diff --git a/CHANGELOG.md b/CHANGELOG.md
index a6a8167c..bca89eb3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,12 +10,17 @@ Version 76:
 API Changes:
 
 * Rename to serializer::keep_alive
+* BodyReader, BodyWriter use two-phase init
 
 Actions Required:
 
 * Use serializer::keep_alive instead of serializer::close and
   take the logical NOT of the return value.
 
+* Modify instances of user-defined BodyReader and BodyWriter
+  types to perfrom two-phase initialization, as per the
+  updated documented type requirements.
+
 --------------------------------------------------------------------------------
 
 Version 75:
diff --git a/doc/concept/BodyReader.qbk b/doc/concept/BodyReader.qbk
index 4ede1415..470370df 100644
--- a/doc/concept/BodyReader.qbk
+++ b/doc/concept/BodyReader.qbk
@@ -36,8 +36,14 @@ In this table:
 
 * `R<T>` is the type `boost::optional<std::pair<T, bool>>`.
 
-[table BodyReader requirements
-[[expression] [type] [semantics, pre/post-conditions]]
+[heading Associated Types]
+
+* __Body__
+* [link beast.ref.beast__http__is_body_reader `is_body_reader`]
+
+[heading  BodyReader requirements]
+[table Valid Expressions
+[[Expression] [Type] [Semantics, Pre/Post-conditions]]
 [
     [`X::const_buffers_type`]
     []
@@ -46,22 +52,33 @@ In this table:
         This is the type of buffer returned by `X::get`.
     ]
 ][
-    [`X(m,ec);`]
+    [`X(m);`]
     []
     [
         Constructible from `m`. The lifetime of `m` is guaranteed
         to end no earlier than after the `X` is destroyed.
+        The reader shall not access the contents of `m` before the
+        first call to `init`, permitting lazy construction of the
+        message.
+
         The constructor may optionally require that `m` is const, which
         has these consequences:
 
         * If `X` requires that `m` is a const reference, then serializers
         constructed for messages with this body type will also require a
-        `const` reference to a message, otherwise:
+        const reference to a message, otherwise:
 
         * If `X` requires that `m` is a non-const reference, then serializers
         constructed for messages with this body type will aso require
         a non-const reference to a message.
-
+    ]
+][
+    [`a.init(ec)`]
+    []
+    [
+        Called once to fully initialize the object before any calls to
+        `get`. The message body becomes valid before entering this function,
+        and remains valid until the reader is destroyed.
         The function will ensure that `!ec` is `true` if there was
         no error or set to the appropriate error code if there was one. 
     ]
@@ -69,9 +86,9 @@ In this table:
     [`a.get(ec)`]
     [`R<X::const_buffers_type>`]
     [
-        Called repeatedly after `init` succeeds. This function returns
-        `boost::none` if all buffers representing the body have been
-        returned in previous calls or if it sets `ec` to indicate an
+        Called one or more times after `init` succeeds. This function
+        returns `boost::none` if all buffers representing the body have
+        been returned in previous calls or if it sets `ec` to indicate an
         error. Otherwise, if there are buffers remaining the function
         should return a pair with the first element containing a non-zero
         length buffer sequence representing the next set of octets in
@@ -82,13 +99,6 @@ In this table:
         The function will ensure that `!ec` is `true` if there was
         no error or set to the appropriate error code if there was one. 
     ]
-][
-    [`is_body_reader<B>`]
-    [`std::true_type`]
-    [
-        An alias for `std::true_type` for `B`, otherwise an alias
-        for `std::false_type`.
-    ]
 ]
 ]
 
@@ -96,4 +106,11 @@ In this table:
 
 [concept_BodyReader]
 
+[heading Models]
+
+* [link beast.ref.beast__http__basic_dynamic_body.reader `basic_dynamic_body::reader`]
+* [link beast.ref.beast__http__basic_file_body__reader `basic_file_body::reader`]
+* [link beast.ref.beast__http__empty_body.reader `empty_body::reader`]
+* [link beast.ref.beast__http__string_body.reader `string_body::reader`]
+
 [endsect]
diff --git a/doc/concept/BodyWriter.qbk b/doc/concept/BodyWriter.qbk
index 830d2509..50ae408d 100644
--- a/doc/concept/BodyWriter.qbk
+++ b/doc/concept/BodyWriter.qbk
@@ -38,20 +38,40 @@ In this table:
 
 * `ec` is a value of type [link beast.ref.beast__error_code `error_code&`].
 
