[compatibility] allow to disable the compatibility layer

depending on a preprocessor define we switch the base classes to be
pure virtual, or implement a dummy category(). If the classes are pure
virtual the old interface with an enum is not possible anymore.

To enable/disable the backwards compatibility check we provide a cmake
flag DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE, which defaults to 1
(e.g. backwards compatibility enabled).

cmake/modules/DuneIstlMacros.cmake

@@ -12,3 +12,7 @@ find_package(ARPACKPP)
 include(AddARPACKPPFlags)
 find_package(SuiteSparse OPTIONAL_COMPONENTS LDL SPQR UMFPACK)
 include(AddSuiteSparseFlags)
+
+# enable / disable backwards compatibility w.r.t. category
+set(DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE 1
+  "Enable/Disable the backwards compatibility of the category enum/method in dune-istl solvers, preconditioner, etc. '1'")

dune/istl/operators.hh

@@ -84,9 +84,13 @@ namespace Dune {
 
     //! Category of the linear operator (see SolverCategory::Category)
     virtual SolverCategory::Category category() const
+#ifdef DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE
     {
       DUNE_THROW(Dune::Exception,"It is necessary to implement the category method in a derived classes, in the future this method will pure virtual.");
     };
+#else
+    = 0;
+#endif
   };
 
 

dune/istl/preconditioner.hh

@@ -77,9 +77,13 @@ namespace Dune {
 
     //! Category of the preconditioner (see SolverCategory::Category)
     virtual SolverCategory::Category category() const
+#ifdef DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE
     {
       DUNE_THROW(Dune::Exception,"It is necessary to implement the category method in a derived classes, in the future this method will pure virtual.");
     }
+#else
+    = 0;
+#endif
 
     //! every abstract base class has a virtual destructor
     virtual ~Preconditioner () {}

dune/istl/solver.hh

@@ -128,9 +128,13 @@ namespace Dune
 
     //! Category of the solver (see SolverCategory::Category)
     virtual SolverCategory::Category category() const
+#ifdef DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE
     {
       DUNE_THROW(Dune::Exception,"It is necessary to implement the category method in a derived classes, in the future this method will pure virtual.");
     }
+#else
+    = 0;
+#endif
 
     //! \brief Destructor
     virtual ~InverseOperator () {}