Specifying a different userdata size for gc purposes?

[ddneilson <ddneilson@...>]

Hi there,
 I've a quick question. I'm currently building a userdata type that is 
implemented as a pointer to a C++ class. However, as far as the lua 
garbage collector is concerned the size of this userdata is 
sizeof(MyClass*) -- the size of a pointer.

 When the C++ class in question uses a lot of memory (think a class for 
an image, for instance) the amount of memory being used by the program 
and the amount of memory that lua thinks is outstanding to be gc'd can 
be extremely out of whack (lua will see the multi-megabyte image as only 
4 bytes, for instance). This wouldn't be a problem if you're only 
creating a couple of these userdata in a lua program, but if you're 
creating a lot of them (as local variables in a loop, for instance) then 
it doesn't take very long at all to accumulate hundreds of megabytes of 
uncollected garbage that lua thinks is only a couple hundred bytes.

 What I'm wondering is if there's any way of telling the lua garbage 
collector that this userdata is actually occupying more than 4 bytes? 
The idea being that then the gc will have a more accurate idea of just 
how much memory is being used for the purpose of deciding when to run a 
collection cycle. Right now, I get around this with a liberal sprinkling 
of collectgarbage("collect") calls throughout my lua code; but, that 
seems like it rather defeats the purpose of a garbage collected environment.

Thanks,
 Daniel