[DougBTX]

I’d say that “standard procedure” would be to do it the same way as it would be done in C: define a struct, allocate one somewhere, then pass a pointer to it as the data pointer. Using the anonymous struct which represents the closure itself seems like skipping a step, the user doesn’t need to spell out which values are stored in the struct.

[zozbot234]

If the language supports closures which capture variables from their surrounding environment, there's no way around using "the closure itself" as your data object. After all, "the user" is not expected to "know" what any given closure is capturing from the environment; part of the point of closures is implementing a sort of information hiding.