On Mon, 29 Jan 2018, Zoë Peterson wrote:
In the code below Bar creates a reference counted object and returns an
interface to it. Foo calls Bar but doesn't assign the result anywhere.
In Delphi, this produces the result:
Foo >
TAutoRelease.Create
Foo <
TAutoRelease.Destroy
But in Free Pascal it produces this:
Foo >
TAutoRelease.Create
TAutoRelease.Destroy
Foo <
If I change Foo to assign the interface to a local variable but don't do
anything with that variable, Free Pascal produces the same results as
Delphi.
Delphi's behavior is what I expected, and allows tricks with triggering
behavior when leaving the current scope. Barry Kelly has one example on
his blog where he talks about implementing C++'s RAII using it:
http://blog.barrkel.com/2010/01/one-liner-raii-in-delphi.html
Is Free Pascal's behavior intentional? Can it be changed to match Delphi?
The question is whether Delphi's behaviour is intentional. You are relying
on undocmented behaviour: when does an intermediate temp variable go out of
scope?
This is not defined, and the behaviours of Delphi and FPC regarding temp
variables do not match. This is well known since many many years.
see e.g.
https://bugs.freepascal.org/view.php?id=30409
As Maciej wrote, it can be changed.
But relying on what is an implementation detail is ill-advised, and
certainly not robust programming.
Michael.
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