Re: [boost] [contract] Without the macros

Subject: Re: [boost] [contract] Without the macros
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2016-06-15 17:42:02

On Wed, Jun 15, 2016 at 8:47 AM, Peter Dimov <lists_at_[hidden]> wrote:
> Lorenzo Caminiti wrote:
>
>>         boost::contract::old_ptr<int> old_x = BOOST_CONTRACT_OLDOF(x);
>
> Hi Lorenzo,
>
> Why is the old value a pointer instead of just a local copy?

Good question. Old values need to be at least optional values because:

1. They are not copied and therefore left empty when
BOOST_CONTRACT_NO_POSTCONDITIONS is #defined.

https://lcaminiti.github.io/boost-contract/doc/html/BOOST_CONTRACT_NO_POSTCONDITIONS.html

2. It is possible to copy them after preconditions are checked
specifying a functor using `.old(...)`. In that case old values need
to be left initialized (i.e., empty) when they are first declared
before the contract code.

https://lcaminiti.github.io/boost-contract/doc/html/boost_contract/advanced_topics.html#boost_contract.advanced_topics.old_values_at_body

So I originally implemented old values using boost::optional. However,
that did not suffice when I implemented subcontracting. The reasons
are specific to the library implementation so hard to clearly explain
in few words... but I'll try:

3. An overriding public function will call the public functions
(plural for multiple inheritance) that is overriding and call each of
them multiple times. That is to check subcontracted preconditions
first, than execute the functor specified to `.old(...)` (if present),
and then to check subcontracted postconditions (because preconditions,
old, and postconditions functors are part of the overridden function
definition so they are only accessible by calling the overridden
functions). All these different calls to the overridden functions
should not copy old values multiple times. Using smart pointers
(instead of just boost::optional values) allows the library to copy an
old value and allocate the related pointer only once then pass this
one pointer around the multiple calls to the overridden functions
needed for subcontracting.

I will add rationales 1-3 above the docs (I forgot to list them in the
current docs).

Thanks,
--Lorenzo