Re: [boost] [outcome] Exception safety guarantees

Subject: Re: [boost] [outcome] Exception safety guarantees
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-05-29 07:48:35

On 29/05/2017 19:36, Andrzej Krzemienski wrote:
> Ok, so who of the people who are following this thread ever did anything
> else with these past-removal objects, other than to destroy them? I am not
> being sarcastic. I just can't imagine anyone doing anything else with them.

Reassignment is pretty common.  Think of an array or queue where items 
are moved in and out of slots.  It's perfectly legal to do this with 
move-assignment directly of T, not just of a storage abstraction (unless 
you're an AllocatorAwareContainer, I guess).

Though it's important to realise that unless you know the type in 
question you can't rely on it itself having a perceptible empty state -- 
if you want to track which slots are used or not then you might need to 
do that separately.  It's always legal for an arbitrary type to copy 
itself even if you ask it to move.

Perhaps this counts as a special case of destruction, though.

For types that do have a well-defined empty state (like most smart 
pointers) it can be safer to make assumptions that the pointer is now 
empty after being moved-from.  (As long as you remember that it's up to 
both the method you're passing the object to and the object itself 
whether the move actually happens -- calling std::move by itself is a 
declaration of intent, not an action.  That trips many people up at first.)