Implement operator[] for random-access to matrix rows

We use the same trick with std::integral_constant as in the MultiTypeBlockVector code.

dune/istl/multitypeblockmatrix.hh

@@ -232,6 +232,42 @@ namespace Dune {
       return FirstRow::size();
     }
 
+    /** \brief Random-access operator
+     *
+     * This method mimicks the behavior of normal vector access with square brackets like, e.g., m[5] = ....
+     * The problem is that the return type is different for each value of the argument in the brackets.
+     * Therefore we implement a trick using std::integral_constant.  To access the first row of
+     * a MultiTypeBlockMatrix named m write
+     * \code
+     *  std::integral_constant<int,0> _0;
+     *  m[_0] = ...
+     * \endcode
+     * The name '_0' used here as a static replacement of the integer number zero is arbitrary.
+     * Any other variable name can be used.  If you don't like the separate variable, you can writee
+     * \code
+     *  m[std::integral_constant<int,0>()] = ...
+     * \endcode
+     */
+    template< int index >
+    typename mpl::at_c<type,index>::type&
+    operator[] ( const std::integral_constant< int, index > indexVariable )
+    {
+      DUNE_UNUSED_PARAMETER(indexVariable);
+      return fusion::at_c<index>(*this);
+    }
+
+   /** \brief Const random-access operator
+     *
+     * This is the const version of the random-access operator.  See the non-const version for a full
+     * explanation of how to use it.
+     */
+    template< int index >
+    const typename mpl::at_c<type,index>::type&
+    operator[] ( const std::integral_constant< int, index > indexVariable ) const
+    {
+      DUNE_UNUSED_PARAMETER(indexVariable);
+      return fusion::at_c<index>(*this);
+    }
     /**
      * assignment operator
      */

dune/istl/test/multitypeblockmatrixtest.cc

@@ -46,21 +46,22 @@ int main(int argc, char** argv) try
 
   MultiTypeBlockMatrix<RowType0,RowType1> multiMatrix;
 
-  fusion::at_c<0>(multiMatrix)[_0].setSize(3,3);
-  fusion::at_c<0>(multiMatrix)[_1].setSize(3,2);
-  fusion::at_c<1>(multiMatrix)[_0].setSize(2,3);
-  fusion::at_c<1>(multiMatrix)[_1].setSize(2,2);
+  //fusion::at_c<0>(multiMatrix)[_0].setSize(3,3);
+  multiMatrix[_0][_0].setSize(3,3);
+  multiMatrix[_0][_1].setSize(3,2);
+  multiMatrix[_1][_0].setSize(2,3);
+  multiMatrix[_1][_1].setSize(2,2);
 
   // lazy solution: initialize the entire matrix with zeros
-  fusion::at_c<0>(multiMatrix)[_0] = 0;
-  fusion::at_c<0>(multiMatrix)[_1] = 0;
-  fusion::at_c<1>(multiMatrix)[_0] = 0;
-  fusion::at_c<1>(multiMatrix)[_1] = 0;
+  multiMatrix[_0][_0] = 0;
+  multiMatrix[_0][_1] = 0;
+  multiMatrix[_1][_0] = 0;
+  multiMatrix[_1][_1] = 0;
 
-  printmatrix(std::cout, fusion::at_c<0>(multiMatrix)[_0], "(0,0)", "--");
-  printmatrix(std::cout, fusion::at_c<0>(multiMatrix)[_1], "(0,1)", "--");
-  printmatrix(std::cout, fusion::at_c<1>(multiMatrix)[_0], "(1,0)", "--");
-  printmatrix(std::cout, fusion::at_c<1>(multiMatrix)[_1], "(1,1)", "--");
+  printmatrix(std::cout, multiMatrix[_0][_0], "(0,0)", "--");
+  printmatrix(std::cout, multiMatrix[_0][_1], "(0,1)", "--");
+  printmatrix(std::cout, multiMatrix[_1][_0], "(1,0)", "--");
+  printmatrix(std::cout, multiMatrix[_1][_1], "(1,1)", "--");
 
   // set up a test vector
   MultiTypeBlockVector<BlockVector<FieldVector<double,3> >, BlockVector<FieldVector<double,1> > > multiVector;
@@ -174,10 +175,10 @@ int main(int argc, char** argv) try
   typedef Dune::MultiTypeBlockVector<BCRSMat,BCRSMat> BCRS_Row;
   typedef Dune::MultiTypeBlockMatrix<BCRS_Row,BCRS_Row> CM_BCRS;
   CM_BCRS A;
-  fusion::at_c<0>(A)[_0] = A11;
-  fusion::at_c<0>(A)[_1] = A12;
-  fusion::at_c<1>(A)[_0] = A21;
-  fusion::at_c<1>(A)[_1] = A22;
+  A[_0][_0] = A11;
+  A[_0][_1] = A12;
+  A[_1][_0] = A21;
+  A[_1][_1] = A22;
 
   x = 1;
   b = 1;