How to pass a nil in place of a const record?

Question

I am trying to call the WIC (Windows Imaging Component) factory method CreateEncoder:

HRESULT CreateEncoder(
  [in]            REFGUID guidContainerFormat,
  [in, optional]  const GUID *pguidVendor,
  [out, retval]   IWICBitmapEncoder **ppIEncoder
);

The general idea would be:

var
   encoder: IWICBitmapEncoder;

CreateEncoder(
      GUID_ContainerFormatIco,  //The GUID for the desired container format
      nil, //No preferred codec vendor
      {out}encoder);

For the second parameter, i want to pass nil; to indicate that i have no encoder preference

But Delphi's translation of the WinCodec.pas is:

function CreateEncoder(const guidContainerFormat: TGuid; const pguidVendor: TGUID;
      out ppIEncoder: IWICBitmapEncoder): HRESULT; stdcall;

You'll notice that the second parameter, the optional TGUID, is declared as const.

So how do i pass it nil?

How can i pass a nil where a const record is required?

I tried the naïve solution:

const
   NULL_GUID: TGUID = ();

factory.CreateEncoder(GUID_ContainerFormatIco, NULL_GUID, {out}encoder);

and while that happens to not fail, it isn't really following the correct rules. It's passing a non-NULL address, that points to a GUID that the API doesn't know about.

I tried the naïve solution:

factory.CreateEncoder(GUID_ContainerFormatIco, TGUID(nil), {out}encoder);

but that's an Invalid typecast.

@DavidHeffernan: +1 That's usually the point when people start to look strange at you. BTDT. — JensG, May 31 '14 at 20:27

score 10 · Accepted Answer · edited May 31 '14 at 22:31

10

You can use something like this:

const
  NULL_PGUID: PGUID = nil;
begin
  factory.CreateEncoder(GUID_ContainerFormatIco, NULL_PGUID^, {out}encoder);
end;

Or as Sertac suggested, the shorter and sweeter version:

factory.CreateEncoder(GUID_ContainerFormatIco, PGUID(nil)^, {out}encoder);

I did a test and David's suggestion of:

factory.CreateEncoder(GUID_ContainerFormatIco, TGUID(nil^), {out}encoder);

Also works.

edited May 31 '14 at 22:31

TLama

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answered May 31 '14 at 01:35

Graymatter

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5

or PGuid(nil)^ to save a variable – Sertac Akyuz May 31 '14 at 01:46
Those are much better than the `TGUID(PGUID(nil)^)` things that i was fiddling with! – Ian Boyd May 31 '14 at 01:55
Ian, your `TGUID(PGUID(nil)^)` was a good attempt! You could just omit that `TGUID` typecast since `PGUID()^` already dereferences a value of `TGUID` so that typecast was useless there. `PGUID(nil)^` can be read as that you're dereferencing a `TGUID` pointer to the `nil` pointer constant. `TGUID(nil^)` as that you're dereferencing `nil` pointer constant and that value you typecast to the `TGUID` type. – TLama May 31 '14 at 21:34

How to pass a nil in place of a const record?

So how do i pass it nil?

1 Answers1