+[heading Associated Types]
+
+* __Body__
+* [link beast.ref.beast__http__is_body_writer `is_body_writer`]
+
 [table Writer requirements
 [[expression] [type] [semantics, pre/post-conditions]]
 [
-    [`X(m,n,ec);`]
+    [`X(m);`]
     []
     [
-        Constructible from `m`. The lifetime of `m` is guaranteed
-        to end no earlier than after the `X` is destroyed. The constructor
-        will be called after a complete header is stored in `m`, and before
-        parsing body octets for messages indicating that a body is present.
+        Constructible from `m`. The lifetime of `m` is guaranteed to
+        end no earlier than after the `X` is destroyed. The constructor
+        will be called after a complete header is stored in `m`, and
+        before parsing body octets for messages indicating that a body
+        is present The writer shall not access the contents of `m` before
+        the first call to `init`, permitting lazy construction of the
+        message.
+
+        The function will ensure that `!ec` is `true` if there was
+        no error or set to the appropriate error code if there was one. 
+    ]
+][
+    [`a.init(n, ec)`]
+    []
+    [
+        Called once to fully initialize the object before any calls to
+        `put`. The message body is valid before entering this function,
+        and remains valid until the writer is destroyed.
         The value of `n` will be set to the content length of the
         body if known, otherwise `n` will be equal to `boost::none`.
         Implementations of [*BodyWriter] may use this information to
         optimize allocation.
+
         The function will ensure that `!ec` is `true` if there was
         no error or set to the appropriate error code if there was one. 
     ]
@@ -89,4 +109,11 @@ In this table:
 
 [concept_BodyWriter]
 
+[heading Models]
+
+* [link beast.ref.beast__http__basic_dynamic_body.writer `basic_dynamic_body::writer`]
+* [link beast.ref.beast__http__basic_file_body__reader `basic_file_body::writer`]
+* [link beast.ref.beast__http__empty_body.writer `empty_body::writer`]
+* [link beast.ref.beast__http__string_body.writer `string_body::writer`]
+
 [endsect]
diff --git a/example/common/const_body.hpp b/example/common/const_body.hpp
index c4a0f743..9549ff49 100644
--- a/example/common/const_body.hpp
+++ b/example/common/const_body.hpp
@@ -68,9 +68,14 @@ struct const_body
 
         template<bool isRequest, class Fields>
         explicit
-        reader(beast::http::message<isRequest, const_body,
-                Fields> const& msg, beast::error_code& ec)
+        reader(beast::http::message<isRequest,
+                const_body, Fields> const& msg)
             : body_(msg.body)
+        {
+        }
+
+        void
+        init(beast::error_code& ec)
         {
             ec.assign(0, ec.category());
         }
diff --git a/example/common/mutable_body.hpp b/example/common/mutable_body.hpp
index 3d441e53..bf080356 100644
--- a/example/common/mutable_body.hpp
+++ b/example/common/mutable_body.hpp
@@ -71,9 +71,14 @@ struct mutable_body
 
