[fpc-devel] Question on constref

[Ondrej Pokorny]

On 02.02.2023 07:42, Sven Barth via fpc-devel wrote:
> The case when you *need* a constant reference. Case in point: the 
> passing of TGuid in IInterface.QueryInterface. Delphi code relies on 
> it being a reference, but “const” does not guarantee that for all 
> platforms.

Maybe I am missing something, could you please explain why 
IInterface.QueryInterface needs constref?

Delphi definition doesn't use it (Delphi 11):

   IInterface = interface
     ['{00000000-0000-0000-C000-000000000046}']
     function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
     function _AddRef: Integer; stdcall;
     function _Release: Integer; stdcall;
   end;

FPC uses it in the definition:

        IUnknown = interface
          ['{00000000-0000-0000-C000-000000000046}']
          function QueryInterface({$IFDEF 
FPC_HAS_CONSTREF}constref{$ELSE}const{$ENDIF} iid : tguid;out obj) : 
longint;{$IFNDEF WINDOWS}cdecl{$ELSE}stdcall{$ENDIF};
          function _AddRef : longint;{$IFNDEF 
WINDOWS}cdecl{$ELSE}stdcall{$ENDIF};
          function _Release : longint;{$IFNDEF 
WINDOWS}cdecl{$ELSE}stdcall{$ENDIF};
        end;

but when I check all the implementations of QueryInterface in objpas.inc 
I see that the iid is paased to TObject.QueryInterface

       function TObject.GetInterface(const iid : tguid;out obj) : boolean;

that has a const without ref.

---

I myself cannot think of any real use case of constref other than hacks 
like the FreeAndNil in recent Delphi versions:

procedure FreeAndNil(const [ref] Obj: TObject);

that needs the ref because it changes it to nil (so is not really a 
const - it used to be an untyped var). As a result FreeAndNil allows you 
to happily pass read-only properties to it that are then nilled.

Maybe somebody knows a real use case?

Ondrej