Dear Matthew,
The extra call to destroy occurs when the command interpreter closes, so must mean that MyBase was not properly destroyed because "next" was not called. This is probably the intended behaviour, but it is difficult to work it out from the examples given in http://media.wu-wien.ac.at/doc/tutorial.html.
you are right, the documentation should be improved. I have just added a paragraph adressing this point.
The destruction of objects is performed by the c-level method destroy of ::xotcl::Object. The method destroy is a method like every other method in xotcl, asides from its side-effects, it is no way magic or special. If an application class overwrites the destroy method, and does not call "next", Object->destroy will not be called, the object will not be destroyed.
Can somebody tell me the creation and destruction processes that go on "behind the scenes" in XOTcl?
You can ask xotcl to tell you, what's going on. Consider the following filter method implementing a trace. Before and after each invocation of a method, the filter spits out a single line. We register the filter for all instances of all objects (as an instfilter for Object):
Object instproc f args { puts "call [self]->[self calledproc]" set r [next] puts " [self]->[self calledproc] returns '$r'" return $r }
Class C Object instfilter f puts ================== C c1 puts ================== c1 destroy puts ================== Object instfilter ""
If we run the script, we get the following output. ================== call ::C->c1 ::C->c1 returns '' call ::C->unknown call ::C->create call ::C->alloc ::C->alloc returns '::c1' call ::xotcl::Class::Parameter->searchDefaults ::xotcl::Class::Parameter->searchDefaults returns '' call ::c1->configure ::c1->configure returns '0' call ::c1->init ::c1->init returns '' ::C->create returns '::c1' ::C->unknown returns '::c1' ================== call ::c1->destroy call ::C->instdestroy ::C->instdestroy returns '' ::c1->destroy returns '' ==================
If we create an Object via "C c1", we call a method c1 of Object. since this is not defined, "C unknown" is invoked. For all classes, the predefined unknown handler "create"s an object with the specified name. create initiates the creation protocol, - starting with "alloc" (registering the object as command, allocating memory), - calling "searchDefaults" (for setting the default values specified via parameters), - calling "configure", to overwrite some of these values and to specify additional values, - calling "init" to perform additional application-level initializations, based on the default and configured values.
destruction is much simpler. "destroy" is called on the object level, "instdestroy" is the counterpart of "alloc" on the class level (the class manages the object, allocates and destroys it, knows its instances, etc.)
Also, it would be helpful if the differences between XOTcl and other OO languages, say C++, were highlighted as I know I have made assumptions which turn out to be wrong.
There are certainly many differences between a highly dynamic oo language and e.g. C++. Maybe some can start writing his gotchas into a wiki page. See as well the distinction between call-oriented vs. object-oriented in
Hop this helps
-gustaf