Re: [boost] compact_optional -- prompting interest

Subject: Re: [boost] compact_optional -- prompting interest
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2015-09-27 18:35:21

On Sun, Sep 27, 2015 at 3:20 PM, Matt Calabrese <rivorus_at_[hidden]> wrote:

> It doesn't. The idea is that the times that you can make such a hook for
> intrusive_optional are where it is not UB to access that storage regardless
> of whether or not the object is constructed. For instance, I use an
> unsigned char to store the value 0 through 51 and I know that this is at
> the start of the storage location for the card. As the creator of the card
> type I can guarantee this (this is why it is intrusive). With that
> knowledge, I can provide some small, default set of functions that operate
> on raw storage (not an instance of the card type) but that can do so safely
> whether the object is constructed or not since I can also via hooks
> guarantee that there will be an unsigned char in that location regardless.
>

Expanding on this, while it may sound hairy, in practice it should be able
to be not too difficult to compose. For instance, if I make a type that
contains a card, yet I did not create that card type itself, assuming that
I know "card" supports intrusive optional, I would be able to easily point
my own intrusive_optional hooks to the location of the "card" that I
contain, and forward the functionality along. This leaves you with a
higher-level type that again takes up no extra storage for the optional
version, and the original type doesn't need a notion of emptiness.

-- 
-Matt Calabrese