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Subject: Re: [boost] [contract] move operations and class invariants
From: Barrett Adair (barrettellisadair_at_[hidden])
Date: 2017-11-29 07:52:22

You could add a `bool moved_from` field and OR it in your invariants. If
you can afford the cost of contracts, then maybe you can also afford the
overhead of this extra field.

On Nov 29, 2017 1:14 AM, "Julien Blanc via Boost" <boost_at_[hidden]>

> Le 2017-11-29 05:39, Lorenzo Caminiti via Boost a écrit :
>> Hello all,
>> This is not really a question about Boost.Contract, but more a
>> question on how contract programming interacts with C++ move
>> operations.
>> C++ requires that moved-from objects can still be destructed. Because
>> contract programming requires class invariants to hold at destructor
>> entry, it follows that moved-from objects must still satisfy class
>> invariants.
> This is not only about destructors, but more generally about reusing
> moved-from objects.
> That sounds restrictive... and it might force the class invariants to
>> be empty. For example, for vector a class invariant is size() <=
>> capacity(). Should that sill hold after the vector has been moved?
>> That means I can still call size() and capacity() on a moved-from
>> object, which might not be the case.
> If it can’t hold, that should be part of the contract. IMHO moving-from
> can result in two situations :
> 1) reverting back to an empty state
> 2) going to an invalid state
> The bad thing with 2) is that currently you have no way to compile-time
> check it, and it will lead to crashes. 1) is safer, but has problems with
> RAII, since the object no longer hold any resource.
> If some sort of moved() function could be called on a moved-from
>> object, the invariants could be programmed as follow to work around
>> this issue:
> I'm not really sure... What do you think? Do you know if this topic
>> "C++ move & class invariants" has already been discussed somewhere?
> I’m not aware of any litterature on this.
> At first glance, i’ll separate two things :
> - value types (such as std::vector), for which move semantic is mainly a
> performance matter. For those types, 1) (empty state) makes a lot of sense.
> - entity types (which holds resource), for which move semantic is really
> an ownership transfer. For those types, 1) doesn’t make a lot of sense, and
> 2) may lead to hard to debug crashes. In the current state of the language,
> I’d rather make these types not movable, and use unique_ptr to transfer
> ownership.
> Regards,
> Julien
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