Re: [eigen] Matrix::RowXpr::Nested is a reference...

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Ah yes and for plain matrices, you really want to pass them by const
reference, so you'd still have to edit ei_nested to handle that case
separately, with a condition like:

  ei_is_same_type<T,  typename T::PlainMatrixType>::ret

Good luck ;)

Benoit

2009/10/2 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> 2009/10/1 Hauke Heibel <hauke.heibel@xxxxxxxxxxxxxx>:
>> Is that correct as is?
>
> Yes. Check in Core/util/XprHelper.h:
>
> template< blabla > struct ei_nested
> {
>  blabla
>
>  typedef typename ei_meta_if<
>    ei_must_nest_by_value<T>::ret,
>    T,
>    typename ei_meta_if<
>      (int(ei_traits<T>::Flags) & EvalBeforeNestingBit)
>      || ( int(CostEval) <= int(CostNoEval) ),
>      PlainMatrixType,
>      const T&
>    >::ret
>  >::ret type;
> };
>
> See the const T& above ? That's the solution used when doing
> ei_nested<RowXpr> by default.
>
>> In the following case
>>
>> Replicated<MatrixXd::RowXpr,2,1>(matrix.row(0))
>>
>> it means that it fails since matrix.row(0) returns a temporary to which a
>> reference is stored in Replicate.
>
> Yes, this is true.
>
> In most cases this is OK as the temporary is alive long enough.
>
> There are some corner cases where this is NOT OK for example if you
> want to write a function that returns such a Replicate object.
>
> For these corner cases, we have .nestByValue() which allows you to
> force that the row() object is nested by value.
>
> Now I don't remember for sure the exact reasons why we nest by
> reference by default, as indeed these are lightweight objects. I think
> that was just to avoid useless copying of objects which we feared
> would confuse the compiler. It's crucial that the compiler understands
> that it can inline all these objects and not actually construct them
> in memory.
>
> So it's that's only that, then, feel free with replacing const T& by T
> here and checking if the compiler still generates good code. If yes,
> then that sounds like great news, as we would then be able to get rid
> of NestByValue.
>
> Benoit
>
>>
>> I used this piece of code to verify my assumption:
>>
>> #include "Eigen/Core"
>>
>> #include "boost/type_traits/is_reference.hpp"
>>
>> void main()
>> {
>>   typedef Eigen::MatrixXd::RowXpr::Nested type;
>>
>>   std::cout << typeid(type).name();
>>   if (boost::is_reference<type>::type::value == true) std::cout << "&";
>>   std::cout << std::endl;
>> }
>>
>> Cheers,
>> Hauke
>>
>