[fpc-pascal] class declaration questions

[Mattias Gaertner]

On Fri, 12 Oct 2007 18:41:19 -0300
Joao Morais <post at joaomorais.com.br> wrote:

> Felipe Monteiro de Carvalho wrote:
> > On 10/12/07, Marc Santhoff <M.Santhoff at t-online.de> wrote:
> >> Does object pascal allow a class declaration marked as an abstract
> >> class? Like in java for making sure the class can never be
> >> instantiated itself, only it's descendants can be?
> > 
> > The compiler will issue a warning if you instantiate a class with
> > abstract methods
> 
> Only if creating an instance straight from the class, eg:
> 
> This will warn:
> 
> begin
>    VMyObj := TMyAbstract.Create;
> 
> This wont:
> 
> var
>    VMyClass: TMyAbstractClass;
> begin
>    VMyClass := TMyAbstract;
>    VMyObj := VMyClass.Create;

Correct. The compiler can only warn about compile time errors, not
runtime errors.
For runtime you can use

constructor TMyAbstract.Create;
begin
  if ClassType=TMyAbstract then raise Exception.Create('abstract
class');
  inherited Create;
end;


Mattias