Add buffer_traits.hpp, buffers_type

include/boost/beast/core.hpp

@@ -13,6 +13,7 @@
 #include <boost/beast/core/detail/config.hpp>
 
 #include <boost/beast/core/bind_handler.hpp>
+#include <boost/beast/core/buffer_traits.hpp>
 #include <boost/beast/core/buffered_read_stream.hpp>
 #include <boost/beast/core/buffers_adapter.hpp>
 #include <boost/beast/core/buffers_cat.hpp>

include/boost/beast/core/buffer_traits.hpp

@@ -0,0 +1,73 @@
+//
+// Copyright (c) 2018 Vinnie Falco (vinnie dot falco 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_BUFFER_TRAITS_HPP
+#define BOOST_BEAST_BUFFER_TRAITS_HPP
+
+#include <boost/beast/core/detail/config.hpp>
+#include <boost/asio/buffer.hpp>
+#include <type_traits>
+
+namespace boost {
+namespace beast {
+
+/** Type alias used to obtain the underlying buffer type of a buffer sequence.
+
+    This metafunction is used to determine the underlying buffer type for
+    a given buffer sequence. The equivalent type of the alias will vary
+    depending on the template type argument:
+
+    @li If the template argument is a <em>MutableBufferSequence</em>,
+        the resulting type alias will be `net::mutable_buffer`, else
+
+    @li If the template argument is a <em>ConstBufferSequence</em>,
+        the resulting type alias will be `net::const_buffer`, otherwise
+
+    @li The resulting type alias will be `void`.
+
+    @par Example
+    The following code returns the first buffer in a buffer sequence,
+    or generates a compilation error if the argument is not a buffer
+    sequence:
+    @code
+    template <class BufferSequence>
+    buffers_type <BufferSequence>
+    buffers_front (BufferSequence const& buffers)
+    {
+        static_assert(
+            net::is_const_buffer_sequence<BufferSequence>::value,
+            "BufferSequence requirements not met");
+        auto const first = net::buffer_sequence_begin (buffers);
+        if (first == net::buffer_sequence_end (buffers))
+            return {};
+        return *first;
+    }
+    @endcode
+
+    @param T The buffer sequence type to use.
+*/
+template<class T>
+#if BOOST_BEAST_DOXYGEN
+struct buffers_type : __see_below__ {};
+//using buffers_type = __see_below__;
+#else
+using buffers_type = typename
+    std::conditional<
+        net::is_mutable_buffer_sequence<T>::value,
+        net::mutable_buffer,
+        typename std::conditional<
+            net::is_const_buffer_sequence<T>::value,
+            net::const_buffer,
+            void>::type>::type;
+#endif
+
+} // beast
+} // boost
+
+#endif