3.5 Special kinds of object

A few methods in Object support the creation of particular objects. This include:

• finalizable objects

• weak objects (i.e. objects whose contents are not considered, during garbage collection, when scanning the heap for live objects).

• read-only objects (like literals found in methods)

• fixed objects (guaranteed not to move across garbage collections)

They are:

Method: Object makeWeak

Marks the object so that it is considered weak in subsequent GC passes. The garbage collector will consider dead an object which has references only inside weak objects, and will replace references to such an "almost-dead" object with nils.

Method: Object addToBeFinalized

Marks the object so that, as soon as it becomes unreferenced, its finalize method is called. Before finalize is called, the VM implicitly removes the objects from the list of finalizable ones. If necessary, the finalize method can mark again the object as finalizable, but by default finalization will only occur once. Note that a finalizable object is kept in memory even when it has no references, because tricky finalizers might "resuscitate" the object; automatic marking of the object as not to be finalized has the nice side effect that the VM can simply delay the releasing of the memory associated to the object, instead of being forced to waste memory even after finalization happens. An object must be explicitly marked as to be finalized every time the image is loaded; that is, finalizability is not preserved by an image save. This was done because in most cases finalization is used together with CObjects that would be stale when the image is loaded again, causing a segmentation violation as soon as they are accessed by the finalization method.

Method: Object removeToBeFinalized

Removes the to-be-finalized mark from the object. As I noted above, the finalize code for the object does not have to do this explicitly.

Method: Object finalize

This method is called by the VM when there are no more references to the object (or, of course, if it only has references inside weak objects).

Method: Object isReadOnly

This method answers whether the VM will refuse to make changes to the objects when methods like become:, basicAt:put:, and possibly at:put: too (depending on the implementation of the method). Note that GNU Smalltalk won't try to intercept assignments to fixed instance variables, nor assignments via instVarAt:put:. Many objects (Characters, nil, true, false, method literals) are read-only by default.

Method: Object makeReadOnly: aBoolean

Changes the read-only or read-write status of the receiver to that indicated by aBoolean.

Method: Object basicNewInFixedSpace

Same as #basicNew, but the object won't move across garbage collections.

Method: Object basicNewInFixedSpace:

Same as #basicNew:, but the object won't move across garbage collections.

Method: Object makeFixed

Ensure that the receiver won't move across garbage collections. This can be used either if you decide after its creation that an object must be fixed, or if a class does not support using #new or #new: to create an object

Note that, although particular applications will indeed have a need for fixed, read-only or finalizable objects, the #makeWeak primitive is seldom needed and weak objects are normally used only indirectly, through the so called weak collections. These are easier to use because they provide additional functionality (for example, WeakArray is able to determine whether an item has been garbage collected, and WeakSet implements hash table functionality); they are:

• WeakArray
• WeakSet
• WeakKeyLookupTable
• WeakValueLookupTable
• WeakIdentitySet
• WeakKeyIdentityDictionary
• WeakValueIdentityDictionary