[fpc-pascal] various ways of passing a class instance as a parameter

[Martin Schreiber]

On Tuesday 15 November 2011 11.00:34 Graeme Geldenhuys wrote:

> >> procedure foo(const AClass: TStringList);
> > 
> > Take a copy of the AClass instance pointer, AClass is readonly.
> 
> This one confused me a bit. I thought the whole object would be
> read-only, but in fact it is just the AClass instance pointer which
> cannot be modified. The properties of AClass are still read-write.

The wording was bad. Should be:
"
> procedure foo(AClass: TStringList);
>   or

Take a copy of the AClass instance pointer.

> procedure foo(var AClass: TStringList);

Take the address of the instance variable. The instance pointer pointed by the 
address can be changed by the procedure, so the type of the instance variable 
must match TStringList exactly otherwise the procedure could store a wrong 
class into the instance variable. Example:

procedure foo(var AClass: TList);
begin
 aclass.free;
 aclass:= Tlist.create;
end;
[...]
var
 cl1: TStringList.
begin
 foo(cl1);       //does not compile
 //now there would be a TList in a TstringList variable

>   or
> procedure foo(const AClass: TStringList);
> 
Take a copy of the AClass instance pointer, AClass instance pointer is 
readonly.

>   or
> procedure foo(constref AClass: TStringList);

Take the address of the instance variable, AClass instance pointer pointed by 
the address is readonly.
" 
Martin
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freepascal.org/pipermail/fpc-pascal/attachments/20111115/2e527c35/attachment.html>