diff --git a/tests/MessageBuffer.cpp b/tests/MessageBuffer.cpp
new file mode 100644
index 000000000..e0c0f73c4
--- /dev/null
+++ b/tests/MessageBuffer.cpp
@@ -0,0 +1,60 @@
+/*
+  Copyright (c) 2020 Equinor ASA
+
+  This file is part of the Open Porous Media project (OPM).
+
+  OPM is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  OPM is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with OPM.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <sstream>
+
+class MessageBuffer
+{
+private:
+    std::stringstream str_{};
+
+public:
+    template <class T>
+    void read(T& value)
+    {
+        this->str_.read(reinterpret_cast<char *>(&value), sizeof value);
+    }
+
+    template <class T>
+    void write(T const& value)
+    {
+        this->str_.write(reinterpret_cast<const char *>(&value), sizeof value);
+    }
+
+    void write(const std::string& str)
+    {
+        const int size = str.size();
+        this->write(size);
+        for (int k = 0; k < size; ++k) {
+            this->write(str[k]);
+        }
+    }
+
+    void read(std::string& str)
+    {
+        int size = 0;
+        this->read(size);
+        str.resize(size);
+        for (int k = 0; k < size; ++k) {
+            this->read(str[k]);
+        }
+    }
+};
+
+
diff --git a/tests/test_data_GuideRateValue.cpp b/tests/test_data_GuideRateValue.cpp
index c4d1487e5..97f46c314 100644
--- a/tests/test_data_GuideRateValue.cpp
+++ b/tests/test_data_GuideRateValue.cpp
@@ -23,48 +23,10 @@
 
 #include <opm/output/data/GuideRateValue.hpp>
 
+#include "tests/MessageBuffer.cpp"
+
 BOOST_AUTO_TEST_SUITE(GuideRate_Values)
 
-namespace {
-    class MessageBuffer
-    {
-    private:
-        std::stringstream str_{};
-
-    public:
-        template <class T>
-        void read(T& value)
-        {
-            this->str_.read(reinterpret_cast<char *>(&value), sizeof value);
-        }
-
-        template <class T>
-        void write(T const& value)
-        {
-            this->str_.write(reinterpret_cast<const char *>(&value), sizeof value);
-        }
-
-        void write(const std::string& str)
-        {
-            const int size = str.size();
-            this->write(size);
-            for (int k = 0; k < size; ++k) {
-                this->write(str[k]);
-            }
-        }
-
-        void read(std::string& str)
-        {
-            int size = 0;
-            this->read(size);
-            str.resize(size);
-            for (int k = 0; k < size; ++k) {
-                this->read(str[k]);
-            }
-        }
-    };
-}
-
 BOOST_AUTO_TEST_CASE(Construct)
 {
     using GRValue = ::Opm::data::GuideRateValue;