         template<bool isRequest, class Fields>
         explicit
-        reader(beast::http::message<isRequest, mutable_body,
-                Fields> const& msg, beast::error_code& ec)
+        reader(beast::http::message<isRequest,
+                mutable_body, Fields> const& msg)
             : body_(msg.body)
+        {
+        }
+
+        void
+        init(beast::error_code& ec)
         {
             ec.assign(0, ec.category());
         }
@@ -99,15 +104,20 @@ struct mutable_body
     public:
         template<bool isRequest, class Fields>
         explicit
-        writer(beast::http::message<isRequest, mutable_body, Fields>& m,
-            boost::optional<std::uint64_t> const& content_length,
-                beast::error_code& ec)
+        writer(beast::http::message<
+                isRequest, mutable_body, Fields>& m)
             : body_(m.body)
         {
-            if(content_length)
+        }
+
+        void
+        init(boost::optional<
+            std::uint64_t> const& length, beast::error_code& ec)
+        {
+            if(length)
             {
-                if(*content_length > (std::numeric_limits<
-                        std::size_t>::max)())
+                if(*length > (std::numeric_limits<
+                    std::size_t>::max)())
                 {
                     ec = beast::http::error::buffer_overflow;
                     return;
@@ -115,7 +125,7 @@ struct mutable_body
                 try
                 {
                     body_.reserve(static_cast<
-                        std::size_t>(*content_length));
+                        std::size_t>(*length));
                 }
                 catch(std::exception const&)
                 {
@@ -129,7 +139,7 @@ struct mutable_body
         template<class ConstBufferSequence>
         std::size_t
         put(ConstBufferSequence const& buffers,
-                beast::error_code& ec)
+            beast::error_code& ec)
         {
             using boost::asio::buffer_size;
             using boost::asio::buffer_copy;
diff --git a/include/beast/http/buffer_body.hpp b/include/beast/http/buffer_body.hpp
index d889bd4e..3b4f660a 100644
--- a/include/beast/http/buffer_body.hpp
+++ b/include/beast/http/buffer_body.hpp
@@ -101,9 +101,14 @@ struct buffer_body
 
         template<bool isRequest, class Fields>
         explicit
-        reader(message<isRequest, buffer_body,
-                Fields> const& msg, error_code& ec)
+        reader(message<isRequest,
+                buffer_body, Fields> const& msg)
             : body_(msg.body)
+        {
+        }
+
+        void
+        init(error_code& ec)
         {
             ec.assign(0, ec.category());
         }
@@ -152,10 +157,13 @@ struct buffer_body
     public:
         template<bool isRequest, class Fields>
         explicit
-        writer(message<isRequest, buffer_body, Fields>& m,
-                boost::optional<std::uint64_t> const&,
-                    error_code& ec)
+        writer(message<isRequest, buffer_body, Fields>& m)
             : body_(m.body)
+        {
+        }
+
+        void
+        init(boost::optional<std::uint64_t> const&, error_code& ec)
         {
             ec.assign(0, ec.category());
         }
diff --git a/include/beast/http/dynamic_body.hpp b/include/beast/http/dynamic_body.hpp
index 43206e17..35fbaddc 100644
--- a/include/beast/http/dynamic_body.hpp
+++ b/include/beast/http/dynamic_body.hpp
@@ -55,9 +55,14 @@ struct basic_dynamic_body
 
         template<bool isRequest, class Fields>
         explicit
-        reader(message<isRequest, basic_dynamic_body,
-                Fields> const& m, error_code& ec)
+        reader(message<isRequest,
+                basic_dynamic_body, Fields> const& m)
             : body_(m.body)
+        {
+        }
+
+        void
+        init(error_code& ec)
         {
             ec.assign(0, ec.category());
         }
@@ -82,10 +87,13 @@ struct basic_dynamic_body
     public:
         template<bool isRequest, class Fields>
         explicit
-        writer(message<isRequest, basic_dynamic_body, Fields>& msg,
-            boost::optional<std::uint64_t> const&,
-                error_code& ec)
+        writer(message<isRequest, basic_dynamic_body, Fields>& msg)
             : body_(msg.body)
+        {
+        }
+
+        void
+        init(boost::optional<std::uint64_t> const&, error_code& ec)
         {
             ec.assign(0, ec.category());
         }
diff --git a/include/beast/http/empty_body.hpp b/include/beast/http/empty_body.hpp
index 5ac2ca81..9468502c 100644
--- a/include/beast/http/empty_body.hpp
+++ b/include/beast/http/empty_body.hpp
@@ -54,8 +54,13 @@ struct empty_body
 
