Why are weak pointers useful?

问题:

I've been reading up on garbage collection looking for features to include in my programming language and I came across "weak pointers". From here:

Weak pointers are like pointers, except that references from weak pointers do not prevent garbage collection, and weak pointers must have their validity checked before they are used.

Weak pointers interact with the garbage collector because the memory to which they refer may in fact still be valid, but containing a different object than it did when the weak pointer was created. Thus, whenever a garbage collector recycles memory, it must check to see if there are any weak pointers referring to it, and mark them as invalid (this need not be implemented in such a naive way).

I've never heard of weak pointers before. I would like to support many features in my language, but in this case I cannot for the life of me think of a case where this would be useful. For what would one use weak pointer?

回答1:

A typical use case is storage of additional object attributes. Suppose you have a class with a fixed set of members, and, from the outside, you want to add more members. So you create a dictionary object -> attributes, where the keys are weak references. Then, the dictionary doesn't prevent the keys from being garbage collected; removal of the object should also trigger removal of the values in the WeakKeyDictionary (e.g. by means of a callback).

回答2:

A really big one is caching. Let's think through how a cache would work:

The idea behind a cache is to store objects in memory until memory pressure becomes so great that some of the objects need to be pushed out (or are explicitly invalidated of course). So your cache repository object must hold on to these objects somehow. By holding onto them via weak reference, when the garbage collector goes looking for things to consume because memory is low, the items referred to only by weak reference will appear as candidates for garbage collection. Items in the cache that are currently being used by other code will have hard references still active, so those items will be protected from garbage collection.

In most situations you won't be rolling your own caching mechanism, but it is common to use a cache. Let's suppose you want to have a property which refers to an object in cache, and that property stays in scope for a long time. You would prefer to fetch the object from cache, but if it's not available, you can get it from persisted storage. You also don't want to force that particular object to stay in memory if pressure gets too high. So you can use a weak reference to that object, which will allow you to fetch it if it is available but also allow it to fall out of cache.

回答3:

If your language's garbage collector is incapable of collecting circular data structures, then you can use weak references to enable it to do so. Normally, if you have two objects which have references to each other, but no other outside object has a reference to those two, they would be candidates for garbage collection. But, a naïve garbage collector wouldn't collect them, since they contain references to each other.

To fix this, you make it so one object has a strong reference to the second, but the second has a weak reference to the first. Then, when the last outside reference to the first object goes away, the first object becomes a candidate for garbage collection, followed shortly thereafter by the second, since now its only reference is weak.

回答4:

Another example... not quite caching, but similar: Suppose an I/O library provides an object which wraps a file descriptor and permits access to the file. When the object is collected, the file descriptor is closed. It is desired to be able to list all currently opened files. If you use strong pointers for this list, then files are never closed.

回答5:

Use them when you wanted to keep a cached list of objects but not prevent those objects from getting garbage collected if the "real" owner of the object is done with it.

A web browser might have a history object that keeps references to image objects that the browser loaded elsewhere and saved in the history/disk cache. The web browser might expire one of those images (user cleared the cache, the cache timeout elapsed, etc) but the page would still have the reference/pointer. If the page used a weak reference/pointer the object would go away as expected and the memory would be garbage collected.

回答6:

One important reason for having weak references is to deal with the possibility that an object may serve as a pipeline to connect a source of information or events to one or more listeners. If there aren't any listeners, there's no reason to keep sending information to the pipeline.

Consider, for example, an enumerable collection which allows updates during enumeration. The collection may need notify any active enumerators that it has been changed, so those enumerators can adjust themselves accordingly. If some enumerators get abandoned by their creators, but the collection holds strong references to them, those enumerators will continue to exist (and process update notifications) as long as the collection exists. If the collection itself will exist for the lifetime of the application, those enumerators will effectively become a permanent memory leak.

If the collection holds weak references to the enumerators, this problem can be largely solved. If an enumerator is abandoned, it will be eligible for garbage collection, even though the collection still holds a weak reference to it. The next time the collection is changed, it can look through its list of weak references, send updates to the ones that are still valid, and remove from its list the ones that are not.

It would be possible to achieve many of the effects of weak references using finalizers along with some extra objects, and it's possible to make such implementations more efficient than those using weak references, but there are many pitfalls and it's hard to avoid bugs. It's much easier to make a correct approach using WeakReference. The approach may not be optimally efficient, but it won't fail badly.

回答7:

Weak Pointers keep whatever holds them from becoming a form of "life support" for the object the pointer points to.

