Pointer to generic record

Question

Note that this is not a duplicate of Pointer to generic type. It's a followup question.

I know it is possible to define a pointer to any generic type.
It just that Delphi makes it complicated. It was meant to be impossible, but due to a compiler bug the option slipped through.
This is what the linked question answers.

My question is:

How do I define a pointer to a generic record without encapsulating it in a surrounding class?

Example code:

TGenericRecord<T> = record
  Data: integer;
  Procedure SomeMethod; inline; <<<< inlining is vital here. 
end;

I want to get a type safe pointer to TGenericRecord.
I do not want to wrap the record in a surrounding class because in my experiments I've found that that disables the inlining.

How do I get a typesafe generic pointer to this record.

Use case

{class} function create(size: integer): PGenericRecord{<T>}

I want to be able to create records on the heap in addition to the stack.

You could declare Type PT = ^T; inside the record and use it as var m: TMiniStack<Integer>.PT; with m := TMiniStack<Integer>.Create;. — LU RD
– LU RD, Commented Aug 28, 2014 at 11:19
No it's not, because that question does A: not provide the answer and B: I already know how to make a pointer to operate within the generic type itself. I'm asking a specific followup question to the one you're referring to. — Johan
– Johan, Commented Aug 28, 2014 at 11:20
@LURD, I'm not interested in a ^T, I want a ^TMiniStack<T>. — Johan
– Johan, Commented Aug 28, 2014 at 11:21
It does provide an answer as Barry clearly explains that it's not possible to declare a pointer to a generic type. — Stefan Glienke
– Stefan Glienke, Commented Aug 28, 2014 at 11:24

Rudy Velthuis · Accepted Answer · 2014-08-28 13:48:36Z

4

I think your best bet probably looks like this:

type 
  TMyStaticClass<T> = class
  public
    type
      TRec = record
        ....
      end;
      PRec = ^TRec;
  public
    class function NewRec: PRec; static;
  end;

I don't have a compiler handy to check whether or not this even compiles but I feel that it should.....

edited Aug 28, 2014 at 13:48

Rudy Velthuis

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answered Aug 28, 2014 at 11:26

David Heffernan

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3 Comments

Rudy Velthuis Over a year ago

It compiles (after adding an Integer to the record and adding an implementation for NewRec and correcting the syntax error - <T> should be after the name, not after class).

Rudy Velthuis Over a year ago

I would have posted a similar solution. The only disadvantage is that you have to address this type as TMyStaticClass<SomeType>.PRec instead of a simple PRec. I guess one can create an alias, once SomeType is known.

David Heffernan Over a year ago

@rudy I think that is unavoidable

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