eigen/doc/CustomizingEigen.dox

namespace Eigen {

/** \page CustomizingEigen

<h1>Customizing Eigen</h1>

Eigen2 can be extended in several way, for instance, by defining global methods, \ref ExtendingMatrixBase "by adding custom methods to MatrixBase", etc.

\section ExtendingMatrixBase Extending MatrixBase

In this section we will see how to add custom methods to MatrixBase. Since all expressions and matrix types inherit MatrixBase, adding a method to MatrixBase make it immediately available to all expressions ! A typical use case is, for instance, to make Eigen compatible with another API.

You certainly know that in C++ it is not possible to add methods to an extending class. So how that's possible ? Here the trick is to include in the declaration of MatrixBase a file defined by the preprocessor token \c EIGEN_MATRIXBASE_PLUGIN:
\code
class MatrixBase {
  // ...
  #ifdef EIGEN_MATRIXBASE_PLUGIN
  #include EIGEN_MATRIXBASE_PLUGIN
  #endif
};
\endcode
Therefore to extend MatrixBase with you own methods you just have to create a file with your method declaration and define EIGEN_MATRIXBASE_PLUGIN before you include any Eigen's header file.

Here is an example of such an extension file: \n
\b MatrixBaseAddons.h
\code
inline Scalar at(uint i, uint j) const { return this->operator()(i,j); }
inline Scalar& at(uint i, uint j) { return this->operator()(i,j); }
inline Scalar at(uint i) const { return this->operator[](i); }
inline Scalar& at(uint i) { return this->operator[](i); }

inline RealScalar squaredLength() const { return norm2(); }
inline RealScalar length() const { return norm(); }
inline RealScalar invLength(void) const { return fast_inv_sqrt(norm2()); }

template<typename OtherDerived>
inline Scalar squaredDistanceTo(const MatrixBase<OtherDerived>& other) const
{ return (derived() - other.derived()).norm2(); }

template<typename OtherDerived>
inline RealScalar distanceTo(const MatrixBase<OtherDerived>& other) const
{ return ei_sqrt(derived().squaredDistanceTo(other)); }

inline void scaleTo(RealScalar l) { RealScalar vl = norm(); if (vl>1e-9) derived() *= (l/vl); }

inline Transpose<Derived> transposed() {return transpose();}
inline const Transpose<Derived> transposed() const {return transpose();}

inline uint minComponentId(void) const  { int i; minCoeff(&i); return i; }
inline uint maxComponentId(void) const  { int i; maxCoeff(&i); return i; }

template<typename OtherDerived>
void makeFloor(const MatrixBase<OtherDerived>& other) { derived() = derived().cwise().min(other.derived()); }
template<typename OtherDerived>
void makeCeil(const MatrixBase<OtherDerived>& other) { derived() = derived().cwise().max(other.derived()); }

const typename Cwise<Derived>::ScalarAddReturnType
operator+(const Scalar& scalar) const { return cwise() + scalar }

friend const typename Cwise<Derived>::ScalarAddReturnType
operator+(const Scalar& scalar, const MatrixBase<Derived>& mat) { return mat + scalar; }
\endcode

Then one can the following declaration in the config.h or whatever prerequisites header file of his project:
\code
#define EIGEN_MATRIXBASE_PLUGIN "MatrixBaseAddons.h"
\endcode

\section PreprocessorDirectives Preprocessor directives

 - \b EIGEN_DONT_VECTORIZE disables explicit vectorization when defined.
 - \b EIGEN_UNROLLING_LIMIT defines the maximal instruction counts to enable meta unrolling of loops. Set it to zero to disable unrolling. The default is 100.
 - \b EIGEN_TUNE_FOR_L2_CACHE_SIZE represents the maximal size in Bytes of L2 blocks. Since several blocks have to stay concurently in L2 cache, this value should correspond to at most 1/4 of the size of L2 cache.

*/

}