[Lazarus] Check if an abstract method is implemented or not

[Michael Van Canneyt]

On Thu, 24 Jan 2013, 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>);

>  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?

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

Michael.
> 
> Thanks!
> Shannon
> 
> 
> 2013/1/24 Michael Van Canneyt <michael at freepascal.org>
> 
>
>       On Thu, 24 Jan 2013, xrfang wrote:
>
>             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?
> 
> 
> 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.
> --
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