Add detail::lean_tuple

Provides a subset of std::tuple functionality, for internal use
to reduce the resources required during compilation.

Does not support reference-like semantics on assignment.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

include/boost/beast/core/detail/lean_tuple.hpp

@@ -0,0 +1,87 @@
+//
+// Copyright (c) 2018 Damian Jarek (damian dot jarek93 at gmail dot com)
+//
+// 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)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+#ifndef BOOST_BEAST_DETAIL_LEAN_TUPLE_HPP
+#define BOOST_BEAST_DETAIL_LEAN_TUPLE_HPP
+
+#include <boost/mp11/integer_sequence.hpp>
+#include <boost/mp11/algorithm.hpp>
+#include <boost/type_traits/remove_cv.hpp>
+#include <boost/type_traits/copy_cv.hpp>
+#include <cstdlib>
+#include <utility>
+
+namespace boost {
+namespace beast {
+namespace detail {
+
+template<std::size_t I, class T>
+struct lean_tuple_element
+{
+    T t;
+};
+
+template<class... Ts>
+struct lean_tuple_impl;
+
+template<class... Ts, std::size_t... Is>
+struct lean_tuple_impl<
+        boost::mp11::index_sequence<Is...>, Ts...>
+  : lean_tuple_element<Is, Ts>...
+{
+    template<class... Us>
+    explicit lean_tuple_impl(Us&&... us)
+      : lean_tuple_element<Is, Ts>{std::forward<Us>(us)}...
+    {
+    }
+};
+
+template<class... Ts>
+struct lean_tuple : lean_tuple_impl<
+    boost::mp11::index_sequence_for<Ts...>, Ts...>
+{
+    template<class... Us>
+    explicit lean_tuple(Us&&... us)
+      : lean_tuple_impl<
+            boost::mp11::index_sequence_for<Ts...>, Ts...>{
+          std::forward<Us>(us)...}
+    {
+    }
+};
+
+template<std::size_t I, class T>
+T&
+get(lean_tuple_element<I, T>& te)
+{
+    return te.t;
+}
+
+template<std::size_t I, class T>
+T const&
+get(lean_tuple_element<I, T> const& te)
+{
+    return te.t;
+}
+
+template<std::size_t I, class T>
+T&&
+get(lean_tuple_element<I, T>&& te)
+{
+    return std::move(te.t);
+}
+
+template <std::size_t I, class T>
+using tuple_element_t = typename boost::copy_cv<
+    mp11::mp_at_c<typename remove_cv<T>::type, I>, T>::type;
+
+} // detail
+} // beast
+} // boost
+
+#endif

test/beast/core/CMakeLists.txt

@@ -46,6 +46,7 @@ add_executable (tests-beast-core
     type_traits.cpp
     detail/base64.cpp
     detail/clamp.cpp
+    detail/lean_tuple.cpp
     detail/sha1.cpp
     detail/variant.cpp
     detail/varint.cpp

test/beast/core/Jamfile

@@ -36,6 +36,7 @@ local SOURCES =
     type_traits.cpp
     detail/base64.cpp
     detail/clamp.cpp
+    detail/lean_tuple.cpp
     detail/sha1.cpp
     detail/variant.cpp
     detail/varint.cpp

test/beast/core/detail/lean_tuple.cpp

@@ -0,0 +1,54 @@
+//
+// Copyright (c) 2018 Damian Jarek (damian dot jarek93 at gmail dot com)
+//
+// 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)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+// Test that header file is self-contained.
+#include <boost/beast/core/detail/lean_tuple.hpp>
+
+#include <boost/beast/_experimental/unit_test/suite.hpp>
+
+namespace boost {
+namespace beast {
+namespace detail {
+
+class lean_tuple_test : public beast::unit_test::suite
+{
+public:
+    void
+    run()
+    {
+        struct explicit_constructible
+        {
+            explicit_constructible(std::nullptr_t)
+                : i_(0)
+            {
+            }
+
+            explicit explicit_constructible(int i)
+                : i_(i)
+            {
+            }
+
+            int i_;
+        };
+
+        lean_tuple<explicit_constructible, int> t{nullptr, 42};
+        BEAST_EXPECT(detail::get<1>(t) == 42);
+        BEAST_EXPECT(detail::get<0>(t).i_ == 0);
+
+        t = lean_tuple<explicit_constructible, int>{explicit_constructible(42), 43};
+        BEAST_EXPECT(detail::get<1>(t) == 43);
+        BEAST_EXPECT(detail::get<0>(t).i_ == 42);
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(beast,core,lean_tuple);
+
+} // detail
+} // beast
+} // boost