         template<bool isRequest, class Fields>
         explicit
-        reader(message<isRequest, empty_body,
-            Fields> const&, error_code& ec)
+        reader(message<isRequest,
+            empty_body, Fields> const&)
+        {
+        }
+
+        void
+        init(error_code& ec)
         {
             ec.assign(0, ec.category());
         }
@@ -77,9 +82,12 @@ struct empty_body
     {
         template<bool isRequest, class Fields>
         explicit
-        writer(message<isRequest, empty_body, Fields>&,
-            boost::optional<std::uint64_t> const&,
-                error_code& ec)
+        writer(message<isRequest, empty_body, Fields>&)
+        {
+        }
+
+        void
+        init(boost::optional<std::uint64_t> const&, error_code& ec)
         {
             ec.assign(0, ec.category());
         }
diff --git a/include/beast/http/file_body.hpp b/include/beast/http/file_body.hpp
index ea59e270..17970126 100644
--- a/include/beast/http/file_body.hpp
+++ b/include/beast/http/file_body.hpp
@@ -235,9 +235,17 @@ public:
     // always have the `file_body` as the body type.
     //
     template<bool isRequest, class Fields>
-    reader(
-        message<isRequest, basic_file_body, Fields> const& m,
-        error_code& ec);
+    reader(message<
+        isRequest, basic_file_body, Fields> const& m);
+
+    // Initializer
+    //
+    // This is called before the body is serialized and
+    // gives the reader a chance to do something that might
+    // need to return an error code.
+    //
+    void
+    init(error_code& ec);
 
     // This function is called zero or more times to
     // retrieve buffers. A return value of `boost::none`
@@ -261,18 +269,29 @@ template<class File>
 template<bool isRequest, class Fields>
 basic_file_body<File>::
 reader::
-reader(
-    message<isRequest, basic_file_body, Fields> const& m,
-    error_code& ec)
+reader(message<isRequest, basic_file_body, Fields> const& m)
     : body_(m.body)
 {
     // The file must already be open
     BOOST_ASSERT(body_.file_.is_open());
 
     // Get the size of the file
-    remain_ = body_.file_.size(ec);
-    if(ec)
-        return;
+    remain_ = body_.file_size_;
+}
+
+// Initializer
+template<class File>
+void
+basic_file_body<File>::
+reader::
+init(error_code& ec)
+{
+    // The error_code specification requires that we
+    // either set the error to some value, or set it
+    // to indicate no error.
+    //
+    // We don't do anything fancy so set "no error"
+    ec.assign(0, ec.category());
 }
 
 // This function is called repeatedly by the serializer to
@@ -353,9 +372,18 @@ public:
     template<bool isRequest, class Fields>
     explicit
     writer(
-        message<isRequest, basic_file_body, Fields>& m,
-        boost::optional<std::uint64_t> const& content_length,
-        error_code& ec);
+        message<isRequest, basic_file_body, Fields>& m);
+
+    // Initializer
+    //
+    // This is called before the body is parsed and
+    // gives the writer a chance to do something that might
+    // need to return an error code. It informs us of
+    // the payload size (`content_length`) which we can
+    // optionally use for optimization.
+    //
+    void
+    init(boost::optional<std::uint64_t> const&, error_code& ec);
 
     // This function is called one or more times to store
     // buffer sequences corresponding to the incoming body.
@@ -387,11 +415,18 @@ template<class File>
 template<bool isRequest, class Fields>
 basic_file_body<File>::
 writer::
-writer(
-    message<isRequest, basic_file_body, Fields>& m,
+writer(message<isRequest, basic_file_body, Fields>& m)
+    : body_(m.body)
+{
+}
+
+template<class File>
+void
+basic_file_body<File>::
+writer::
+init(
     boost::optional<std::uint64_t> const& content_length,
     error_code& ec)
-    : body_(m.body)
 {
     // The file must already be open for writing
     BOOST_ASSERT(body_.file_.is_open());
@@ -401,7 +436,11 @@ writer(
     // to see if there is enough room to store the body.
     boost::ignore_unused(content_length);
 
-    // This is required by the error_code specification
+    // The error_code specification requires that we
+    // either set the error to some value, or set it
+    // to indicate no error.
+    //
+    // We don't do anything fancy so set "no error"
     ec.assign(0, ec.category());
 }
 
diff --git a/include/beast/http/impl/parser.ipp b/include/beast/http/impl/parser.ipp
index 9f5ab5a2..73bacd9a 100644
--- a/include/beast/http/impl/parser.ipp
+++ b/include/beast/http/impl/parser.ipp
@@ -14,12 +14,20 @@
 namespace beast {
 namespace http {
 
