CODEWITHSUNDEEP
javadelphiabstract-class
Theo
Theo

Reputation: 464

Delphi : Constructing Abstract Class like Java

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;

Upvotes: 2

Views: 415

Answers (2)

David Heffernan
David Heffernan

Reputation: 612964

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.

Upvotes: 5

Gloegg
Gloegg

Reputation: 380

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.

Upvotes: 4

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