows

-- 1. the object's lua_thread's stack contains "self", ready to be used.

-- 2. call self.textChanged(self, text)

function SpecialEdit:textChanged(text)

  -- just to show that self is accessed by the callback with custom values ("owner").

  self.owner:doSomething()

  -- call a native method

  -- "hide" is found in mimas.LineEdit_mt       ---> LineEdit_mt.hide(self)

  -- the userdata is found in self              ---> LineEdit *c_obj = self.super    // C pseudo-code

  -- call the class method                      ---> c_obj->hide()

  self:hide(nil)

end

In order to handle the callbacks, I would create a single lua thread (which could keep "self" in it's stack).

What do you think ?

Gaspard

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                                                               Gaspard