Re: [eigen] Minimal ei_traits?

[Manoj Rajagopalan <rmanoj@xxxxxxxxx>]

I am attempting a SymmetricMatrix implementation modeled on what Eigen already 
does - hence the ei_traits question. I need this class because I really want 
to store only the triangular part - my application runs out of memory 
quickly. I am trying the rectangular packed format that Gael pointed out in 
an earlier mail.

   Any suggestions, particularly on the class hierarchy, would be welcome. I 
am attaching a graphviz-generated diagram for comments from everyone. Maybe I 
can even submit a patch if I get it working :-)

Thanks,
Manoj



On Wednesday 30 June 2010 03:36:11 pm Benoit Jacob wrote:
> There is, and you can find that in Macros.h, in
> EIGEN_DENSE_PUBLIC_INTERFACE, which is basically where ei_traits are
> used:
>
> #define EIGEN_DENSE_PUBLIC_INTERFACE(Derived) \
>   typedef typename Eigen::ei_traits<Derived>::Scalar Scalar; /*!<
> \brief Numeric type, e.g. float, double, int or std::complex<float>.
> */ \
>   typedef typename Eigen::NumTraits<Scalar>::Real RealScalar; /*!<
> \brief The underlying numeric type for composed scalar types. \details
> In cases where Scalar is e.g. std::complex<T>, T were corresponding to
> RealScalar. */ \
>   typedef typename Base::PacketScalar PacketScalar; \
>   typedef typename Base::CoeffReturnType CoeffReturnType; /*!< \brief
> The return type for coefficient access. \details Depending on whether
> the object allows direct coefficient access (e.g. for a MatrixXd),
> this type is either 'const Scalar&' or simply 'Scalar' for objects
> that do not allow direct coefficient access. */ \
>   typedef typename Eigen::ei_nested<Derived>::type Nested; \
>   typedef typename Eigen::ei_traits<Derived>::StorageKind StorageKind; \
>   typedef typename Eigen::ei_traits<Derived>::Index Index; \
>   enum { RowsAtCompileTime = Eigen::ei_traits<Derived>::RowsAtCompileTime,
> \ ColsAtCompileTime = Eigen::ei_traits<Derived>::ColsAtCompileTime, \
> MaxRowsAtCompileTime =
> Eigen::ei_traits<Derived>::MaxRowsAtCompileTime, \
>         MaxColsAtCompileTime =
> Eigen::ei_traits<Derived>::MaxColsAtCompileTime, \
>         Flags = Eigen::ei_traits<Derived>::Flags, \
>         CoeffReadCost = Eigen::ei_traits<Derived>::CoeffReadCost, \
>         SizeAtCompileTime = Base::SizeAtCompileTime, \
>         MaxSizeAtCompileTime = Base::MaxSizeAtCompileTime, \
>         IsVectorAtCompileTime = Base::IsVectorAtCompileTime }; \
>   using Base::derived; \
>   using Base::const_cast_derived;
>
>
> Notice that NOT all of the above lines refer to stuff in ei_traits.
> Those that do, tell you what ei_traits must provide.
>
> But let me take this occasion to insist that the ONLY place where you
> have to use ei_traits, is if you are hacking Eigen and adding or
> modifying some base class underneath some expression class. If you are
> just trying to get compile-time information on a given Eigen
> expression type (or even if you are just coding some new expression
> class for Eigen), you can access these typedefs and enum values right
> off this type:
>
> do:
>
>      enum { myvalue = Derived::RowsAtCompileTime };
>      typedef typename Derived::Scalar mytype;
>
> and NOT (unless you are hacking Eigen's own base classes):
>
>      enum { myvalue = ei_traits<Derived>::RowsAtCompileTime };
>      typedef typename ei_traits<Derived>::Scalar mytype;
>
> The only case where you have to do the latter, is when Derived is not
> fully defined. You might ask how could this possibly happen? This
> happens when you are inside of a template Base<Derived> class and
> Derived inherits Base<Derived>. See the link I just mentioned about
> the CRTP.
>
> Now I know that there are many places in Eigen where we use ei_traits,
> and that's unnecessary, so that's bad. A patch fixing that is always
> welcome. Only use ei_traits if it's needed in order to compile.
>
> Benoit
>
> 2010/6/30 Manoj Rajagopalan <rmanoj@xxxxxxxxx>:
> > hi eigen developers,
> >
> >   It is a little difficult to trace the various ei_traits because of
> > their distribution and levels of inheritance? Is there a minimum set of
> > typedefs, enums etc. that any ei_traits must
> > declare/define/statically-compute?
> >
> > thanks,
> > Manoj

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