Hello,

Any idea how I know if a class, in a class pointer, overrides a virtual method? Eg:

tfooclass = class of tfoo;
tfoo = class
   procedure sample; virtual;
end;
tboo1 = class(tfoo)
end;
tboo2 = class(tfoo)
   procedure sample; override;
end;

...

vfooclass := tboo1;
// vfooclass doesn't implement sample.
vfooclass := tboo2;
// vfooclass implements sample.

At this moment I know that:

1. I could cast a method pointer with tmethod if sample was a class method;

2. I could cast this same method pointer if I had an instance of vfooclass.

But is there another way beyond creating an instance?

Thanks.

Any idea how I know if a class, in a class pointer, overrides a virtual method? Eg:

<snip>

You need to implement a virtual method, even if it does nothing. Are you sure you're not meaning abstract method?

e.g.

TBlah = class
  public
     procedure blah; virtual;
     procedure blahblah; virtual; abstract;
 end;

Will only compile if "TBlah.blah" has a body.... where as TBlah.blahblah" does not require itself to be implemnted.

At this moment I know that: 1. I could cast a method pointer with tmethod if sample was a class

method;

2. I could cast this same method pointer if I had an instance of

vfooclass.

But is there another way beyond creating an instance?

You can always hack the RTTI/VMT, but that might not be a safe way.

Might it be that you need to have a more complex inheritence tree? So add a few levels?

tfooBaseclass = class of tfooBase;
tfooclass = class of tfoo;
tfooBase = class
private
  procedure sample; virtual;
end;
tfoo = class(tfooBase)
public
  procedure sample; override; //a base implementation
end;
tboo1 = class(tfooBase)
   //sample is still private and not publicly accessable
end;
tboo2 = class(tfoo)
public
   procedure sample; override; //whatever
end;
if (x is tfoo ) then {supports the sample};
if (x is tfooBase ) then {does not make public the sample};

You then know that tfooBaseclass never implement sample, but that all tfooClass have that method. It all seems a little confusing though. I'm not really clear what you are trying to achieve. Inheritence wants you to either make methods static, or virtual. If it is static, you can't override it, only "hide" (Replace) it.

You coule also use interfaces. Then you don't need a base class:

ifoo = interface
private
  procedure sample; virtual;
end;
tboo1 = class( ifoo)
   procedure sample;
end;
tboo2 = class
end;
if (tboo is ifoo) then {supports the ifoo interface};

Hello Matt, thanks for your fast input.

No, no... I just want to know if the class referenced by vfooclass overrides the "sample" method, implementing something different from the virtual implementation that, btw, isn't abstract and it's empty.

I though about this, dunno it's better than creating an useless instance.

Might it be that you need to have a more complex inheritence tree? So add a few levels?

Even more complex? No, no! =)

This is a piece of an OPF framework. I want to know if a business class implement something in a virtual method. Such class belongs to the application and the framework doesn't have direct access to it. If the class doesn't implement I don't need to retrieve an instance from the database, btw an expensive task for absolutely nothing.

So, at this moment I know that I can:

1. Use a safe, magic and efficient way, but I need to know if it exists;
2. Create a "light" instance using vfooclass.NewInstance method;
3. hack the vmt.

Which approach do you use?

Thanks.

I have worked with a bespoke OPF a few years ago. The way we got around this was to use a multitier approach and literally have the client ask the server questions in this kind of situation. The server was multithreaded and it cached information, so it usually didn't have much of an impact. So long as the network traffic was minimal and only happened once per session (the reply being cached) it worked well.

We also used a complex inheritance tree with interfaces being passed back from the server rather than class references. There were also tight rules about implementing methods. Methods with no implementation were not allowed in persistently stored objects.

I don't think there's much more really you can do. Other than caching the RTTI info for each class you use in this way.

HTH