5 ms·

In my opinion, if you understand how memory management works, ARC is definitely superior to GC. There are things it's not very good at, typically things that us

In my opinion, if you understand how memory management works, ARC is definitely superior to GC. There are things it's not very good at, typically things that usual reference counting is not very good at either, such as handling cyclical data structures.

danbruc 12y ago
Resource management is a necessary evil but usually not a primary concern of most applications. Therefore in an ideal world it should be abstracted away and you should not have to care about it ever. In this respect I consider a full blown garbage collector as closer to the ideal than automatic reference counting. But automatic reference counting is of course also a kind of automatic garbage collection mechanism, just with a bit of a different focus. All solutions are not optimal, but unless you have really many allocations and dealloctions going on, have tight resource constraints or hard timing constraints, current garbage collectors are really good enough. Not that you can not shoot your feet by accidentally keeping large object graphs alive, but that is easy to detect, though sometimes hard to fix. Some developers seem to have some kind of resentments against garbage collectors because real developers of course manage their resources on their own, but many of their arguments are not well founded. Garbage collectors nondeterministically pausing program execution to free unused resources is a common point, but they usually don't realize that automatic reference counting has the same problem - you never know when a reference count drops to zero and triggers the deallocation of the object possibly with a large object graph attached.
- jahewson 12y ago
  Not true. A reference counted program is deterministic, it is known in advance for a given set of inputs exactly when deallocation will occur. Also, its not really fair to sweep "hard timing constraints" under the rug as a minority use case when these are essential for smooth modern GUIs. Sure, GC is pretty much ideal in terms of absolving developers of resource management responsibilities, but it comes with some significant downsides which can easily effect the final product and should be understated. Rust is a great example of a new language which avoids GC for this reason.
  - pjmlp 12y ago
    You cannot know how much the deallocation impact will be, if a big datastructure is getting its count to 0.
    - jahewson 12y ago
      Of course you can know that, because you know the type of every object. If you're deallocating a big data structure, then the impact will be big. Its easy to choose not to deallocate such objects at an inconvinient time by holding references to them. Compare this with GC where the collection can occur at any time after deallocation, e.g. during an animation.
      - danbruc 12y ago
        You can do almost the same with a garbage collector - keep references alive until a convenient point is reached, than kill those last references and manually force a collection. Not that it seems a good idea, but if you need a level of control as described by you, then you are already actively fighting against automatic reference counting and there seems no point to use any kind of automatic resource management in the first place, at least for the relevant objects.
        jahewson 12y ago
        It's not possible to do that in Java, though it would work in C#. Retaining references really is not "fighting against automatic reference counting", it's the opposite: the reference clearly and unambiguously defines the lifetime of the object, including deallocation. This is by design and ARC can still be used for any shared references to the object. Forcing GC on the other hand, would be fighting against it because you're circumventing its usual operation.
        Skinney 12y ago
        It wouldn't work reliably in C# either. Both platforms allow you to inform the GC that now would be a good time to collect, but in both systems the GC can ignore your advice. C# has structs which will try to allocate on the stack, but that is potentially a big (or impossible) refactoring on your program (unless you plan ahead).
- blub 12y ago
  Garbage collectors don't solve the problem of "resource" management, but of memory management. Languages like Java have mechanisms such as try with resources (not available in Android) or try finally blocks which seem error prone to me, since the programmer has to know that that particular resource should be released explicitly. They also make developers lazy, they get used to not thinking about object lifetimes and the costs of allocation. YMMV of course.
  - mike_hearn 12y ago
    Try with resources is available in Jellybean+

danbruc 12y ago

Resource management is a necessary evil but usually not a primary concern of most applications. Therefore in an ideal world it should be abstracted away and you should not have to care about it ever. In this respect I consider a full blown garbage collector as closer to the ideal than automatic reference counting. But automatic reference counting is of course also a kind of automatic garbage collection mechanism, just with a bit of a different focus. All solutions are not optimal, but unless you have really many allocations and dealloctions going on, have tight resource constraints or hard timing constraints, current garbage collectors are really good enough. Not that you can not shoot your feet by accidentally keeping large object graphs alive, but that is easy to detect, though sometimes hard to fix. Some developers seem to have some kind of resentments against garbage collectors because real developers of course manage their resources on their own, but many of their arguments are not well founded. Garbage collectors nondeterministically pausing program execution to free unused resources is a common point, but they usually don't realize that automatic reference counting has the same problem - you never know when a reference count drops to zero and triggers the deallocation of the object possibly with a large object graph attached.

jahewson 12y ago
Not true. A reference counted program is deterministic, it is known in advance for a given set of inputs exactly when deallocation will occur. Also, its not really fair to sweep "hard timing constraints" under the rug as a minority use case when these are essential for smooth modern GUIs. Sure, GC is pretty much ideal in terms of absolving developers of resource management responsibilities, but it comes with some significant downsides which can easily effect the final product and should be understated. Rust is a great example of a new language which avoids GC for this reason.
- pjmlp 12y ago
  You cannot know how much the deallocation impact will be, if a big datastructure is getting its count to 0.
  - jahewson 12y ago
    Of course you can know that, because you know the type of every object. If you're deallocating a big data structure, then the impact will be big. Its easy to choose not to deallocate such objects at an inconvinient time by holding references to them. Compare this with GC where the collection can occur at any time after deallocation, e.g. during an animation.
    - danbruc 12y ago
      You can do almost the same with a garbage collector - keep references alive until a convenient point is reached, than kill those last references and manually force a collection. Not that it seems a good idea, but if you need a level of control as described by you, then you are already actively fighting against automatic reference counting and there seems no point to use any kind of automatic resource management in the first place, at least for the relevant objects.
      - jahewson 12y ago
        It's not possible to do that in Java, though it would work in C#. Retaining references really is not "fighting against automatic reference counting", it's the opposite: the reference clearly and unambiguously defines the lifetime of the object, including deallocation. This is by design and ARC can still be used for any shared references to the object. Forcing GC on the other hand, would be fighting against it because you're circumventing its usual operation.
        Skinney 12y ago
        It wouldn't work reliably in C# either. Both platforms allow you to inform the GC that now would be a good time to collect, but in both systems the GC can ignore your advice. C# has structs which will try to allocate on the stack, but that is potentially a big (or impossible) refactoring on your program (unless you plan ahead).
blub 12y ago
Garbage collectors don't solve the problem of "resource" management, but of memory management. Languages like Java have mechanisms such as try with resources (not available in Android) or try finally blocks which seem error prone to me, since the programmer has to know that that particular resource should be released explicitly. They also make developers lazy, they get used to not thinking about object lifetimes and the costs of allocation. YMMV of course.
- mike_hearn 12y ago
  Try with resources is available in Jellybean+