1namespace Eigen { 2 3/** \eigenManualPage TutorialMapClass Interfacing with raw buffers: the Map class 4 5This page explains how to work with "raw" C/C++ arrays. 6This can be useful in a variety of contexts, particularly when "importing" vectors and matrices from other libraries into %Eigen. 7 8\eigenAutoToc 9 10\section TutorialMapIntroduction Introduction 11 12Occasionally 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. 13 14\section TutorialMapTypes Map types and declaring Map variables 15 16A Map object has a type defined by its %Eigen equivalent: 17\code 18Map<Matrix<typename Scalar, int RowsAtCompileTime, int ColsAtCompileTime> > 19\endcode 20Note that, in this default case, a Map requires just a single template parameter. 21 22To 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: 23\code 24Map<MatrixXf> mf(pf,rows,columns); 25\endcode 26where \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 27\code 28Map<const Vector4i> mi(pi); 29\endcode 30where \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. 31 32Note 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). 33 34Map is flexible enough to accomodate a variety of different data representations. There are two other (optional) template parameters: 35\code 36Map<typename MatrixType, 37 int MapOptions, 38 typename StrideType> 39\endcode 40\li \c MapOptions specifies whether the pointer is \c #Aligned, or \c #Unaligned. The default is \c #Unaligned. 41\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: 42<table class="example"> 43<tr><th>Example:</th><th>Output:</th></tr> 44<tr> 45<td>\include Tutorial_Map_rowmajor.cpp </td> 46<td>\verbinclude Tutorial_Map_rowmajor.out </td> 47</table> 48However, Stride is even more flexible than this; for details, see the documentation for the Map and Stride classes. 49 50\section TutorialMapUsing Using Map variables 51 52You can use a Map object just like any other %Eigen type: 53<table class="example"> 54<tr><th>Example:</th><th>Output:</th></tr> 55<tr> 56<td>\include Tutorial_Map_using.cpp </td> 57<td>\verbinclude Tutorial_Map_using.out </td> 58</table> 59 60All %Eigen functions are written to accept Map objects just like other %Eigen types. However, when writing your own functions taking %Eigen types, this does \em not happen automatically: a Map type is not identical to its Dense equivalent. See \ref TopicFunctionTakingEigenTypes for details. 61 62\section TutorialMapPlacementNew Changing the mapped array 63 64It is possible to change the array of a Map object after declaration, using the C++ "placement new" syntax: 65<table class="example"> 66<tr><th>Example:</th><th>Output:</th></tr> 67<tr> 68<td>\include Map_placement_new.cpp </td> 69<td>\verbinclude Map_placement_new.out </td> 70</table> 71Despite appearances, this does not invoke the memory allocator, because the syntax specifies the location for storing the result. 72 73This syntax makes it possible to declare a Map object without first knowing the mapped array's location in memory: 74\code 75Map<Matrix3f> A(NULL); // don't try to use this matrix yet! 76VectorXf b(n_matrices); 77for (int i = 0; i < n_matrices; i++) 78{ 79 new (&A) Map<Matrix3f>(get_matrix_pointer(i)); 80 b(i) = A.trace(); 81} 82\endcode 83 84*/ 85 86} 87