On Thu, 2 Feb 2023, Ondrej Pokorny via fpc-devel wrote:
On 02.02.2023 10:22, Michael Van Canneyt via fpc-devel wrote:
On Thu, 2 Feb 2023, Ondrej Pokorny via fpc-devel wrote:
On 02.02.2023 10:15, Michael Van Canneyt via fpc-devel wrote:
On Thu, 2 Feb 2023, Ondrej Pokorny via fpc-devel wrote:
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.
Which is a clear violation of the concept of Const... :/
Exactly.
Probably an ill-advised "fix" to the problem that FreeAndNil accepted an
untyped var
and you could basically pass anything.
Yes. Solving one problem by creating a different one. The best would be if
the FreeAndNil declaration alternated between constref and untyped var
between builds. In that case you would be able to hunt both the issues :)
Seriously, having a [loose] modifier would be much more useful, IMO:
FreeAndNil(var [loose] Obj: TObject);
That would allow you to pass any TObject descendant.
It would solve this problem, but introduces another problem, since it would be
usable anywhere:
Procedure DoSomething(var [loose] Obj: TObject);
var
a : TSomeObject;
begin
a:=TSomeobject.Create;
DoSomething(a);
end;
after DoSomething, A may contain a class that is not a TSomeObject at all,
leading to more problems.
In userspace, the best seems
Function FreeAndNil<T :TObject>(Obj : T) : T;
begin
Obj.Free;
Result:=Nil;
end;
With automatic type inference for generics, this allows you to do
A:=FreeAndNil(A);
Which is quite acceptable IMO. If you could add inline to the generic
definition the overhead would be minimal.
As it is, FreeAndNil() seems like another candidate for a compiler intrinsic ;-)
Michael.
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