[BCRSMatrix] Improve error handling for implicit build mode parameters

This patch adds checks to make sure the user has actually set valid
parameters before trying to allocate the matrix in implicit mode.
We use a negative value for the overflow fraction as an indicator for
this.

As part of the cleanup, this patch also makes sure that avg and
overflowsize are always initialized to a well-defined value.

dune/istl/bcrsmatrix.hh

@@ -682,15 +682,20 @@ namespace Dune {
 
     //===== constructors & resizers
 
+    // we use a negative overflowsize to indicate that the implicit
+    // mode parameters have not been set yet
+
     //! an empty matrix
     BCRSMatrix ()
       : build_mode(unknown), ready(notbuilt), n(0), m(0), nnz(0),
-        r(0), a(0)
+        r(0), a(0),
+        avg(0), overflowsize(-1.0)
     {}
 
     //! matrix with known number of nonzeroes
     BCRSMatrix (size_type _n, size_type _m, size_type _nnz, BuildMode bm)
       : build_mode(bm), ready(notbuilt)
+      , avg(0), overflowsize(-1.0)
     {
       allocate(_n, _m, _nnz);
     }
@@ -698,6 +703,7 @@ namespace Dune {
     //! matrix with unknown number of nonzeroes
     BCRSMatrix (size_type _n, size_type _m, BuildMode bm)
       : build_mode(bm), ready(notbuilt)
+      , avg(0), overflowsize(-1.0)
     {
       allocate(_n, _m);
     }
@@ -716,6 +722,14 @@ namespace Dune {
     BCRSMatrix (size_type _n, size_type _m, size_type _avg, double _overflowsize, BuildMode bm)
       : build_mode(bm), ready(notbuilt), avg(_avg), overflowsize(_overflowsize)
     {
+      if (bm != implicit)
+        DUNE_THROW(BCRSMatrixError,"Only call this constructor when using the implicit build mode");
+      // Prevent user from setting a negative overflowsize:
+      // 1) It doesn't make sense
+      // 2) We use a negative overflow value to indicate that the parameters
+      //    have not been set yet
+      if (_overflowsize < 0.0)
+        DUNE_THROW(BCRSMatrixError,"You cannot set a negative overflow fraction");
       implicit_allocate(_n,_m);
     }
 
@@ -726,6 +740,7 @@ namespace Dune {
      */
     BCRSMatrix (const BCRSMatrix& Mat)
       : n(Mat.n), nnz(0)
+      , avg(Mat.avg), overflowsize(Mat.overflowsize)
     {
       // deep copy in global array
       size_type _nnz = Mat.nnz;
@@ -808,6 +823,12 @@ namespace Dune {
      */
     void setImplicitBuildModeParameters(size_type _avg, double _overflow)
     {
+      // Prevent user from setting a negative overflowsize:
+      // 1) It doesn't make sense
+      // 2) We use a negative overflow value to indicate that the parameters
+      //    have not been set yet
+      if (_overflow < 0.0)
+        DUNE_THROW(BCRSMatrixError,"You cannot set a negative overflow fraction");
       avg = _avg;
       overflowsize = _overflow;
     }
@@ -1923,6 +1944,9 @@ namespace Dune {
      */
     void implicit_allocate(size_type _n, size_type _m)
     {
+      // check to make sure the user has actually set the parameters
+      if (overflowsize < 0)
+        DUNE_THROW(InvalidStateException,"You have to set the implicit build mode parameters before starting to build the matrix");
       //calculate size of overflow region, add buffer for row sort!
       size_type osize = (size_type) (_n*avg)*overflowsize + 4*avg;
       allocationSize = _n*avg + osize;