From 2a0e76b08235c0e6c6a0686aab66ca1cf0688feb Mon Sep 17 00:00:00 2001
From: ESeNonFossiIo <esenonfossiio@gmail.com>
Date: Tue, 2 Jun 2015 12:39:38 +0200
Subject: [PATCH] null operator

---
 include/deal.II/lac/linear_operator.h         | 42 +++++++++++++++++++
 tests/lac/linear_operator_01.cc               |  7 ++++
 .../linear_operator_01.with_cxx11=on.output   |  1 +
 3 files changed, 50 insertions(+)

diff --git a/include/deal.II/lac/linear_operator.h b/include/deal.II/lac/linear_operator.h
index e74aa2cf9a..5992a91765 100644
--- a/include/deal.II/lac/linear_operator.h
+++ b/include/deal.II/lac/linear_operator.h
@@ -682,6 +682,48 @@ identity_operator(const std::function<void(Range &, bool)> &reinit_vector)
 }
 
 
+/**
+ * @relates LinearOperator
+ *
+ * Returns a LinearOperator that is the null operator of the vector space
+ * @p Range.
+ *
+ * The function takes an <code>std::function</code> object @ref
+ * reinit_vector as an argument to initialize the
+ * <code>reinit_range_vector</code> and <code>reinit_domain_vector</code>
+ * objects of the LinearOperator object.
+ *
+ * @ingroup LAOperators
+ */
+template <typename Range>
+LinearOperator<Range, Range>
+null_operator(const std::function<void(Range &, bool)> &reinit_vector)
+{
+  LinearOperator<Range, Range> return_op;
+
+  return_op.reinit_range_vector = reinit_vector;
+  return_op.reinit_domain_vector = reinit_vector;
+
+  return_op.vmult = [](Range &v, const Range &u)
+  {
+    v = 0;
+  };
+
+  return_op.vmult_add = [](Range &v, const Range &u)
+  {};
+
+  return_op.Tvmult = [](Range &v, const Range &u)
+  {
+    v = 0;
+  };
+
+  return_op.Tvmult_add = [](Range &v, const Range &u)
+  {};
+
+  return return_op;
+}
+
+
 namespace internal
 {
   namespace LinearOperator
diff --git a/tests/lac/linear_operator_01.cc b/tests/lac/linear_operator_01.cc
index 31eb357d3b..2ed27d9611 100644
--- a/tests/lac/linear_operator_01.cc
+++ b/tests/lac/linear_operator_01.cc
@@ -229,4 +229,11 @@ int main()
   auto test3 = identity_operator(test2.reinit_range_vector) + test2;
   test3.vmult(w, u);
   deallog << "(1 + 2 * 4) * " << u.value << " = " << w.value << std::endl;
+
+  // null operator
+
+  auto test4 = null_operator(test2.reinit_range_vector);
+  test4.vmult(w, u);
+  deallog << " 0 * " << u.value << " = " << w.value << std::endl;
+
 }
diff --git a/tests/lac/linear_operator_01.with_cxx11=on.output b/tests/lac/linear_operator_01.with_cxx11=on.output
index 2bceae3a83..c910ee55b3 100644
--- a/tests/lac/linear_operator_01.with_cxx11=on.output
+++ b/tests/lac/linear_operator_01.with_cxx11=on.output
@@ -17,3 +17,4 @@ DEAL::(4 * 4) * 24.0000 = 384.000
 DEAL::(2 * 4) * 24.0000 = 192.000
 DEAL::(2 * 4) * 1 * 24.0000 = 192.000
 DEAL::(1 + 2 * 4) * 24.0000 = 216.000
+DEAL:: 0 * 24.0000 = 0.00000
-- 
2.39.5