Re: question: userdata cross references ?

[Romulo <romuloab@...>]

On 12/18/09, Francesco Abbate <gslshell@gmail.com> wrote:
> Hi all,
>
> I have a small question that should not make people angry :-)
>
> Recently I'm writing a lot of library C code for Lua and I've got the
> need of something tha Lua seems to lack: userdata cross-references.
>
> I will try to explain my idea. Let suppose that an userdata of type A
> allocate internally some resources that will be freed only when the
> object will be garbage-collected with the __gc metamethods. Let use
> suppose that I want to create another type of userdata, let's say of
> type B, that make reference to some resources allocated by an object
> of the type A. This scenario pose a problem because is the object of
> type A is garbage-collected when the object of type B is still alive
> it will raise some serious errors.
>
> For me this features will be very useful to create lightweight object
> that points to the resources owned by other objects without the needs
> of creating unnecessary copies of the data.
>
> As far as I know there is no obvious solution to this problem but I
> would like to check with more expert people here.
>
> One solution could be to create a Lua table that embed the object of
> type B and keep a reference to the object of type A so that this
> latter will not be freed. This solution is functional but cumbersome
> because force you to create a Lua table to wrap your object and to
> attach a metatable to the table instead of to your userdata itself.
>
> I was thinking that it would be nice if the userdata could store one
> or more reference to other userdata objects in order to ensure that
> the GC does not free the main object that own the resource if other
> objects that need the same resource are still alive. In my idea the C
> API could be something like:
>
> /* let us suppose that in stack position 'index' we have a userdata of type
> A */
> struct b *bobj = lua_newuserdata(sizeof(struct b));
> lua_adduserdataref (L, index); /* add to userdata on top of the stack
> a reference to userdata at 'index' */
> /* and keep the stack unchanged */
>
> This kind of feature could be implemented by adding an extra space of
> dimension to keep a pointer at the end of the userdata. If there are
> no references this pointer will be null or otherwise it will point to
> a linked list of references to Lua objects (userdata). Then the GC
> should check the userdata cross-references just the same as it checks
> for cross-references between ordinary Lua tables.
>
> Does this make sense ? Is there a more simple/obvious solution to my problem
> ?
>
> In my experience the kind of feature I propose would be a big plus to
> simplify my library code by eliminating some boiler-plate solutions I
> was forced to adopt.
>
> Francesco
>

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