namespace Eigen { /** \page TutorialMapClass Tutorial page 10 - Interfacing with C/C++ arrays and external libraries: the %Map class \ingroup Tutorial \li \b Previous: \ref TutorialSparse \li \b Next: \ref TODO This tutorial page explains how to work with "raw" C++ arrays. This can be useful in a variety of contexts, particularly when "importing" vectors and matrices from other libraries into Eigen. \b Table \b of \b contents - \ref TutorialMapIntroduction - \ref TutorialMapTypes - \ref TutorialMapUsing - \ref TutorialMapPlacementNew \section TutorialMapIntroduction Introduction Occasionally you may have a pre-defined array of numbers that you want to use within Eigen as a vector or matrix. While one option is to make a copy of the data, most commonly you probably want to re-use this memory as an Eigen type. Fortunately, this is very easy with the Map class. \section TutorialMapTypes Map types and declaring Map variables A Map object has a type defined by its Eigen equivalent: \code Map > \endcode Note that, in this default case, a Map requires just a single template parameter. To construct a Map variable, you need two other pieces of information: a pointer to the region of memory defining the array of coefficients, and the desired shape of the matrix or vector. For example, to define a matrix of \c float with sizes determined at compile time, you might do the following: \code Map mf(pf,rows,columns); \endcode where \c pf is a \c float \c * pointing to the array of memory. A fixed-size read-only vector of integers might be declared as \code Map mi(pi); \endcode where \c pi is an \c int \c *. In this case the size does not have to be passed to the constructor, because it is already specified by the Matrix/Array type. Note that Map does not have a default constructor; you \em must pass a pointer to intialize the object. However, you can work around this requirement (see \ref TutorialMapPlacementNew). Map is flexible enough to accomodate a variety of different data representations. There are two other (optional) template parameters: \code Map \endcode \li \c MapOptions specifies whether the pointer is \c #Aligned, or \c #Unaligned. The default is \c #Unaligned. \li \c StrideType allows you to specify a custom layout for the memory array, using the Stride class. One example would be to specify that the data array is organized in row-major format:
Example:Output:
\include Tutorial_Map_rowmajor.cpp \verbinclude Tutorial_Map_rowmajor.out
However, Stride is even more flexible than this; for details, see the documentation for the Map and Stride classes. \section TutorialMapUsing Using Map variables You can use a Map object just like any other Eigen type:
Example:Output:
\include Tutorial_Map_using.cpp \verbinclude Tutorial_Map_using.out
However, when writing functions taking Eigen types, it is important to realize that a Map type is \em not identical to its Dense equivalent. See \ref TopicFunctionTakingEigenTypesMultiarguments for details. \section TutorialMapPlacementNew Changing the mapped array It is possible to change the array of a Map object after declaration, using the C++ "placement new" syntax:
Example:Output:
\include Map_placement_new.cpp \verbinclude Map_placement_new.out
Despite appearances, this does not invoke the memory allocator, because the syntax specifies the location for storing the result. This syntax makes it possible to declare a Map object without first knowing the mapped array's location in memory: \code Map A(NULL); // don't try to use this matrix yet! VectorXf b(n_matrices); for (int i = 0; i < n_matrices; i++) { new (&A) Map(get_matrix_pointer(i)); b(i) = A.trace(); } \endcode \li \b Next: \ref TODO */ }