In GC languages a common approach is to have "finalizers" to make something like this a possible and sometimes convenient way of dealing with a foreign API, I wonder if what you saw was something similar? The idea is to allocate the foreign memory, then make a finalizer which is just a hook that will (eventually) call the foreign free only when some object (like a wrapper for the foreign memory) is collected. Something similar could be hidden behind some preprocessor macros, with guarantees only until the next OUR_MALLOC...

The problems for the GC languages tend to be fewer but if the wrapper is out of scope but someone grabbed and maintains a hold on the foreign memory directly, they're playing with fire as for when the GC will execute the finalizer hook and make that memory invalid. It's also a frustrating technique when foreign APIs -- particularly in certain graphics contexts -- require allocation threads to be the same as freeing threads, and of course depending on the implementation of free and the GC it might be an expensive operation to have a bunch of them suddenly happen at once when all you were expecting was a new native object and not a bunch of GC work behind the scenes.