[/
    Copyright (c) 2013-2017 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)
]

[section:BodyReader BodyReader requirements]

A [*BodyReader] provides an online algorithm to obtain a sequence of zero
or more buffers from a body during serialization. The implementation creates
an instance of this type when needed, and calls into it one or more times to
retrieve buffers holding body octets. The interface of [*BodyReader] is
intended to obtain buffers for these scenarios:

* A body that does not entirely fit in memory.
* A body produced incrementally from coroutine output.
* A body represented by zero or more buffers already in memory.
* A body whose size is not known ahead of time.
* Body data generated dynamically from other threads.
* Body data computed algorithmically.

In this table:

* `X` denotes a type meeting the requirements of [*BodyReader].

* `B` denotes a __Body__ where
      `std::is_same<X, B::reader>::value == true`.

* `a` denotes a value of type `X`.

* `m` denotes a value of type `message const&` where
      `std::is_same<decltype(m.body), Body::value_type>:value == true`.

* `ec` is a value of type [link beast.ref.error_code `error_code&`].

* `R<T>` is the type `boost::optional<std::pair<T, bool>>`.

[table BodyReader requirements
[[expression] [type] [semantics, pre/post-conditions]]
[
    [`X::const_buffers_type`]
    []
    [
        A type which meets the requirements of __ConstBufferSequence__.
        This is the type of buffer returned by `X::get`.
    ]
]
[
    [`X::is_deferred`]
    []
    [
    ]
]
[
    [`X(m);`]
    []
    [
        Constructible from `m`. The lifetime of `m` is guaranteed
        to end no earlier than after the `X` is destroyed.
    ]
]
[
    [`a.init(ec)`]
    []
    [
        Called immediately after construction. If the function sets an
        error code in `ec`, the serialization is aborted and the error
        is propagated to the caller.
    ]
]
[
    [`a.content_length()`]
    [`std::uint64_t`]
    [
        If this member is present, it is called after initialization
        and before calls to provide buffers. The serialized message will
        have the Content-Length field set to the value returned from
        this function. If this member is absent, the serialized message
        body will be chunk-encoded for HTTP versions 1.1 and later, else
        the serialized message body will be sent unmodified, with the
        error `boost::asio::error::eof` returned to the caller, to notify
        they should close the connection to indicate the end of the message.
        This function must be `noexcept`.
    ]
]
[
    [`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
        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
        the body, while the second element is a `bool` meaning `true`
        if there may be additional buffers returned on a subsequent call,
        or `false` if the buffer returned on this call is the last
        buffer representing the body.
    ]
]
[
    [`is_body_reader<B>`]
    [`std::true_type`]
    [
        An alias for `std::true_type` for `B`, otherwise an alias
        for `std::false_type`.
    ]
]
]

[note
    Definitions for required [*BodyReader] member functions should
    be declared inline so the generated code can become part of the
    implementation.
]

Exemplar:
```
struct reader
{
public:
    /** Controls when the implementation requests buffers.

        If false, the implementation will request the first buffer
        immediately and try to send both the header and the body
        buffer in a single call to the stream's `write_some`
        function.
    */
    using is_deferred = std::false_type;

    /** The type of buffer returned by `get`.
    */
    using const_buffers_type = boost::asio::const_buffers_1;

    /** Construct the reader.

        @param msg The message whose body is to be written.
    */
    template<bool isRequest, class Body, class Headers>
    explicit
    reader(message<isRequest, Body, Headers> const& msg);

    /** Initialization.

        Called once immediately after construction.

        @param ec Set to the error, if any occurred.
    */
    void
    init(error_code& ec);

    /** Returns the content length.

        If this member is present, the implementation will set the
        Content-Length field accordingly. If absent, the implementation
        will use chunk-encoding or terminate the connection to indicate
        the end of the message.
    */
    std::uint64_t
    content_length();

    /** Returns the next buffer in the body.

        @li If the return value is `boost::none` (unseated optional) and
            `ec` does not contain an error, this indicates the end of the
            body, no more buffers are present.

        @li If the optional contains a value, the first element of the
            pair represents a @b ConstBufferSequence containing one or
            more octets of the body data. The second element indicates
            whether or not there are additional octets of body data.
            A value of `true` means there is more data, and that the
            implementation will perform a subsequent call to `get`.
            A value of `false` means there is no more body data.

        @li If `ec` contains an error code, the return value is ignored.
    */
    boost::optional<std::pair<const_buffers_type, bool>>
    get(error_code& ec);
};
```

[endsect]