[/
    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(m)`]
    [`std::uint64_t`]
    [
        If present, returns the serialized size of `m.body` not including
        any chunked transfer encoding. When this function is provided,
        [link beast.ref.http__message.prepare `message::prepare`]
        will automatically set the content length field based on the
        value. Otherwise, the chunked transfer encoding will be set.
    ]
][
    [`is_body<X>`]
    [`std::true_type`]
    [
        An alias for `std::true_type` for `X`, otherwise an alias
        for `std::false_type`.
    ]
]
]

Exemplar:
```
struct body
{
    using value_type = implementation_defined;

    /// The algorithm used for extracting buffers
    class reader;

    /// The algorithm used for inserting buffers
    class writer;

    /// Returns the body's payload size
    template<bool isRequest, class Fields>
    static
    std::uint64_t
    size(message<isRequest, empty_body, Fields> const& m);
}
```

[endsect]