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.

doc/reference/adaptors.qbk

@@ -172,6 +172,7 @@ rng | boost::adaptors::adaptor_generator
 [include adaptors/indirected.qbk]
 [include adaptors/map_keys.qbk]
 [include adaptors/map_values.qbk]
+[include adaptors/ref_unwrapped.qbk]
 [include adaptors/replaced.qbk]
 [include adaptors/replaced_if.qbk]
 [include adaptors/reversed.qbk]

doc/reference/adaptors/ref_unwrapped.qbk

@@ -0,0 +1,32 @@
+[/
+    Copyright 2015 Robin Eckert
+    Distributed under 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)
+/]
+[section:ref_unwrapped ref_unwrapped]
+
+[table
+    [[Syntax] [Code]]
+    [[Pipe] [`rng | boost::adaptors::ref_unwrapped`]]
+    [[Function] [`boost::adaptors::ref_unwrap(rng)`]]
+]
+
+This adaptor produces a range than applies `.get()` on all values in
+the range. It is useful for iterating ranges of
+`std::reference_wrapper` values or values using similar semantics.
+
+The adaptor is C++11 (and above) only.
+
+* [*Precondition:] The `value_type` of the range has a `.get() const`.
+* [*Postcondition:] For all elements `x` in the returned range, `x` is the result of `y.get()` where `y` is the corresponding element in the original range.
+* [*Range Category:] __single_pass_range__
+* [*Range Return Type:] `boost::unwrap_ref_range<decltype(rng)>`
+* [*Returned Range Category:] The range category of `rng`.
+
+[section:ref_unwrapped_example ref_unwrapped example]
+[import ../../../test/adaptor_test/ref_unwrapped_example.cpp]
+[ref_unwrapped_example]
+[endsect]
+
+This would produce the output `123`.
+[endsect]

include/boost/range/adaptor/ref_unwrapped.hpp

@@ -0,0 +1,102 @@
+// Boost.Range library
+//
+//  Copyright Robin Eckert 2015.
+//  Copyright Thorsten Ottosen, Neil Groves 2006 - 2008. 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/
+//
+
+#ifndef BOOST_RANGE_ADAPTOR_REF_UNWRAPPED_HPP
+#define BOOST_RANGE_ADAPTOR_REF_UNWRAPPED_HPP
+
+#include <boost/range/adaptor/transformed.hpp>
+#include <boost/range/reference.hpp>
+#include <boost/range/concepts.hpp>
+
+#include <utility>
+
+#if !defined(BOOST_NO_CXX11_DECLTYPE)
+
+namespace boost
+{
+    namespace range_detail
+    {
+        struct ref_unwrapped_forwarder {};
+
+        template<class SinglePassRange>
+        struct unwrap_ref
+        {
+            typedef BOOST_DEDUCED_TYPENAME
+                          range_reference<SinglePassRange>::type argument_type;
+
+            using result_type = decltype(std::declval<argument_type>().get() );
+
+            result_type operator()( argument_type &&r ) const
+            {
+                return r.get();
+            }
+        };
+
+
+        template<class SinglePassRange>
+        class unwrap_ref_range
+            : public transformed_range<unwrap_ref<SinglePassRange>,
+                                       SinglePassRange>
+        {
+            using base = transformed_range<unwrap_ref<SinglePassRange>,
+                                           SinglePassRange>;
+        public:
+            using transform_fn_type = unwrap_ref<SinglePassRange>;
+            using source_range_type = SinglePassRange;
+
+            unwrap_ref_range(transform_fn_type fn, source_range_type &rng)
+                : base(fn, rng)
+            {
+            }
+
+            unwrap_ref_range(const base &other) : base(other) {}
+        };
+
+        template<class SinglePassRange>
+        inline unwrap_ref_range<SinglePassRange>
+        operator|(SinglePassRange& r, ref_unwrapped_forwarder)
+        {
+            BOOST_RANGE_CONCEPT_ASSERT((
+                SinglePassRangeConcept<SinglePassRange>));
+
+            return operator|( r,
+                boost::adaptors::transformed(unwrap_ref<SinglePassRange>()));
+        }
+
+    }
+
+    using range_detail::unwrap_ref_range;
+
+    namespace adaptors
+    {
+        namespace
+        {
+            const range_detail::ref_unwrapped_forwarder ref_unwrapped =
+                                       range_detail::ref_unwrapped_forwarder();
+        }
+
+        template<class SinglePassRange>
+        inline unwrap_ref_range<SinglePassRange>
+        ref_unwrap(SinglePassRange& rng)
+        {
+            BOOST_RANGE_CONCEPT_ASSERT((
+                SinglePassRangeConcept<SinglePassRange>));
+
+            return unwrap_ref_range<SinglePassRange>(
+                range_detail::unwrap_ref<SinglePassRange>(), rng );
+        }
+    } // 'adaptors'
+
+}
+
+#endif
+
+#endif

test/Jamfile.v2

@@ -59,6 +59,8 @@ test-suite range :
     [ range-test adaptor_test/indexed ]
     [ range-test adaptor_test/indirected ]
     [ range-test adaptor_test/map ]
+    [ range-test adaptor_test/ref_unwrapped ]
+    [ range-test adaptor_test/ref_unwrapped_example ]
     [ range-test adaptor_test/replaced ]
     [ range-test adaptor_test/replaced_if ]
     [ range-test adaptor_test/reversed ]

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

test/adaptor_test/ref_unwrapped_example.cpp

@@ -0,0 +1,47 @@
+// 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/
+//
+//[ref_unwrapped_example
+#include <boost/range/adaptor/ref_unwrapped.hpp>
+#include <iostream>
+#include <vector>
+
+struct example
+{
+  int value;
+};
+
+int main(int argc, const char* argv[])
+{
+//<-
+#if !defined(BOOST_NO_CXX11_DECLTYPE) \
+ && !defined(BOOST_NO_CXX11_RANGE_BASED_FOR) \
+ && !defined(BOOST_NO_CXX11_UNIFIED_INITIALIZATION_SYNTAX) \
+ && !defined(BOOST_NO_CXX11_AUTO_DECLARATIONS)
+//->
+    using boost::adaptors::ref_unwrapped;
+
+    example one{1};
+    example two{2};
+    example three{3};
+
+    std::vector<std::reference_wrapper<example> > input{one, two, three};
+
+    for (auto&& entry : input | ref_unwrapped)
+    {
+      std::cout << entry.value;
+    }
+
+    return 0;
+//<-
+#endif
+//->
+}
+//]