[/
    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:Body Body]

A [*Body] type is supplied as a template argument to the __message__ class. It
controls both the type of the data member of the resulting message object, and
the algorithms used during parsing and serialization.

In this table:

* `X` is a type meeting the requirements of [*Body].

* `m` is a value of type `message<b, X, F>` where `b` is a `bool` value
  and `F` is a type meeting the requirements of [*Fields].

[table Body requirements
[[expression] [type] [semantics, pre/post-conditions]]
[
    [`X::value_type`]
    []
    [
        The type of the `message::body` member.
        If this is not movable or not copyable, the containing message
        will be not movable or not copyable.
    ]
][
    [`X::writer`]
    []
    [
        If present, indicates that the body can hold a message body
        parsing result. The type must meet the requirements of
        __BodyWriter__. The implementation constructs an object of
        this type to obtain buffers into which parsed body octets
        are placed.
    ]
][
    [`X::reader`]
    []
    [
        If present, indicates that the body is serializable. The type
        must meet the requirements of __BodyReader__. The implementation
        constructs an object of this type to obtain buffers representing
        the message body for serialization.
    ]
][
    [`X::size(X::value_type v)`]
    [`std::uint64_t`]
    [
        This static member function is optional. It returns the payload
        size of `v` not including any chunked transfer encoding. The
        function shall not exit via an exception.

        When this function is present:

        * The function shall not fail

        * A call to
        [link beast.ref.beast__http__message.payload_size `message::payload_size`]
        will return the same value as `size`.

        * A call to
        [link beast.ref.beast__http__message.prepare_payload `message::prepare_payload`]
        will remove "chunked" from the Transfer-Encoding field if it appears
        as the last encoding, and will set the Content-Length field to the
        returned value.

        Otherwise, when the function is omitted:

        * A call to
        [link beast.ref.beast__http__message.payload_size `message::payload_size`]
        will return `boost::none`.

        * A call to
        [link beast.ref.beast__http__message.prepare_payload `message::prepare_payload`]
        will erase the Content-Length field, and add "chunked" as the last
        encoding in the Transfer-Encoding field if it is not already present.
    ]
][
    [`is_body<X>`]
    [`std::true_type`]
    [
        An alias for `std::true_type` for `X`, otherwise an alias
        for `std::false_type`.
    ]
]
]

[heading Exemplar]

[concept_Body]

[heading Models]

* [link beast.ref.beast__http__basic_dynamic_body `basic_dynamic_body`]
* [link beast.ref.beast__http__buffer_body `buffer_body`]
* [link beast.ref.beast__http__dynamic_body `dynamic_body`]
* [link beast.ref.beast__http__empty_body `empty_body`]
* [link beast.ref.beast__http__string_body `string_body`]
* [link beast.ref.beast__http__string_view_body `string_view_body`]

[endsect]