Re: Using obj:method syntax [was Re: changes in 'require']

[herviou <herviou@...>]

Russell Y. Webb wrote:

> On 15/07/2005, at 3:09 PM, Ive wrote:
>
> > In my .lua file I got the following:
> >
> > actor = CreateActor("ActorCubeName");
> > primitive = CreateCube("CubeName", 1, 1, 1);
> > ActorSetPrimitive(actor, primitive);
> > ...
> > now... I ll rather do something like this:
> >
> > actor = CreateActor("ActorCube");
> > primitive = CreateCube("CubeName", 1, 1, 1);
> > actor->SetPrimitive(primitive);
>
>
> Assuming actor is a table (if it it's then your need to use a table), 
> try this instead of your last line above:
>
> actor.SetPrimitive = ActorSetPrimitive
> actor:SetPrimative(primative)
>
> Otherwise,
> a = { }
> a.ptr = actor
> a.Set = function(a, prim) ActorSetPrimitive(a.ptr, prim) end
> a:Set(primative)
>
> Basically x:y(...) expands to x.y(x, ...)
>
> Hope that helps.  I may have missed the point since your actor->xyz 
> looks more like C++ than Lua.
>
> Russ

Hi,
I have done such jobs last week, exactly to do the same (wrapping C++ 
methods to lua and have an object style notation in Lua). I have choosen 
an other solution (more complicated than the one above) in which you can 
have inheritance (on the lua side) of your wrapped c++ class.
To do so you need (in c++) to create metatable for your wrapped c++ 
class (see http://www.lua.org/pil/16.html) in which you will store all 
the methods you need to wrapp (here you will have to use lua_newuserdata 
instead of lua_lightuserdata). You will also need to set 2 special field 
in this metatable to enable the object style notation and also the 
garbage collection of this share object when needed. Those fields are 
__index and __gc.
This solution is a little bit tricky (see the online documentation for 
help) but is not without creating problems. I explain: when you use 
lua_lightuserdata to push your object pointer this is easy to have the 
equality in lua (same pointer means equality) but when you use 
lua_newuserdata lua create a new memory block in which you will store 
your ptr (encapsulate your object ptr in a struct) and the next time you 
will share the same object ptr with the help of lua_newuserdata you will 
have another memory block (not the same)... so, the equality will not be 
possible easily... The solution is to create a metatable in which you 
store the table create with lua_newuserdata with a unique key that is 
your object pointer. So the next time you will need to share an object 
you just have to look in this metatable for the key and get the value 
(the table of your already shared object).

With all this hard work, you will be able to write in Lua without adding 
anything else:
actor=CreateActor:new("ActorCube")
primitive = Primitive:createCube("CubeName", 1, 1, 1);
actor:setPrimitive(primitive);

and you will also be able to look for methods of your actor (a c++ class 
wrapp!!) by doing:
mthd = getmetatable(actor)
for i in pairs(mthd) do
   io.write(i.."\n")
   if i=="setColor" then
       primitive:setColor(1,0,0)
   end
end
Groovy baby...

Have a look on the online documentation to learn more about the object 
oriented programming in lua, also have a look on the user wiki. But you 
have to know that it will take some times to build a system like the one 
above: you will also have to build a look up system for you c++ class 
and so on and so forth. Have a look on tolua++.

Hope that helps.

dave

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