Provide stream that ignores everything written to it

Use this stream when you want a straight code path, but also be able
to disable output at will.

opm/core/utility/NullStream.cpp

@@ -0,0 +1,20 @@
+// Copyright (C) 2013 Uni Research AS
+// This file is licensed under the GNU General Public License v3.0
+
+#include <opm/core/utility/NullStream.hpp>
+#include <ostream>
+#include <streambuf>
+
+// buffer that ignores everything
+// see <http://forums.codeguru.com/showthread.php?460071-ostream-bit-bucket>
+struct NullBuf : public std::streambuf {};
+static NullBuf null_buf_impl;
+
+// link the stream up to the black hole buffer
+struct NullStream : public std::ostream {
+    NullStream () : std::ostream (&null_buf_impl) {}
+};
+
+// create a singleton and point the reference to it
+static NullStream null_stream_impl;
+std::ostream& Opm::null_stream = null_stream_impl;

opm/core/utility/NullStream.hpp

@@ -0,0 +1,30 @@
+#ifndef OPM_NULLSTREAM_HEADER_INCLUDED
+#define OPM_NULLSTREAM_HEADER_INCLUDED
+
+// Copyright (C) 2013 Uni Research AS
+// This file is licensed under the GNU General Public License v3.0
+
+#include <iosfwd>
+
+namespace Opm {
+
+/**
+ * Output stream that ignores everything written to it.
+ *
+ * Use this stream if you want to disable output without having a
+ * lot of conditionals in your code.
+ *
+ * Since the null stream has no state, there is no point in
+ * instantiating your own; simply use this reference instead.
+ *
+ * @example
+ * @code{.cpp}
+ *   std::ostream& outp = (quiet ? Opm::null_stream : std::cerr);
+ *   outp << "Hello, World!" << std::endl;
+ * @endcode
+ */
+extern std::ostream& null_stream;
+
+} /* namespace Opm */
+
+#endif /* OPM_NULLSTREAM_HEADER_INCLUDED */