+template<bool isRequest, class Body, class Allocator>
+parser<isRequest, Body, Allocator>::
+parser()
+    : wr_(m_)
+{
+}
+
 template<bool isRequest, class Body, class Allocator>
 template<class Arg1, class... ArgN, class>
 parser<isRequest, Body, Allocator>::
 parser(Arg1&& arg1, ArgN&&... argn)
     : m_(std::forward<Arg1>(arg1),
         std::forward<ArgN>(argn)...)
+    , wr_(m_)
 {
 }
 
@@ -30,8 +38,9 @@ parser(parser<isRequest, OtherBody, Allocator>&& p,
         Args&&... args)
     : base_type(std::move(p))
     , m_(p.release(), std::forward<Args>(args)...)
+    , wr_(m_)
 {
-    if(p.wr_)
+    if(wr_inited_)
         BOOST_THROW_EXCEPTION(std::invalid_argument{
             "moved-from parser has a body"});
 }
diff --git a/include/beast/http/impl/serializer.ipp b/include/beast/http/impl/serializer.ipp
index 0e8f86c8..89f5c6da 100644
--- a/include/beast/http/impl/serializer.ipp
+++ b/include/beast/http/impl/serializer.ipp
@@ -59,6 +59,7 @@ template<bool isRequest, class Body,
 serializer<isRequest, Body, Fields, ChunkDecorator>::
 serializer(value_type& m)
     : m_(m)
+    , rd_(m_)
 {
 }
 
@@ -67,6 +68,7 @@ template<bool isRequest, class Body,
 serializer<isRequest, Body, Fields, ChunkDecorator>::
 serializer(value_type& m, ChunkDecorator const& d)
     : m_(m)
+    , rd_(m_)
     , d_(d)
 {
 }
@@ -96,12 +98,12 @@ next(error_code& ec, Visit&& visit)
 
     case do_init:
     {
-        if(split_)
-            goto go_header_only;
-        rd_.emplace(m_, ec);
+        rd_.init(ec);
         if(ec)
             return;
-        auto result = rd_->get(ec);
+        if(split_)
+            goto go_header_only;
+        auto result = rd_.get(ec);
         if(ec == error::need_more)
             goto go_header_only;
         if(ec)
@@ -129,18 +131,12 @@ next(error_code& ec, Visit&& visit)
         break;
 
     case do_body:
-        if(! rd_)
-        {
-            rd_.emplace(m_, ec);
-            if(ec)
-                return;
-        }
         s_ = do_body + 1;
         BEAST_FALLTHROUGH;
 
     case do_body + 1:
     {
-        auto result = rd_->get(ec);
+        auto result = rd_.get(ec);
         if(ec)
             return;
         if(! result)
@@ -161,12 +157,12 @@ next(error_code& ec, Visit&& visit)
     s_ = do_init_c;
     case do_init_c:
     {
-        if(split_)
-            goto go_header_only_c;
-        rd_.emplace(m_, ec);
+        rd_.init(ec);
         if(ec)
             return;
-        auto result = rd_->get(ec);
+        if(split_)
+            goto go_header_only_c;
+        auto result = rd_.get(ec);
         if(ec == error::need_more)
             goto go_header_only_c;
         if(ec)
@@ -237,18 +233,12 @@ next(error_code& ec, Visit&& visit)
         break;
 
     case do_body_c:
-        if(! rd_)
-        {
-            rd_.emplace(m_, ec);
-            if(ec)
-                return;
-        }
         s_ = do_body_c + 1;
         BEAST_FALLTHROUGH;
 
     case do_body_c + 1:
     {
-        auto result = rd_->get(ec);
+        auto result = rd_.get(ec);
         if(ec)
             return;
         if(! result)
diff --git a/include/beast/http/parser.hpp b/include/beast/http/parser.hpp
index e630a996..108223ef 100644
--- a/include/beast/http/parser.hpp
+++ b/include/beast/http/parser.hpp
@@ -60,21 +60,17 @@ class parser
         parser<isRequest, Body, Allocator>>;
 
     message<isRequest, Body, basic_fields<Allocator>> m_;
-    boost::optional<typename Body::writer> wr_;
+    typename Body::writer wr_;
     std::function<void(parser&, error_code&)> cb_;
+    bool wr_inited_ = false;
 
 public:
     /// The type of message returned by the parser
     using value_type =
         message<isRequest, Body, basic_fields<Allocator>>;
 