Say you had a Viewport class, and 2 UI classes, and a buch of Widget classes. You want your UI to control the lifespan of the Widgets it creates, so your UI keeps SharedPtrs to all the Widgets it controls. For as long as your UI object is alive, none of the Widgets it refrences will be garbage collected (thanks to SharedPtr).

However, the Viewport is your class that actually does the drawing, so your UI needs to pass the Viewport a pointer to the Widgets so that it can draw them. For whatever reason, you want to change your active UI class to the other one. Lets consider two scenarios, one where the UI passed the Viewport WeakPtrs and one where it passed SharedPtrs (pointing to the Widgets).

If you had passed the Viewport all the Widgets as WeakPointers, as soon as the UI class was deleted there would be no more SharedPointers to the Widgets, so they would be garbage collected, the Viewport's references to the objects wouldn't keep them on "life support", which is exactly what you want because you aren't even using that UI anymore, much less the Widgets it created.

Now, consider you had passed the Viewport a SharedPointer, you delete the UI, and the Widgets are NOT garbage collected! Why? because the Viewport, which is still alive has an array (vector or list, whatever) full of SharedPtrs to the Widgets. The Viewport has in effect became a form of "life support" for them, even though you had deleted the UI that was controlling the widgets for another UI object.

Generally, a language/system/framework will garbage collect anything unless there is a "strong" reference to it somewhere in memory. Imagine if everything had a strong reference to everything, nothing would ever get garbage collected! Sometimes you want that behavior sometimes you don't. If you use a WeakPtr, and there are no Shared/StrongPtrs left pointing at the object (only WeakPtrs), then the objects will be garbage collected despite the WeakPtr references, and the WeakPtrs (should be) set to NULL (or deleted, or something).

Again, when you use a WeakPtr you're basically allowing the object you're giving it too to be able to access the data, but the WeakPtr won't prevent garbage collection of the object it points to like a SharedPtr would. When you think SharedPtr, think "life support", WeakPtr, NO "life support." Garbage collection won't (generally) occur until the object has zero life support.

回答8:

Weak references can for example be used in caching scenarios - you can access data through weak references, but if you don't access the data for a long time or there is high memory pressure, the GC can free it.

回答9:

The reason for garbage collection at all is that in a language like C where memory management is totally under explicit control of the programmer, when object ownership is passed around, especially between threads or, even harder, between processes sharing memory, avoiding memory leaks and dangling pointers can become very hard. If that weren't hard enough, you also have to deal with the need to have access to more objects than will fit in memory at one time—you need to have a way to have free up some objects for a while so that other objects can be in memory.

So, some languages (e.g., Perl, Lisp, Java) provide a mechanism where you can just stop "using" an object and the garbage collector will eventually discover this and free up memory used for the object. It does this correctly without the programmer worrying about all the ways they can get it wrong (albeit there are lots of ways programmers can screw this up).

If you conceptually multiply the number of times you access an object by the time that it takes to compute the value of an object, and possibly multiply again by the cost of not having the object readily available or by the size of an object since keeping a large object around in memory can prevent keeping several smaller objects around, you could classify objects into three categories.

Some objects are so important that you want to explicitly manage their existence—they will not be managed by the garbage collector or they must never be collected until explicitly freed. Some objects are cheap to compute, are small, are not accessed frequently or have similar characteristics that allow them to be garbage collected at any time.

The third class, objects which are expensive to be recomputed but could be recomputed, are accessed somewhat frequently (perhaps for a short burst of time), are of large size, and so on are a third class. You'd like to keep them in memory as long as possible because they might be reused again, but you don't want to run out of memory needed for critical objects. These are candidates for weak references.

You want these objects kept around as long as possible if they aren't conflicting with critical resources, but they should be dropped if memory is needed for a critical resource because it can be recomputed again when needed. These are hat weak pointers are for.

An example of this might be pictures. Say you have a photo web page with thousands of pictures to display. You need to know how many pictures to lay out and maybe you have to do a database query to get the list. The memory to hold a list of a few thousand items is probably very small. You want to do the query once and keep it around.

You can only physically show perhaps a few dozen pictures at a time, though, in a pane of a web page. You don't need to fetch the bits for the pictures that the user can't be looking at. When the user scrolls the page, you'll gather the actual bits for the pictures visible. Those pictures could require many megabytes to show them. If the user scrolls back and forth between a few scroll positions, you'd like not to have to refetch those megabytes over and over again. But you can't keep all the pictures in memory all the time. So you use weak pointers.

If the user just looks at a few pictures over and over again, they may stay in cache and you don't have to refetch them. But if they scroll enough, you need to free up some memory so the visible pictures can be fetched. With a weak reference, you check the reference just before you use it. If its still valid, you use it. If its not, you make the expensive calculation (fetch) to get it.