Re: [boost] GSoC: Enforcing Code Feature Requirements

Subject: Re: [boost] GSoC: Enforcing Code Feature Requirements
From: Vicente Botet Escriba (vicente.botet_at_[hidden])
Date: 2010-03-31 09:21:22

joel falcou-3 wrote:
> 
>> Joel, maybe you can elaborate how the Concept/SFINAE approach manages
>> with this problem.
> 
> Sure,
> 
> in my head, we can define Concepts callee ThreadSafe, ExceptionSafe etc 
> and just use BOOST_CONCEPT_REQUIRE on those (or the real C++0x concept 
> maps) and have Concept Violation error when you try to call one function 
> with improper settings. Then, IIRC there was planned to have SFINAE 
> based on concept so we cna alos have function prunning (ie nice 'no such 
> function f() defined' inthose case.
> 
> Using BOOST.CC, I already do that for parallel computing enforcement: i 
> have a type traits that I can specialize per obejct or function type 
> basis and the concept class just check for this concept then I write:
> 
> template<class F,class A>
> BOOST_CONCEPT_REQUIRE( (CalalbleObject<F>)(DataParallel<F>)
>                       , std::vector<boost::result_of<F(A)>::type>
> 		     )
> map( F f, std::vector );
> 
> if f is not a CallableObjec supporting data-parallelism (here I just 
> check that is_dataparallel<F>::value is true) then the function is not 
> permitted.
> 
> Such a mechanism can be applied to what's describe in the article, or am 
> I mistaken ?
> 
> 

Well, the problem is that the constraints on features must be checked on the
caller function. So you will need to add a parameter or a template parameter
for the caller function. This is what I try to avoid.

In your case we will need to have

template<class F,class A, typename CALLER>
BOOST_CONCEPT_REQUIRE(
(CalalbleObject<F>)(DataParallel<F>)(ThreadSafe<CALLER>)
                      , std::vector<boost::result_of<F(A)>::type>
                     )
map( F f, std::vector );

and the user will need to call the function map as follows

void g() {
  map<void g()>(foo, v);
}

On the other side this can not be applied to 3pp function which don't have
specified the constraint using SFINAE.

Hoping my concern will be clear now.
Vicente
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