Re: [boost] [contract] extra type requirements for contracts

Subject: Re: [boost] [contract] extra type requirements for contracts
From: lcaminiti (lorcaminiti_at_[hidden])
Date: 2011-09-20 10:57:26

Dave Abrahams wrote:
> 
> on Mon Sep 19 2011, lcaminiti <lorcaminiti-AT-gmail.com> wrote:
> 
>> This is a general issue with contracts. In general, an arbitrary complex
>> (const) expression can be used to assert preconditions, postconditions,
>> or
>> invariants. Such a complex assertion might introduce an arbitray set of
>> extra requirements on the types used, requirements that might not be
>> necessary to implement the function body. The extra type requirements
>> introduced by the contracts are especially problematic when programming
>> templates because the used types are generic and it is not know a priori
>> to
>> the function programmer if the generic types will or not satisfy the
>> requirements.
> 
> If the contract can't be expressed within the constraints, the
> constraints are broken, period.
> 

But this means that if you add contracts to vector<T> then you cannot use it
unless T is EqualityComparable... I don't think that will be acceptable to
Boost.Contract users... will it be acceptable?

> Now, however, the standard contains special hand-wavey language that
> says, essentially, "when we use a == b to describe a postcondition we
> don't literally mean ==.  We mean that a and b are equivalent" (whatever
> that means).  So even if T supports == it doesn't necessarily mean the
> standard is going to use it.  I think what it really means is that b is
> indistinguishable from any other copy of a.
> 
> ...which doesn't help you at all in terms of deciding what to actually
> /do/, I know... hopefully, though, it muddies the water a bit just in
> case it was too clear for you up till now ;-)
> 

I will study this part of the standard.

Thanks,
--Lorenzo

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