Hi,
I need to build linked list constants, something like "
type
pbty = ^bty;

 aty = record
  b: pbty;
 end;
 paty = ^aty;
 bty = record
  a: paty;
 end;
const
 b0: bty = (a: @a0); //Error: Identifier not found "a0"
 a0: aty = (b: @b0);
"
Is it possible to make a forward declaration for a0?

Martin

Is it possible to make a forward declaration for a0?

No, forward type declaration is possible (with the restriction that the type must be based on pointer and the real declaration starts in the same type block), but forward variable declaration is not. You won't gain much benefit from doing the assignment at declaration than doing it at program start.

Really? No gain in the beloved readability? And it seems to me a "const" should not be changed by code. ;-)

Martin

Really? No gain in the beloved readability?

Not for me, I always initialize code in "executable" section, keeping the declaration section clean. That's what "readable" to me.

And it seems to me a "const" should not be changed by code. ;-)

Even if it's allowed, you're declaring typed constant, which is not a real constant and has the same effect as initialized variable (e.g. you can change the value later on). It only differs (from initialized var) when the assignment happens inside a routine.

Delphi has {$J-} in order to set them readonly.
Another problem with Free Pascal in this context is that it is AFAIK not possible to define forward types which could be used as parameters for procedures used as procedure variables in later type definitions.

Martin

Do you know whether Delphi has by default $J+ or $J-?

Another problem with Free Pascal in this context is that it is AFAIK not possible to define forward types which could be used as parameters for procedures used as procedure variables in later type definitions.

In case of records, classes and objects you can work around this with nested types (Note: for records the modeswitch advancedrecords is required in non-Delphi modes):

=== code begin ===

type
   TMyRecord = record
   public type
      TTest = procedure(aRec: TMyRecord);
   public var
     Test: TTest;
   end;

=== code end ===

And I just noticed: this example does not work in Delphi...

Regards,
Sven

Then we don't need to change the default :)

Regards,
Sven

{$J+} in Delphi 7.

Ouch! ;-)

Martin