Delphi : Constructing Abstract Class like Java

Question

Just curious, can i construct an abstract class in Delphi like Java Style?

Java Declaration:

public abstract class Foo {
  public abstract void test();
}

Way to use :

public void testCsontruct() {
  Foo clsfoo = new Foo(){
    public void test(){
      //....do something here

    }
  };
}

Delphi declaration :

Tfoo = class
public
  procedure test; virtual; abstract;
end;

Way to use :

procedure testUse;
var
  foo:Tfoo;
begin
  foo:=Tfoo.Create; //can we implement the test method here ? how?
end;

Ive been searching a while for that and this is the nearest i found: stackoverflow.com/questions/6786401/… — deterministicFail
– deterministicFail, Commented Dec 11, 2013 at 15:24

David Heffernan · Accepted Answer · 2013-12-12 07:20:35Z

5

That Java functionality is the anonymous class. They are useful in a number of scenarios. As well as that which you illustrate, they are commonly used to capture variables in the enclosing scope.

There is nothing quite like that syntax in Delphi. Anonymous methods come close, but there's no syntax to allow you to replace a virtual method of an instantiated object with an anonymous methods.

You need to sub-class to provide a concrete implementation of the virtual method. You can sub-class at compile time in the usual way, or at run time using virtual method interceptors or some similar VMT trickery.

edited Dec 12, 2013 at 7:20

answered Dec 11, 2013 at 14:46

David Heffernan

616k46 gold badges1.1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Gloegg · Accepted Answer · 2013-12-11 15:00:00Z

4

You can do that with virtual method interceptors:

uses
  Rtti;
...
begin
  foo := TFoo.Create;
  vmi := TVirtualMethodInterceptor.Create(foo.ClassType);

  vmi.OnBefore :=
      procedure(Instance: TObject; Method: TRttiMethod; const Args: TArray<TValue>; out DoInvoke: Boolean;
      out Result: TValue)
    begin
      if Method.Name = 'test' then
      begin
        DoInvoke := false;
        // do something here
      end;
    end;
  vmi.Proxify(foo);

  foo.test;

  foo.Free;
  vmi.Free;
end;

However, I would prefer an actual implementation in a subclass.

answered Dec 11, 2013 at 15:00

Gloegg

3803 silver badges12 bronze badges

3 Comments

Theo Over a year ago

Thanks for share. I prefer do sub-class as it's much more simple.

David Heffernan Over a year ago

Actually this is a form of sub-classing. The class is modified at runtime. This is the view point that the contents of the VMT are part of the class definition, so therefore changing the VMT is changing the class.

Stefan Glienke Over a year ago

@DavidHeffernan Not only a form of sub-classing, it is sub-classing because the class that is created by the TVirtualMethodInterceptor actually inherits from the class you passed in (just print out the class hierarchy).

Collectives™ on Stack Overflow

Delphi : Constructing Abstract Class like Java

2 Answers 2

Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related