what is the meaning of `Class of ` type declaration?

Question

While going through one of my code, I am stuck on one statement which is as below.

TMyObjectClass = class of TMyObject;

I am a bit confused, and wondering what is the meaning of this statement. As TMyObjectClass has no declaration above the statement.

and TMyObject is having declaration as below:

TMyObject = class(TObject) private //some private member declaration Public // some public variables end;

So, my question is what is the meaning of the statement TMyObjectClass = class of TMyObject;

and How TMyObjectClass works?

I am a bit new to Delphi, so please help me to get some idea about these type of declaration and there workarounds.

Answer 1

This is a Class Reference.

They are used to work with meta classes. The canonical example is the Delphi streaming framework which uses

TComponentClass = class of TComponent;

This allows for dynamic binding to virtual constructors. The TComponent constructor is virtual. The streaming framework needs to instantiate classes derived from TComponent. It does so something like this:

var
  ComponentClass: TComponentClass;
  Component: TComponent;
....
ComponentClass := GetComponentClassSomehowDoesntMatterHow;
Component := ComponentClass.Create(Owner);

Now, because TComponent.Create is virtual, this is bound in a polymorphic fashion. If TComponentClass is TButton, then TButton.Create is called. If TComponentClass is TPanel, then TPanel.Create is called. And so on.

The most important thing to realise is that the class that is constructed is determined only at runtime. Note that many languages lack this capability, most notably C++.