The thing I don't understand is why anyone would pass a pointer to a GC'ed object into a 3rd party library (that's in a different language) and expect the GC to track the pointer there?
Passing memory into code that uses a different memory manager is always a case where automatic memory management shouldn't be used. IE, when I'm using a 3rd party library in a different language, I don't expect it to know enough about my language's memory model to be able to effectively clean up pointers that I pass to it.
You can pass a pointer to a foreign library, but this requires temporarily making the pointee object a GC root because that library code is essentially sharing ownership of it with the GC.
> The thing I don't understand is why anyone would pass a pointer to a GC'ed object into a 3rd party library
The promise of GC is to free the programmer from the burden of memory management. If I can't give (perhaps fractional) ownership of a data structure to a library and expect its memory to be reclaimed at the appropriate time, have I freed myself from the burden of memory management?
Unless you are using malloc; and/or you don't need to do anything when the pointer is freed, (the pointer doesn't reference anything else that needs to be freed or released,) there's no way that the library written outside of your runtime knows how to free your memory.
Or to put it in a different way: Passing pointers to a native library is a small amount of what your application does and you still benefit from the garbage collector when you are running inside of your own language.
Passing memory into code that uses a different memory manager is always a case where automatic memory management shouldn't be used. IE, when I'm using a 3rd party library in a different language, I don't expect it to know enough about my language's memory model to be able to effectively clean up pointers that I pass to it.