Re: SWIG/Lua/C++ object instance

[Joey Mukherjee <joey@...>]

Sorry for my late reply, but I can not quite get this to work.  I am  
missing something fundamental, but what?

I have two C++ objects I want to read/write from Lua.  One is a  
collection of the other.  I want to be able to create/access objects  
of the subsidiary type, but (currently) only want to access the  
overall collection type (i.e. not create them).

For example:

class BoxPlots {

public:
    void AddPlot (Plot *plot);
    void AddPlots (const std::vector <Plot *> &plots);
    Plot *GetPlot (const unsigned int index) const;
    const unsigned int GetNumberOfPlots () const;
};

class Plot {
...
};

The BoxPlots is a collection of Plot's.   I want to create new Plot's  
and access existing Plot's.  I want to access the existing BoxPlots,  
but I don't necessarily want to create them from Lua.  The C++  
variable that will be "bound" to Lua is of type BoxPlots and from  
then using the GetPlot method, return a Plot to Lua or creating new  
ones etc...

I feel like I need to add something to my .i file to be able to  
create the function like you mention below.  I tried somewhat merging  
your advice and Andrew Zhilin's; however, I was unable to figure how  
to access all my types from Lua.  Maybe some example Lua code would  
help?

I'm not sure if I've explained myself well enough, but I hope you get  
the idea.  I did initially take Roger's advice and made my own  
binding which did work, but I'd rather automate this process since I  
hope to do more in the future.

Thanks to you all!

Joey

On May 6, 2007, at 9:26 PM, mark gossage wrote:

> Hello Joey,
>
> I do almost all my SWIG-LUA work from the lua side. Swig is really  
> good for just wrappering up a C/C++ library to get it readable by  
> lua. Getting the C++ to talk to lua is fairly easy, but not well  
> documented.
>
> You idea of lua_pushlightuserdata(), was close, but not there.
> You probably want something like this:
>
> Foo* p= new Foo();
> SWIG_NewPointerObj(L,p,SWIGTYPE_p_Foo,1);
> lua_setglobal (L, "p");
>
> The SWIG_NewPointerObj() creates a userdata (not a lightuserdata)  
> for the foo object & pushes it to the stack. The last param (in  
> this case 1) is whether you want lua to manage the memory (0 for  
> no, 1 for yes).
>
> The SWIG_NewPointerObj() and SWIGTYPE_p_Foo are both found in the  
> wrapping file.
>
> Once you have that you should be able to do in lua:
> print(p)
> print(swig_type(p))
> p:some_function()
>
> Let me know if you have any other questions.
> Regards,
> Mark
>
> > I create an instance of my class variable in C++ and want to use Lua
> > to use some of the routines in my C++ class so I used SWIG to
> > generate the bindings for Lua and I think I have that right.
> >
> > However, I can not figure out how to make Lua use my C++ variable
> > instance instead of creating a new one.  I was looking at the Wiki
> > and I am very close to the CppObjectBinding method, except I am using
> > SWIG.
> >
> > I have a :
> >
> >      lua_pushlightuserdata (global_Lua_state, this);
> >      lua_setglobal (global_Lua_state, "this");
> >
> > to put the "this" pointer on the stack.  I want to interface that
> >
> > with Lua:
> >
> >
> > print ("In the Lua script")
> > b = plot.Plot(this)  -- wrong!  How do I make b = this, but be of a
> > plot.Plot () type.
> > print (this)
> > print (b)
> > print (b:DumpV (0))
> > print (this:DumpV(0))
> >
> > Two addresses are printed out, but they are different (which I
> > expect).  I want to do this:DumpV (0), but I get a:
> >
> > In the Lua script
> > userdata: bc4dd8
> > userdata: b53330
> > dump.lua:7: attempt to index global 'this' (a userdata value)
> >
> > I feel I am missing something simple, but I just can't get the next
> > step.  I feel like if I go to the direct way of binding like in the
> > wiki CppObjectBinding, I can get this to work, but then I have to do
> > all the bindings myself so I would like to stick with SWIG.
> >
> > Thanks for any ideas!
> >
> > Joey