2.11 Special kinds of objects

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

• finalizable objects
• weak and ephemeron objects (i.e. objects whose contents are considered specially, during the heap scanning phase of garbage collection).
• read-only objects (like literals found in methods)
• fixed objects (guaranteed not to move across garbage collections)

They are:

Method on Object: makeWeak

Marks the object so that it is considered weak in subsequent garbage collection 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, and then send the mourn message to the object.

Method on Object: makeEphemeron

Marks the object so that it is considered specially in subsequent garbage collection passes. Ephemeron objects are sent the message mourn when the first instance variable is not referenced or is referenced only through another instance variable in the ephemeron.

Ephemerons provide a very versatile base on which complex interactions with the garbage collector can be programmed (for example, finalization which is described below is implemented with ephemerons).

Method on 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 operating system resources that would be stale when the image is loaded again. For CObjects, in particular, freeing them would cause a segmentation violation.

Method on 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 on 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 on 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 on Object: makeReadOnly: aBoolean

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

Method on Object: basicNewInFixedSpace

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

Method on Object: basicNewInFixedSpace:

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

Method on 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
• WeakKeyDictionary
• WeakValueLookupTable
• WeakIdentitySet
• WeakKeyIdentityDictionary
• WeakValueIdentityDictionary

Versions of GNU Smalltalk preceding 2.1 included a WeakKeyLookupTable class which has been replaced by WeakKeyDictionary; the usage is completely identical, but the implementation was changed to use a more efficient approach based on ephemeron objects.