A friend and I were discussing C (our language of choice for embedded programming) and the things we like about it and somehow the subject of closures came up. I started thinking about what it would take to implement closures in C. Would you do it in the compiler as a language extension or just build them up by hand using in C itself in a cross-platform way?
And then it struck me: I’ve already been using, in straight, boring old C, the concept of closures. And I’ve been using them practically forever. I would say almost since I first learned C on my Macintosh back in high school.
The original Mac API (which still mostly lives on in Mac OS X’s “carbon” API) has an element in almost every major structure called “refCon”. The Apple documentation sorted of glazed over them (or maybe I glazed over when reading about them) and so I puzzled over these for a long time until I realized that they were pieces of memory that the Mac system would never look at or change. They are for the application writer to use as they see fit.
Apple also uses them when you register a function that the system will call at some later time. Here’s an example I grabbed from the current carbon window manager documentation:
OSStatus InstallWindowContentPaintProc(WindowRef window, WindowPaintUPP paintProc, WindowPaintProcOptions options, void *refCon);
What might you use one for? Well typically you’d create a structure with whatever local data needs to be associated with the call back function and put the pointer to that structure into the refCon. The callback function (since you’re writing it) knows to cast the refCon to the appropriate structure pointer so that it can get access to the fields of the structure.
This is by no means some great thing that Apple discovered. Almost every C API that has callbacks uses the same sort of concept. Here’s a function prototype from glib (one of the gnome base libraries):
guint g_io_add_watch(GIOChannel *channel, GIOCondition condition, GIOFunc func, gpointer user_data);
Here “user_data” serves the same purpose as “refCon” did in the first example. The function you pass in “func” will get “user_data” passed to it (unmolested) by glib.
I’ve used this in almost every API I’ve ever written that has callbacks. Here’s another example from an embedded project I’m working on:
void add_timer(int (*timer)(), void *cookie, int timeout_ms);
I always name my refCons “cookie”, for some reason. Arguably “user_data” is a more descriptive name, but I sort of like the arbitrariness of “cookie”. Or maybe it just makes me think of cookies… MMMM… Cooookies…
It turns out that all those “cookie”, “user_data”, and “refCon” pointers that I had been using since I first learned C transform the user supplied function pointers into “closures”. They do it explicitly and without any syntactic sugar, but they are 100% the same conceptually. In effect you allocate some memory, put references to some of your local variables into the structure and then pass the structure along with the function pointer to be called back later. The callback can use the structure all it wants which ends up being equivalent to a real closure having direct access to those same local variables.
So… Closures aren’t anything new. C has had closures forever. Yes, you have to manually allocate and deallocate the memory for your variables without syntactic sugar… But, hey, that’s C!