[fpc-devel] Question on constref

[Sven Barth]

Ondrej Pokorny via fpc-devel <fpc-devel at lists.freepascal.org> schrieb am
Do., 2. Feb. 2023, 10:06:

> 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?
>

This isn't about Delphi, but about Windows (and XPCOM): there the IID
parameter is declared as "REFIID" which is essentially a pointer. On x86
with stdcall this is not a problem, because there "const TGUID" will be
passed as reference, same on x86_64 and also ARM64. However on ARM this is
*not* the case (and which is how that issue was discovered). So to avoid
having to add a cast to a PGUID to every QueryInterface call we instead
decided to introduce "constref" which results in fewer modifications for
existing code.

Maybe somebody knows a real use case?
>

There are cases were you really need a reference and want it to be const.
Or were you want to avoid a copy *for sure* (e.g. Embedded targets).

Only because you can't see a reason for it doesn't mean that it doesn't
exist.

Regards,
Sven

>
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