meaning of getpeer & setpeer

Question

In cocos2dx Sample, there's code like this:

function UIButtonTest.extend(target)
local t = tolua.getpeer(target)
if not t then
    t = {}
    tolua.setpeer(target, t)
end
setmetatable(t, UIButtonTest)
return target
end

What is setpper & getpeer for?

Answer 1

Those are tolua functions. The tolua manual (for example here) has explanations for them.

tolua.setpeer (object, peer_table) (lua 5.1 only)

Sets the table as the object's peer table (can be nil). The peer table is where all the custom lua fields for the object are stored. When compiled with lua 5.1, tolua++ stores the peer as the object's environment table, and uses uses lua_gettable/settable (instead of lua_rawget/set for lua 5.0) to retrieve and store fields on it. This allows us to implement our own object system on our table (using metatables), and use it as a way to inherit from the userdata object. Consider an alternative to the previous example:

-- a 'LuaWidget' class
LuaWidget = {}
LuaWidget.__index = LuaWidget

function LuaWidget:add_button(caption)
    -- add a button to our widget here. 'self' will be the userdata Widget
end

local w = Widget()
local t = {}
setmetatable(t, LuaWidget) -- make 't' an instance of LuaWidget

tolua.setpeer(w, t) -- make 't' the peer table of 'w'

set_parent(w) -- we use 'w' as the object now

w:show() -- a method from 'Widget'
w:add_button("Quit") -- a method from LuaWidget (but we still use 'w' to call it)

When indexing our object, the peer table (if present) will be consulted first, so we don't need to implement our own __index metamethod to call the C++ functions.

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tolua.getpeer (object) (lua 5.1 only)

Retrieves the peer table from the object (can be nil).