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
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