Finalization and external pointers
On Mon, 3 May 2004, Duncan Murdoch wrote:
I'm adding things to the Windows RGui so that there's more control of the interface from within R. One thing I'm considering is giving access to the Graphapp window objects using external pointers. This raises the issue of finalization on both sides: - If someone creates a pointer referring to a window, then that pointer should be changed to NULL when the window is closed. - If garbage collection destroys a pointer referring to a window, then the window should know not to change that pointer to NULL later. Are there other examples like this I can look at? I'd like to follow existing conventions rather than invent my own. And a related question: is there a writeup anywhere on the R_RegisterFinalizerEx function? What does the onexit argument do?
I added some notes I have on weak references and finalization at http://www.stat.uiowa.edu/~luke/R/weakfinex.html The notes are a bit old but I think still apply, such as they are. The onexit argument sets a flag; when R exits normally it will attempt to run the finalizers of all references with this flag set. The simple examples in these notes may be of use, or not. The Haskell reference may also be worth a look as one writeup of the issues. Making sure you have the right match of object lifetimes with object identities is probably the trickiest issue--do you need to make sure two ways of asking for an R reference to the same physical window return the same R pointer object? Probably you do; that will affect the design in that the pointer object you use has to exist as long as the physical window does. Also keep in mind that these things could get saved in a workspace (they will be restored with NULL pointers). Best, luke
Luke Tierney University of Iowa Phone: 319-335-3386 Department of Statistics and Fax: 319-335-3017 Actuarial Science 241 Schaeffer Hall email: luke@stat.uiowa.edu Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu