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Re: Automatially move from $n (was: C++11 move semantics)

From: Hans Åberg
Subject: Re: Automatially move from $n (was: C++11 move semantics)
Date: Tue, 18 Sep 2018 00:55:13 +0200

> On 18 Sep 2018, at 00:20, Frank Heckenbach <address@hidden> wrote:
> Hans Åberg wrote:
>>>> Yes, indeed C++ does not support that,
>>> Are you replying to your own statement now? I never claimed (or
>>> cared) whether C++ supports GC.
>> C++ does not support the implementation of a (tracing) GC, because
>> the information needed, though available to the compiler, is not
>> available from the language. The fact that it does not have a GC
>> is another topic.
> I know and (again) I don't care because I don't want to use GC
> anyway. You're basically discussing with yourself here.

It exemplifies the of limitations C++. See below.

>>> At runtime, yes. Basically an extended unique_ptr could detect this
>>> automatically.
>> This looks like becoming you option. You might use it for
>> debugging only.
> My option? I'm not having this problem.

So then why bother bringing it up in the first place?

> Again, I only brought up the
> make_pair thing as a counterexample to your claim that an automatic
> break would avoid any possible double-move.

You gave a counterexample of something else than what I discussed.

> But it might be a solution to someone else's problem (or another
> problem of mine some other day ;).

Until that day, it's too esoteric to be worth implementing, in my opinion.

>>> I'd rather see a compile-time check, even if it's a
>>> bit primitive, i.e. gives false positives.
>> Even though the compiler may have access to the information to
>> check that, you don't have access to that from the language
>> itself. Parsing the language is a long haul, even though there
>> were some here wanting help with that.
> No, I don't want to go there. As said before, I see two viable
> compile-time options so far:
> - Within Bison, hopefully rather easy to implement, but with some
>  false positives.
> - Within(!) the compiler, more general, but a job for compiler
>  experts, not me.

You won't get that for the same reason as in the GC case above.

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