-    /// Constructor (default)
-    parser()
-    {
-        // avoid `parser()=default` otherwise value-init
-        // for members can happen (i.e. a big memset on
-        // static_buffer_n).
-    }
+    /// Constructor
+    parser();
 
     /// Copy constructor (disallowed)
     parser(parser const&) = delete;
@@ -285,13 +281,14 @@ private:
         std::uint64_t> const& content_length,
             error_code& ec)
     {
-        wr_.emplace(m_, content_length, ec);
+        wr_.init(content_length, ec);
+        wr_inited_ = true;
     }
 
     std::size_t
     on_data(string_view s, error_code& ec)
     {
-        return wr_->put(boost::asio::buffer(
+        return wr_.put(boost::asio::buffer(
             s.data(), s.size()), ec);
     }
 
@@ -305,10 +302,7 @@ private:
     void
     on_complete(error_code& ec)
     {
-        if(wr_)
-            wr_->finish(ec);
-        else
-            ec.assign(0, ec.category());
+        wr_.finish(ec);
     }
 };
 
diff --git a/include/beast/http/serializer.hpp b/include/beast/http/serializer.hpp
index b5668f19..d69bf711 100644
--- a/include/beast/http/serializer.hpp
+++ b/include/beast/http/serializer.hpp
@@ -134,7 +134,7 @@ public:
     static_assert(is_body_reader<Body>::value,
         "BodyReader requirements not met");
 
-    /** The type of the message referenced by this object.
+    /** The type of message this serializer uses
 
         This may be const or non-const depending on the
         implementation of the corresponding @b BodyReader.
@@ -145,11 +145,9 @@ public:
     using value_type =
         typename std::conditional<
             std::is_constructible<typename Body::reader,
-                message<isRequest, Body, Fields>&,
-                    error_code&>::value &&
+                message<isRequest, Body, Fields>&>::value &&
             ! std::is_constructible<typename Body::reader,
-                message<isRequest, Body, Fields> const&,
-                    error_code&>::value,
+                message<isRequest, Body, Fields> const&>::value,
             message<isRequest, Body, Fields>,
             message<isRequest, Body, Fields> const>::type;
 #endif
@@ -248,8 +246,8 @@ private:
     using pcb8_t = buffer_prefix_view<cb8_t const&>;
 
     value_type& m_;
+    reader rd_;
     boost::optional<typename Fields::reader> frd_;
-    boost::optional<reader> rd_;
     boost::variant<boost::blank,
         cb1_t, cb2_t, cb3_t, cb4_t, cb5_t
     #ifndef BEAST_NO_BIG_VARIANTS
diff --git a/include/beast/http/string_body.hpp b/include/beast/http/string_body.hpp
index 963d48bd..089df6df 100644
--- a/include/beast/http/string_body.hpp
+++ b/include/beast/http/string_body.hpp
@@ -52,9 +52,14 @@ struct string_body
 
         template<bool isRequest, class Fields>
         explicit
-        reader(message<isRequest, string_body,
-                Fields> const& msg, error_code& ec)
+        reader(message<isRequest,
+                string_body, Fields> const& msg)
             : body_(msg.body)
+        {
+        }
+
+        void
+        init(error_code& ec)
         {
             ec.assign(0, ec.category());
         }
@@ -80,23 +85,27 @@ struct string_body
     public:
         template<bool isRequest, class Fields>
         explicit
-        writer(message<isRequest, string_body, Fields>& m,
-            boost::optional<std::uint64_t> content_length,
-                error_code& ec)
+        writer(message<isRequest, string_body, Fields>& m)
             : body_(m.body)
         {
-            if(content_length)
+        }
+
+        void
+        init(boost::optional<
+            std::uint64_t> const& length, error_code& ec)
+        {
+            if(length)
             {
-                if(*content_length > (std::numeric_limits<
-                        std::size_t>::max)())
+                if(*length > (
+                    std::numeric_limits<std::size_t>::max)())
                 {
                     ec = error::buffer_overflow;
                     return;
                 }
                 try
                 {
-                    body_.reserve(static_cast<
-                        std::size_t>(*content_length));
+                    body_.reserve(
+                        static_cast<std::size_t>(*length));
                 }
                 catch(std::exception const&)
                 {
diff --git a/include/beast/http/string_view_body.hpp b/include/beast/http/string_view_body.hpp
index eb5960de..0219372e 100644
--- a/include/beast/http/string_view_body.hpp
+++ b/include/beast/http/string_view_body.hpp
@@ -53,9 +53,14 @@ struct string_view_body
 
