A few weeks ago I sent a post about learning OOP via a Java-based book, and
a few of you were kind enough to respond. I'm happy to tell you that I've
made a lot of progress; I understand the OOP mindset a lot better now. It's
exciting, and I thank again those of you who responded.
Now I've got the task of taking all the general things I've learned and
figuring out how the work in Delphi. I'm struggling a bit with understanding
collaboration among objects, and keeping the GUI object separate from the
"Model" object(s). I haven't found many Delphi-specific resources to help
me; I'm open to suggestions (esp. web-based tutorials). Most discussions are
either too basic for me now ("the difference between a class and an object,"
"the definition of inheritance," etc.) or too advanced (it's great to know
that my questions are addressed by understanding things like the Operator
and Listener design patterns, but I'm not there yet).
Along with this general question, I'd like to ask one that's a little more
specific. To make responses easier, suppose you had a TLibrary class whose
only field is a list of TBook objects. TBook, in turn, contains only a
string field for titles. TLibrary.Create makes fBookList, and and
TLibrary.Destroy frees the list. Adding a book to fBookList involves either
(a) a pre-existing TBook instance or (b) creating the instance as part of a
method. ... So far so good.
Now, suppose in the UI you want to generate a list of all the books in the
library. Since all the data manipulation should take place in a TLibrary
object, then the event handler in Form1 should probably do no more than call
a function in the object, something like TLibrary.ShowAllTitles, right? If
the event handler was going to list the books in something like a Memo, then
it makes sense that ShowAllTitles should generate a TStringList. ... So far,
still so good.
My difficulties relate to whether or not I'd be creating memory leaks, and
understanding what needs to be freed by whom. The TStringList object created
in the last paragraph--when is it freed? Once the Memo (in Form1) has been
filled, the TStringList is no longer needed. But you can't just free it at
the end of ShowAllTitles (Form1's Memo needs the reference). And how about
all those TBook instances referenced in TLibrary's fBookList? When are they
freed? [It's probably inappropriate for this list, but these kinds of
questions get me wondering why Delphi doesn't have garbage collection.]
Hope this is clear. Thank you again; this list has been helpful.
Alan C.
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