From 8f1faababd3984d770a334e0cb018d7b9fa67f09 Mon Sep 17 00:00:00 2001
From: Vinnie Falco <vinnie.falco@gmail.com>
Date: Sat, 15 Dec 2018 11:27:14 -0800
Subject: [PATCH] Add buffers_iterator_type trait

---
 CHANGELOG.md                               |  1 +
 doc/qbk/03_core/3_buffers.qbk              |  6 +++
 doc/qbk/quickref.xml                       |  1 +
 include/boost/beast/core/buffer_traits.hpp | 52 ++++++++++++++++++++++
 test/beast/core/buffer_traits.cpp          | 12 +++++
 5 files changed, 72 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 33ccd970..54faa28f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ Version 200
 * Fix and refactor buffers_cat
 * Refactor buffers_prefix
 * Add const and mutable buffer sequence traits
+* Add buffers_iterator_type trait
 
 API Changes:
 
diff --git a/doc/qbk/03_core/3_buffers.qbk b/doc/qbk/03_core/3_buffers.qbk
index 10237d59..a84a6944 100644
--- a/doc/qbk/03_core/3_buffers.qbk
+++ b/doc/qbk/03_core/3_buffers.qbk
@@ -165,6 +165,12 @@ metafunctions which operate on buffers:
 
 [table Buffer Algorithms and Types
 [[Name][Description]]
+[[
+    [link beast.ref.boost__beast__buffers_iterator_type `buffers_iterator_type`]
+][
+    This metafunction is used to determine the type of iterator
+    used by a particular buffer sequence.
+]]
 [[
     [link beast.ref.boost__beast__buffers_type `buffers_type`]
 ][
diff --git a/doc/qbk/quickref.xml b/doc/qbk/quickref.xml
index 297c7e40..fbbb3341 100644
--- a/doc/qbk/quickref.xml
+++ b/doc/qbk/quickref.xml
@@ -236,6 +236,7 @@
           <bridgehead renderas="sect3">Type Traits</bridgehead>
           <simplelist type="vert" columns="1">
             <member><link linkend="beast.ref.boost__beast__buffers_type">buffers_type</link></member>
+            <member><link linkend="beast.ref.boost__beast__buffers_iterator_type">buffers_iterator_type</link></member>
             <member><link linkend="beast.ref.boost__beast__get_lowest_layer">get_lowest_layer</link></member>
             <member><link linkend="beast.ref.boost__beast__has_get_executor">has_get_executor</link></member>
             <member><link linkend="beast.ref.boost__beast__is_async_read_stream">is_async_read_stream</link></member>
diff --git a/include/boost/beast/core/buffer_traits.hpp b/include/boost/beast/core/buffer_traits.hpp
index 74ed663a..033ca778 100644
--- a/include/boost/beast/core/buffer_traits.hpp
+++ b/include/boost/beast/core/buffer_traits.hpp
@@ -12,6 +12,7 @@
 
 #include <boost/beast/core/detail/config.hpp>
 #include <boost/asio/buffer.hpp>
+#include <boost/config/workaround.hpp>
 #include <boost/mp11/function.hpp>
 #include <type_traits>
 
@@ -103,6 +104,57 @@ using buffers_type = typename
         net::const_buffer>::type;
 #endif
 
+namespace detail {
+
+// VFALCO This is a workaround for MSVC.v140
+template<class T>
+struct buffers_iterator_type_helper
+{
+    using type = decltype(
+        net::buffer_sequence_begin(
+            std::declval<T const&>()));
+};
+
+template<>
+struct buffers_iterator_type_helper<
+    net::const_buffer>
+{
+    using type = net::const_buffer const*;
+};
+
+template<>
+struct buffers_iterator_type_helper<
+    net::mutable_buffer>
+{
+    using type = net::mutable_buffer const*;
+};
+
+} // detail
+
+/** Type alias for the iterator type of a buffer sequence type.
+
+    This metafunction is used to determine the type of iterator
+    used by a particular buffer sequence.
+
+    @tparam T The buffer sequence type to use. The resulting
+    type alias will be equal to the iterator type used by
+    the buffer sequence.
+*/
+template <class T>
+#if BOOST_BEAST_DOXYGEN
+struct buffers_iterator_type : __see_below__ {};
+//using buffers_iterator_type = __see_below__;
+#else
+# if BOOST_WORKAROUND(BOOST_MSVC, < 1910)
+using buffers_iterator_type = typename
+    detail::buffers_iterator_type_helper<T>::type;
+# else
+using buffers_iterator_type =
+    decltype(net::buffer_sequence_begin(
+        std::declval<T const&>()));
+# endif
+#endif
+
 } // beast
 } // boost
 
diff --git a/test/beast/core/buffer_traits.cpp b/test/beast/core/buffer_traits.cpp
index d27bff9f..5c4785d8 100644
--- a/test/beast/core/buffer_traits.cpp
+++ b/test/beast/core/buffer_traits.cpp
@@ -107,6 +107,18 @@ public:
             std::array<int, 3>
         >>::value);
 
+    // buffers_iterator_type
+
+    BOOST_STATIC_ASSERT(
+        std::is_same<net::const_buffer const*, buffers_iterator_type<
+            net::const_buffer
+        >>::value);
+
+    BOOST_STATIC_ASSERT(
+        std::is_same<net::mutable_buffer const*, buffers_iterator_type<
+            net::mutable_buffer
+        >>::value);
+
     // javadoc: buffers_type
     template <class BufferSequence>
     buffers_type <BufferSequence>