         template<bool isRequest, class Fields>
         explicit
-        reader(message<isRequest, string_view_body,
-                Fields> const& m, error_code& ec)
+        reader(message<isRequest,
+                string_view_body, Fields> const& m)
             : body_(m.body)
+        {
+        }
+
+        void
+        init(error_code& ec)
         {
             ec.assign(0, ec.category());
         }
diff --git a/include/beast/http/type_traits.hpp b/include/beast/http/type_traits.hpp
index 6a4dd5c6..46f5674a 100644
--- a/include/beast/http/type_traits.hpp
+++ b/include/beast/http/type_traits.hpp
@@ -80,6 +80,7 @@ struct is_body_reader<T, beast::detail::void_t<
     typename T::reader,
     typename T::reader::const_buffers_type,
         decltype(
+    std::declval<typename T::reader&>().init(std::declval<error_code&>()),
     std::declval<boost::optional<std::pair<
             typename T::reader::const_buffers_type, bool>>&>() =
             std::declval<typename T::reader>().get(std::declval<error_code&>()),
@@ -87,11 +88,9 @@ struct is_body_reader<T, beast::detail::void_t<
     is_const_buffer_sequence<
         typename T::reader::const_buffers_type>::value &&
     std::is_constructible<typename T::reader,
-        message<true, T, detail::fields_model>&,
-        error_code&>::value &&
+        message<true, T, detail::fields_model>&>::value &&
     std::is_constructible<typename T::reader,
-        message<false, T, detail::fields_model>&,
-        error_code&>::value
+        message<false, T, detail::fields_model>&>::value
     > {};
 #endif
 
@@ -124,6 +123,9 @@ struct is_body_writer : std::false_type {};
 
 template<class T>
 struct is_body_writer<T, beast::detail::void_t<decltype(
+    std::declval<typename T::writer&>().init(
+        boost::optional<std::uint64_t>(),
+        std::declval<error_code&>()),
     std::declval<std::size_t&>() =
         std::declval<typename T::writer&>().put(
             std::declval<boost::asio::const_buffers_1>(),
@@ -132,10 +134,10 @@ struct is_body_writer<T, beast::detail::void_t<decltype(
         std::declval<error_code&>()),
     (void)0)>> : std::integral_constant<bool,
         std::is_constructible<typename T::writer,
-            message<true, T, detail::fields_model>&,
-            boost::optional<std::uint64_t>,
-            error_code&
-        >::value>
+            message<true, T, detail::fields_model>&>::value &&
+        std::is_constructible<typename T::writer,
+            message<false, T, detail::fields_model>&>::value
+            >
 {
 };
 #endif
diff --git a/test/exemplars.cpp b/test/exemplars.cpp
index 1079475a..95f99971 100644
--- a/test/exemplars.cpp
+++ b/test/exemplars.cpp
@@ -58,10 +58,24 @@ public:
 
         @param ec Set to the error, if any occurred.
     */
-    template<bool isRequest, class Body, class Headers>
+    template<bool isRequest, class Body, class Fields>
     explicit
-    BodyReader(message<isRequest, Body, Headers> const& msg,
-        error_code &ec);
+    BodyReader(message<isRequest, Body, Fields> const& msg);
+
+    /** Initialize the reader
+
+        This is called after construction and before the first
+        call to `get`. The message is valid and complete upon
+        entry.
+
+        @param ec Set to the error, if any occurred.
+    */
+    void
+    init(error_code& ec)
+    {
+        // The specification requires this to indicate "no error"
+        ec.assign(0, ec.category());
+    }
 
     /** Returns the next buffer in the body.
 
