[/
    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]

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.beast__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`]
    []
    [
        The type `std::true_type` if the serialization implementation
        should only attempt to retrieve buffers from the reader after
        the header has been serialized. Otherwise, if this type is
        `std::false_type` the implementation will activate an
        optimization: the first buffer produced during serialization
        will contain both the header and some or all of this body.

        Implementations of [*BodyReader] for which initialization is
        expensive, should use `std::false_type` here, to reduce the
        latency experienced by the remote host when expecting to read
        the HTTP header.
    ]
][
    [`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.
        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.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.
        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.finish(ec)`]
    []
    [
        This function is called after the reader indicates there
        are no more buffers remaining.
        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`.
    ]
]
]

[note
    Definitions for required [*BodyReader] member functions may
    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 serialize both the header and some
        or all of the body in a single buffer.
    */
    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 retrieved.
    */
    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)
    {
        // The specification requires this to indicate "no error"
        ec = {};
    }

    /** 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.

        @param ec Set to the error, if any occurred.
    */
    boost::optional<std::pair<const_buffers_type, bool>>
    get(error_code& ec);

    /** Called after `get` indicates there are no more buffers.

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

[endsect]