[fpc-pascal] Constructors & Destructors 101
roward at mac.com
Fri Apr 3 03:43:46 CEST 2009
I am a little bit unclear about certain things on class constructors
From what I read in the language reference guide (chapter 6) and
runtime utilities document (chapter 29):
1) All classes descend from TObject; even declaring a new class
without using TObject as a qualifier.
2) TObject.Create is static (not declared virtual)
3) TObject.Destroy is declared virtual
4) Under most circumstances one should call the Free method to release
an object since it checks for a nil pointer first. This method then
calls the destructor (Destroy) which, after executing your own clean
up code if any, will then release the memory. However, the object
memory reference will not be set to nil. Testing this out confirms
this and the variable will still contain the memory address of the
previously allocated memory block even though the memory block pointed
to by the variable is not usable for anything. To ensure that the
address is set to nil, one should call the "global" sysutil function
FreeAndNil , (chapter 30 RTL doc). This routine sets the address to
nil before calling the free method which then calls the destructor.
5) Although I can't remember reading it anywhere, I've noticed from my
own demo programs that the addresses of objects are initially set to
If I have all of the above correct, then I have the following questions:
A) The documentation says that for the create constructor:
Description: Create creates a new instance of TObject. Currently it
does nothing. It is also not virtual, so there is in principle no need
to call it directly.
What is it meant by: "no need to call [create] directly?" How do you
invoke the constructor without calling it? ... and ... Why is create
not virtual and the destroy destructor is?
B) The documentation says that for the destroy destructor:
Description: Destroy is the destructor of TObject. It will clean up
the memory assigned to the instance. Descendent classes should
override destroy if they want to do additional clean-up. No other
destructor should be implemented.
What is it meant by: "No other destructor should be
implemented?" ...and... Does it do "something" while the Create
If you had two different create constructors (for whatever reason),
might you not also need two different destroy destructors? What
problems might you get into if you did?
C) Just out of curiosity, am wondering why FreeAndNil is global
procedure instead of a method/destructor of TObject. I am guessing
it is for compatibility with Delphi which may or may not have a reason?
I had other curiosity questions but found a nice web page which covers
the Delphi Object Model.
I have gotten several basic programs to run without worrying about
these questions, but wanted to make sure I am not going to get into
trouble later with some subtle problem cropping up. thanks for any
information on this. - ROW
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