Martin wrote:
Just got a question, to ensure I understand thinks correctly.
Let's start with stuff I definitely know (or think so).
Destructors are virtual/overriden, because they are called on the
instance, and the instance may be assigned to a variable "foo: Tobject",
which would call TObject.Destroy instead of TMyClass.Destroy.
Constructors don't (usually) need this, because the are (usually) called
on a class (that is the classname usually appears hardcoded in the source)
on TComponent constructors are virtual, because when loading from
resource, those objects are instantiated separately (with NewInstance()
), and the constructor is called on an instance. So nothing new, the
usual way of virtual methods
It's not only for streaming, there are more common cases where you might
need virtual constructors:
var
GraphicClass: TGraphicClass;
Graphic: TGraphic;
begin
case Something of
1: GraphicClass := TBitmap;
2: GraphicClass := TPNGImage;
...
end;
Graphic := GraphicClass.Create;
...
end;
Now if I have a variable/value with classinfo?
AnyObject.ClassInfo => has the class info for it's class
BUT
TMyClass.Classinfo, could either be a TmyClass or a TMySubClass
(and per definition ClassInfo is TObject, and could be anything
inherited from Tbject)
Correct.
SO if I wrote
FMyClass := AMyClass.Classinfo.Create;
What will actually happen?
It creates an instance of your class, but calls TObject.Create
If I understand this right, an instance of TMyClass (or TMySubClass , if
AMyClass was a TMySubClass) is created; BUT the constructor
TObject.Create is called (with then TMyClass instance) ?
And it would skip any constructor that was on TMyclass?
Correct.
Then I could of course write
TMyClassType = class of TMyClass;
FMyClass := TMyClassType (AMyClass.Classinfo).Create;
and it would call TMyClass.Create. And if TMyClass wanted to have it's
own constructor, the TMyClass.Create must be virtual?
No, you don't need a virtual constructor here yet, since the
TMyClass.Create will be called. However if you have TMySubClass too with
an own create, you need one.
How close am I do the truth?
Close :)
Marc
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