add "remap" snippet using placement new

Eigen/src/Core/Map.h

@@ -40,6 +40,12 @@
   * It can be used to let Eigen interface without any overhead with non-Eigen data structures,
   * such as plain C arrays or structures from other libraries.
   * 
+  * \b Tips: to change the array of data mapped by a Map object, you can use the C++
+  * placement new syntax:
+  *
+  * Example: \include Map_placement_new.cpp
+  * Output: \verbinclude Map_placement_new.out
+  *
   * This class is the return type of Matrix::Map() but can also be used directly.
   *
   * \sa Matrix::Map()

doc/snippets/Map_placement_new.cpp

@@ -0,0 +1,5 @@
+int data[] = {1,2,3,4,5,6,7,8,9};
+Map<RowVectorXi> v(data,4);
+cout << "The mapped vector v is: " << v << "\n";
+new (&v) Map<RowVectorXi>(data+4,5);
+cout << "Now v is: " << v << "\n";