@@ -85,7 +99,7 @@ public:
     get(error_code& ec)
     {
         // The specification requires this to indicate "no error"
-        ec = {};
+        ec.assign(0, ec.category());
 
         return boost::none; // for exposition only
     }
@@ -109,14 +123,25 @@ struct BodyWriter
     */
     template<bool isRequest, class Body, class Fields>
     explicit
-    BodyWriter(message<isRequest, Body, Fields>& msg,
-        boost::optional<std::uint64_t> content_length,
-            error_code& ec)
+    BodyWriter(message<isRequest, Body, Fields>& msg);
+
+    /** Initialize the writer
+
+        This is called after construction and before the first
+        call to `put`. The message is valid and complete upon
+        entry.
+
+        @param ec Set to the error, if any occurred.
+    */
+    void
+    init(
+        boost::optional<std::uint64_t> const& content_length,
+        error_code& ec)
     {
-        boost::ignore_unused(msg, content_length);
+        boost::ignore_unused(content_length);
 
         // The specification requires this to indicate "no error"
-        ec = {};
+        ec.assign(0, ec.category());
     }
 
     /** Store buffers.
diff --git a/test/http/serializer.cpp b/test/http/serializer.cpp
index 20826307..d39dbcb5 100644
--- a/test/http/serializer.cpp
+++ b/test/http/serializer.cpp
@@ -27,8 +27,10 @@ public:
                 boost::asio::const_buffers_1;
 
             template<bool isRequest, class Fields>
-            reader(message<isRequest, const_body, Fields> const&,
-                error_code&);
+            reader(message<isRequest, const_body, Fields> const&);
+
+            void
+            init(error_code& ec);
 
             boost::optional<std::pair<const_buffers_type, bool>>
             get(error_code&);
@@ -45,8 +47,10 @@ public:
                 boost::asio::const_buffers_1;
 
             template<bool isRequest, class Fields>
-            reader(message<isRequest, mutable_body, Fields>&,
-                error_code&);
+            reader(message<isRequest, mutable_body, Fields>&);
+
+            void
+            init(error_code& ec);
 
             boost::optional<std::pair<const_buffers_type, bool>>
             get(error_code&);
@@ -55,6 +59,7 @@ public:
 
     BOOST_STATIC_ASSERT(std::is_const<  serializer<
         true, const_body>::value_type>::value);
+
     BOOST_STATIC_ASSERT(! std::is_const<serializer<
         true, mutable_body>::value_type>::value);
 
diff --git a/test/http/write.cpp b/test/http/write.cpp
index d072fa2b..bb0a7f63 100644
--- a/test/http/write.cpp
+++ b/test/http/write.cpp
@@ -47,13 +47,18 @@ public:
 
             template<bool isRequest, class Allocator>
             explicit
-            reader(message<isRequest, unsized_body,
-                    Allocator> const& msg, error_code &ec)
+            reader(message<isRequest,
+                    unsized_body, Allocator> const& msg)
                 : body_(msg.body)
+            {
+            }
+
+            void
+            init(error_code& ec)
             {
                 ec.assign(0, ec.category());
             }
-            
+
             boost::optional<std::pair<const_buffers_type, bool>>
             get(error_code& ec)
             {
@@ -87,9 +92,14 @@ public:
 
             template<bool isRequest, class Fields>
             explicit
-            reader(message<isRequest, test_body,
-                    Fields> const& msg, error_code& ec)
+            reader(message<isRequest,
+                    test_body, Fields> const& msg)
                 : body_(msg.body)
+            {
+            }
+
+            void
+            init(error_code& ec)
             {
                 ec.assign(0, ec.category());
             }
@@ -219,9 +229,14 @@ public:
 
             template<bool isRequest, class Allocator>
             explicit
-            reader(message<isRequest, fail_body,
-                    Allocator> const& msg, error_code& ec)
+            reader(message<isRequest,
+                    fail_body, Allocator> const& msg)
                 : body_(msg.body)
+            {
+            }
+
+            void
+            init(error_code& ec)
             {
                 body_.fc_.fail(ec);
             }