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.)