Refactor buffers_prefix:

* Tidy up tests
* Increase code coverage
* Correct behavior for default constructed iterators

include/boost/beast/core/buffers_prefix.hpp

@@ -11,8 +11,8 @@
 #define BOOST_BEAST_BUFFERS_PREFIX_HPP
 
 #include <boost/beast/core/detail/config.hpp>
+#include <boost/beast/core/buffer_traits.hpp>
 #include <boost/beast/core/type_traits.hpp>
-#include <boost/asio/buffer.hpp>
 #include <boost/optional/optional.hpp> // for in_place_init_t
 #include <algorithm>
 #include <cstdint>
@@ -49,13 +49,19 @@ class buffers_prefix_view
         std::size_t dist);
 
 public:
-    /// The type for each element in the list of buffers.
-    using value_type = typename std::conditional<
-        std::is_convertible<typename
-            std::iterator_traits<iter_type>::value_type,
-                net::mutable_buffer>::value,
-                    net::mutable_buffer,
-                        net::const_buffer>::type;
+    /** The type for each element in the list of buffers.
+
+        If the type of the underlying sequence is a mutable buffer
+        sequence, then `value_type` is `net::mutable_buffer`. Otherwise,
+        `value_type` is `net::const_buffer`.
+
+        @see buffers_type
+    */
+#if BOOST_BEAST_DOXYGEN
+    using value_type = __see_below__;
+#else
+    using value_type = buffers_type<ConstBufferSequence>;
+#endif
 
 #if BOOST_BEAST_DOXYGEN
     /// A bidirectional iterator type that may be used to read elements.

include/boost/beast/core/impl/buffers_prefix.hpp

@@ -26,7 +26,7 @@ class buffers_prefix_view<Buffers>::const_iterator
     friend class buffers_prefix_view<Buffers>;
 
     buffers_prefix_view const* b_ = nullptr;
-    std::size_t remain_;
+    std::size_t remain_ = 0;
     iter_type it_;
 
 public:
@@ -49,20 +49,7 @@ public:
     bool
     operator==(const_iterator const& other) const
     {
-        return
-            (b_ == nullptr) ?
-            (
-                other.b_ == nullptr ||
-                other.it_ == other.b_->end_
-            ):(
-                (other.b_ == nullptr) ?
-                (
-                    it_ == b_->end_
-                ): (
-                    b_ == other.b_ &&
-                    it_ == other.it_
-                )
-            );
+        return b_ == other.b_ && it_ == other.it_;
     }
 
     bool
@@ -111,16 +98,18 @@ public:
     }
 
 private:
-    const_iterator(buffers_prefix_view const& b,
-            std::true_type)
+    const_iterator(
+        buffers_prefix_view const& b,
+        std::true_type)
         : b_(&b)
         , remain_(b.remain_)
         , it_(b_->end_)
     {
     }
 
-    const_iterator(buffers_prefix_view const& b,
-            std::false_type)
+    const_iterator(
+        buffers_prefix_view const& b,
+        std::false_type)
         : b_(&b)
         , remain_(b_->size_)
         , it_(net::buffer_sequence_begin(b_->bs_))
@@ -222,18 +211,22 @@ buffers_prefix_view(
 
 template<class Buffers>
 auto
-buffers_prefix_view<Buffers>::begin() const ->
+buffers_prefix_view<Buffers>::
+begin() const ->
     const_iterator
 {
-    return const_iterator{*this, std::false_type{}};
+    return const_iterator{
+        *this, std::false_type{}};
 }
 
 template<class Buffers>
 auto
-buffers_prefix_view<Buffers>::end() const ->
+buffers_prefix_view<Buffers>::
+end() const ->
     const_iterator
 {
-    return const_iterator{*this, std::true_type{}};
+    return const_iterator{
+        *this, std::true_type{}};
 }
 
 template<class Buffers>