Notes on Delphi Constructors and Destructors

Last edit: 05-03-17 Graham Wideman

Delphi

Notes on Delphi Constructors and Destructors
Article created: 98-11-01

The implications of making constructors and destructors virtual or not are not necessarily obvious. First, it's helpful to look at the declaration of TObject, from which all other objects descend, even if they don't say so.

Type
  TObject = class
    constructor Create;
    [...]
    destructor Destroy; virtual;
  end;
[...]
constructor TObject.Create;
begin
end;
destructor TObject.Destroy;
begin
end;

Notes:

TObject.Create and TObject.Destroy both do nothing.
TAnyObject.Create is preceded by allocation of space and zeroing of fields. (AnyInstance.Create is not so treated -- it merely executes AnyInstance.Create). In other words, it is NOT the calling of TObject.Create that causes the allocation and zeroing. Likewise in reverse for Destroy.
Hence, if you are inheriting from TObject (ie: defining a new base class that doesn't inherit from anything other than TObject.) then you are free to call or not call inherited Create and Destroy.

Create

		Ancestor is. . .
		TObject.Create (static)	TSomeObject.Create; static;	TSomeObject.Create; virtual/override;
Descendant Create
Same Args	static	OK, but precludes descendant override.	If called by class reference, then wrong constructor called?	If called by class reference, then wrong constructor called ?
	virtual	Defines new constructor	Defines new constructor	?
	override	Error	Error	Sensible
Different Args	static
	virtual	Defines new constructor	Defines new constructor	Defines new constructor, Sensible
	override	Error	Error	Error

Calling inherited Create		no effect since TObject.Create doesn't do anything.	Sensible	Sensible

Destroy

	Ancestor is. . .
	TObject.Destroy; virtual;	TSomeObject.Destroy; override;	TSomeObject.Destroy; Static
Descendant Destroy
static	Bad because if object is owned by a list (which will destroy it) via an ancestor-type pointer, wrong Destroy will be called.		Bad ancestor.
virtual	Bad	Bad (or error?)
override	Good	Good

Calling inherited Destroy	no effect since TObject.Destroy doesn't do anything.	Usually advisable	Bad ancestor.

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