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