add a new range adaptor boost::adaptors::ref_unwrapped

The new adaptor is like boost::adaptors::indirected, but for
std::reference_wrapper values (instead of pointer). It calls
.get() on every value and returns the result. It is useful for
range-based iteration of ranges of std::reference_wrapper (or
related) types.

test/adaptor_test/ref_unwrapped.cpp

@@ -0,0 +1,101 @@
+// Boost.Range library
+//
+//  Copyright Robin Eckert 2015. Use, modification and distribution is
+//  subject to the Boost Software License, Version 1.0. (See
+//  accompanying file LICENSE_1_0.txt or copy at
+//  http://www.boost.org/LICENSE_1_0.txt)
+//
+//
+// For more information, see http://www.boost.org/libs/range/
+//
+#include <boost/range/adaptor/ref_unwrapped.hpp>
+
+#define BOOST_TEST_MAIN
+
+#include <boost/test/test_tools.hpp>
+#include <boost/test/unit_test.hpp>
+
+#include <vector>
+
+#if !defined(BOOST_NO_CXX11_AUTO_DECLARATIONS) && !defined(BOOST_NO_CXX11_RANGE_BASED_FOR)
+
+namespace boost
+{
+
+    BOOST_AUTO_TEST_CASE(test_mutable)
+    {
+        int one = 1;
+        int two = 2;
+        int three = 3;
+
+        std::vector<std::reference_wrapper<int>> input_values{one, two, three};
+
+        const std::vector<int*> expected{&one, &two, &three};
+        std::vector<int*> actual;
+
+        for (auto&& value : input_values | adaptors::ref_unwrapped)
+        {
+          actual.push_back(&value);
+        }
+
+        BOOST_CHECK_EQUAL_COLLECTIONS(expected.begin(),
+                                      expected.end(),
+                                      actual.begin(),
+                                      actual.end());
+    }
+
+    BOOST_AUTO_TEST_CASE(test_const_range)
+    {
+        int one = 1;
+        int two = 2;
+        int three = 3;
+
+        const std::vector<std::reference_wrapper<int>> input_values{one, two, three};
+
+        const std::vector<int*> expected{&one, &two, &three};
+        std::vector<int*> actual;
+
+        for (auto&& value : input_values | adaptors::ref_unwrapped)
+        {
+          actual.push_back(&value);
+        }
+
+        BOOST_CHECK_EQUAL_COLLECTIONS(expected.begin(),
+                                      expected.end(),
+                                      actual.begin(),
+                                      actual.end());
+    }
+
+    BOOST_AUTO_TEST_CASE(test_const_reference)
+    {
+        const int one = 1;
+        const int two = 2;
+        const int three = 3;
+
+        const std::vector<std::reference_wrapper<const int>> input_values{one, two, three};
+
+        const std::vector<const int*> expected{&one, &two, &three};
+        std::vector<const int*> actual;
+
+        for (auto&& value : input_values | adaptors::ref_unwrapped)
+        {
+          actual.push_back(&value);
+        }
+
+        BOOST_CHECK_EQUAL_COLLECTIONS(expected.begin(),
+                                      expected.end(),
+                                      actual.begin(),
+                                      actual.end());
+    }
+
+
+}
+
+#else
+
+BOOST_AUTO_TEST_CASE(empty)
+{
+  // C++11 only
+}
+
+#endif