Re: [boost] [odeint] Documentation comments

Subject: Re: [boost] [odeint] Documentation comments
From: Karsten Ahnert (karsten.ahnert_at_[hidden])
Date: 2012-09-27 01:11:20

On 09/27/2012 01:42 AM, John Bytheway wrote:
> On 25/09/12 22:38, Mario Mulansky wrote:
>> John Bytheway <jbytheway+boost_at_[hidden]> wrote:
>>> The more general idea of automatically dispatching to the right
>>> algebra is more dangerous, because for example it is entirely
>>> possible to have a type which is both a Fusion sequence and a Range
>>> (but with different semantics in each interpretation). So, I am not
>>> suggesting that (though I wouldn't argue against it either).
>>
>> This would be an edge case where explicit user definition is 
>> required, but i think in 99% an algebra dispatcher would be 
>> sufficient.
> 
> On further reflection I thought of a particular case where this happens.
> Any std::pair<Iterator, Iterator> is both a Fusion Sequence and a Range,
> with very different semantics.  Just something to consider.

Having a range as state_type is difficult maybe even impossible, since
ranges can not be resized. A range models something that already exists
and inside the stepper a temporary must be created. (But a range can be
passed to the do_step method.)

Nevertheless, it is possible the create a dispatcher like

template< class State , class Enabler = void >
struct algebra_dispatcher;

template< class T , class A >
struct algebra_dispatcher< std::vector< T , A > , void >
{
    typedef range_algebra type;
};

// this will create errors for ranges of pairs of iterators
template< class R >
struct algebra_dispatcher< R ,
       typename boost::fusion::is_sequence< R >::type >
{
    typedef fusion_algebra type;
};

The steppers simply get the value of the algebra dispatcher as a default
parameter:

template< class State , ... ,
          class Algebra = typename algebra_dispatcher< State >::type ,
          ... >
struct stepper { ... };

Then, the user will mostly get the right algebra, and in case this is
not possible he can easily specify the desired one. (The same should be
done for the operations.)