Hi All,

I wrote a TPainter abstract class, and a TPaintRect class. ?In the TPaintRect class I have this code:

procedure TPaintRect.OnMouseEnter(Sender: TObject);
var
? i: Integer;
? p: TPainter;
begin
? for i := 0 to painters.Count - 1 do begin
? ? p := TPainter(painters.Objects[i]);
? ? try
? ? ? p.OnMouseEnter(Sender);
? ? except on EAbstractError do ; end;
? end;
? if Assigned(FOnMouseEnter) then FOnMouseEnter(Sender);
end;

While running in IDE, the program will crash because OnMouseEnter is abstract.

My problems are:

1) As I already wrapped it with try-except, I hope it won't trigger IDE exception. But even I turn off "Notify on Lazarus Exception" in debugger options it still pops up, and the popup said RunError(211), NOT EAbstractError. ?The program runs well outside of IDE.

2) Is there a way to detect if an abstract method is implemented or not, without trying to call it and try...except?

Thanks,
Shannon

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Hi, I don't test it in fp. But in Delphi I just create virtual method with empty body in this situation.
24.01.2013 15:11 ???????????? "xrfang" ???????:

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The following will do it:

uses sysutils;
Type
   TParent = Class
     Procedure MyMethod; virtual; abstract;
   end;
   TChild = class(TParent)
   end;
var
   C : TParent;
begin
   c:=TChild.Create;
   If TMethod(@C.MyMethod).Code=Pointer(@system.AbstractError) then
     Writeln('Not implemented')
   else
     Writeln('Implemented')
end.

When run, it will print 'Not implemented'.

But you should not instantiate objects with abstract methods to begin with.

The compiler warns you if you do.
(compile the above program with warnings/hints to see it)

Michael.

I was about to say the same thing.

Regards,
- Graeme -

This is a clean solution. And as a matter of fact, the compiler does NOT warn me while instantiate class with abstract method. This is different than Java, in which you must implement all abstract method while inherits from an abstract class.

Why I shall not instantiate a sub-class that has not fully implemented abstract methods of its parent class?

Thanks!
Shannon

2013/1/24 Michael Van Canneyt

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Xiangrong Fang wrote

This is a clean solution. And as a matter of fact, the compiler does NOT warn me while instantiate class with abstract method.??

It definitely does, unless you use a class pointer to do it of course.

In the below example:

   TParentClass = Class of TParent;
Var
   PC : TParentClass;
begin
   PC:=TChild;
   C:=PC.Create;
end;

You will not get a warning. But if you run the example as I described it, you get:

home: >fpc -S2 -vwh te.pp
te.pp(15,19) Warning: Constructing a class "TChild" with abstract method "MyMethod"
te.pp(5,15) Hint: Found abstract method: procedure MyMethod(<TParent>);

Obviously:
Because then you run the risk that you try to execute a method that is not implemented.

Michael.

Sorry I overlooked the message. Complie DO emit warnings.

As the use of sub-class is well controlled, I will check in every place the abstract method is called, so this is ok for me.

Thanks a lot.

2013/1/24 Michael Van Canneyt

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Xiangrong Fang hat am 24. Januar 2013 um 14:03 geschrieben:

This is a clean solution. And as a matter of fact, the compiler does NOT warn me while instantiate class with abstract method.

Normally the compiler warns for each abstract method. For example, this creates 5 warnings:
TStrings.Create;
If not then please create a bug report.

Well, you can shoot yourself in the foot. ;)

Mattias

One more report:

I finally decided to use the solution suggested by Fei, i.e. empty method in base class instead of Abstract method. Becuase comparing address to @system.AbstractError caused trouble if I put my class into a shared library.

Thanks.

2013/1/24 Mattias Gaertner

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