AlignmentMatch=(int(traits<PlainObjectType>::Alignment)==int(Unaligned))||(DerivedAlignment>=int(Alignment)),// FIXME the first condition is not very clear, it should be replaced by the required alignment
* \brief A matrix or vector expression mapping an existing expression
*
* \tparam PlainObjectType the equivalent matrix type of the mapped data
* \tparam Options specifies the pointer alignment in bytes. It can be: \c #Aligned128, , \c #Aligned64, \c #Aligned32, \c #Aligned16, \c #Aligned8 or \c #Unaligned.
* The default is \c #Unaligned.
* \tparam StrideType optionally specifies strides. By default, Ref implies a contiguous storage along the inner dimension (inner stride==1),
* but accepts a variable outer stride (leading dimension).
* This can be overridden by specifying strides.
* The type passed here must be a specialization of the Stride template, see examples below.
*
* This class provides a way to write non-template functions taking Eigen objects as parameters while limiting the number of copies.
* A Ref<> object can represent either a const expression or a l-value:
* \code
* // in-out argument:
* void foo1(Ref<VectorXf> x);
*
* // read-only const argument:
* void foo2(const Ref<const VectorXf>& x);
* \endcode
*
* In the in-out case, the input argument must satisfy the constraints of the actual Ref<> type, otherwise a compilation issue will be triggered.
* By default, a Ref<VectorXf> can reference any dense vector expression of float having a contiguous memory layout.
* Likewise, a Ref<MatrixXf> can reference any column-major dense matrix expression of float whose column's elements are contiguously stored with
* the possibility to have a constant space in-between each column, i.e. the inner stride must be equal to 1, but the outer stride (or leading dimension)
* can be greater than the number of rows.
*
* In the const case, if the input expression does not match the above requirement, then it is evaluated into a temporary before being passed to the function.
* Here are some examples:
* \code
* MatrixXf A;
* VectorXf a;
* foo1(a.head()); // OK
* foo1(A.col()); // OK
* foo1(A.row()); // Compilation error because here innerstride!=1
* foo2(A.row()); // Compilation error because A.row() is a 1xN object while foo2 is expecting a Nx1 object
* foo2(A.row().transpose()); // The row is copied into a contiguous temporary
* foo2(2*a); // The expression is evaluated into a temporary
* foo2(A.col().segment(2,4)); // No temporary
* \endcode
*
* The range of inputs that can be referenced without temporary can be enlarged using the last two template parameters.
* Here is an example accepting an innerstride!=1:
* \code
* // in-out argument:
* void foo3(Ref<VectorXf,0,InnerStride<> > x);
* foo3(A.row()); // OK
* \endcode
* The downside here is that the function foo3 might be significantly slower than foo1 because it won't be able to exploit vectorization, and will involve more
* expensive address computations even if the input is contiguously stored in memory. To overcome this issue, one might propose to overload internally calling a
* template function, e.g.:
* \code
* // in the .h:
* void foo(const Ref<MatrixXf>& A);
* void foo(const Ref<MatrixXf,0,Stride<> >& A);
*
* // in the .cpp:
* template<typename TypeOfA> void foo_impl(const TypeOfA& A) {
* ... // crazy code goes here
* }
* void foo(const Ref<MatrixXf>& A) { foo_impl(A); }
* void foo(const Ref<MatrixXf,0,Stride<> >& A) { foo_impl(A); }
* \brief Represents a sequence of transpositions (row/column interchange)
*
* \param SizeAtCompileTime the number of transpositions, or Dynamic
* \param MaxSizeAtCompileTime the maximum number of transpositions, or Dynamic. This optional parameter defaults to SizeAtCompileTime. Most of the time, you should not have to specify it.
*
* This class represents a permutation transformation as a sequence of \em n transpositions
* \f$[T_{n-1} \ldots T_{i} \ldots T_{0}]\f$. It is internally stored as a vector of integers \c indices.
* Each transposition \f$ T_{i} \f$ applied on the left of a matrix (\f$ T_{i} M\f$) interchanges
* the rows \c i and \c indices[i] of the matrix \c M.
* A transposition applied on the right (e.g., \f$ M T_{i}\f$) yields a column interchange.
*
* Compared to the class PermutationMatrix, such a sequence of transpositions is what is
* computed during a decomposition with pivoting, and it is faster when applying the permutation in-place.
*
* To apply a sequence of transpositions to a matrix, simply use the operator * as in the following example:
* \code
* Transpositions tr;
* MatrixXf mat;
* mat = tr * mat;
* \endcode
* In this example, we detect that the matrix appears on both side, and so the transpositions
* are applied in-place without any temporary or extra copy.
* \brief Represents a sequence of transpositions (row/column interchange)
*
* \tparam SizeAtCompileTime the number of transpositions, or Dynamic
* \tparam MaxSizeAtCompileTime the maximum number of transpositions, or Dynamic. This optional parameter defaults to SizeAtCompileTime. Most of the time, you should not have to specify it.
*
* This class represents a permutation transformation as a sequence of \em n transpositions
* \f$[T_{n-1} \ldots T_{i} \ldots T_{0}]\f$. It is internally stored as a vector of integers \c indices.
* Each transposition \f$ T_{i} \f$ applied on the left of a matrix (\f$ T_{i} M\f$) interchanges
* the rows \c i and \c indices[i] of the matrix \c M.
* A transposition applied on the right (e.g., \f$ M T_{i}\f$) yields a column interchange.
*
* Compared to the class PermutationMatrix, such a sequence of transpositions is what is
* computed during a decomposition with pivoting, and it is faster when applying the permutation in-place.
*
* To apply a sequence of transpositions to a matrix, simply use the operator * as in the following example:
* \code
* Transpositions tr;
* MatrixXf mat;
* mat = tr * mat;
* \endcode
* In this example, we detect that the matrix appears on both side, and so the transpositions
* are applied in-place without any temporary or extra copy.
