[/
    Copyright (c) 2016-2019 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)

    Official repository: https://github.com/boostorg/beast
]

[section Custom Body Types]
[block'''<?dbhtml stop-chunking?>''']

User-defined types are possible for the message body, where the type meets the
__Body__ requirements. This simplified class declaration
shows the customization points available to user-defined body types:
```
/// Defines a Body type
struct body
{
    /// This determines the return type of the `message::body` member function
    using value_type = ...;

    /// An optional function, returns the body's payload size (which may be zero)
    static
    std::uint64_t
    size(value_type const& v);

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

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

The meaning of the nested types is as follows

[table Body Type Members
[[Name][Description]]
[
    [`value_type`]
    [
        Determines the type of the
        [link beast.ref.boost__beast__http__message.body `message::body`]
        member.
    ]
][
    [`reader`]
    [
        An optional nested type meeting the requirements of __BodyReader__,
        which provides the algorithm for storing a forward range of buffer
        sequences in the body representation.
        If present, this body type may be used with a __parser__.
    ]
][
    [`writer`]
    [
        An optional nested type meeting the requirements of __BodyWriter__,
        which provides the algorithm for converting the body representation
        to a forward range of buffer sequences.
        If present this body type may be used with a __serializer__.
    ]
]
]

[heading Value Type]

The `value_type` nested type allows the body to define the declaration of
the body type as it appears in the message. This can be any type. For
example, a body's value type may specify `std::vector<char>` or 
`std::list<std::string>`. A custom body may even set the value type to
something that is not a container for body octets, such as a
[@boost:/libs/filesystem/doc/reference.html#class-path `boost::filesystem::path`].
Or, a more structured container may be chosen. This declares a body's
value type as a JSON tree structure produced from a
[@boost:/doc/html/json/input_output.html#json.input_output.parsing.streaming_parser `boost::json::stream_parser`]:
```
#include <boost/json/stream_parser.hpp>

struct Body
{
    using value_type = boost::json::value;

    class reader;
    class writer;
};
```

As long as a suitable reader or writer is available to provide the
algorithm for transferring buffers in and out of the value type,
those bodies may be parsed or serialized.



[section:file_body File Body __example__]

Use of the flexible __Body__ concept customization point enables authors to
preserve the self-contained nature of the __message__ object while allowing
domain specific behaviors. Common operations for HTTP servers include sending
responses which deliver file contents, and allowing for file uploads. In this
example we build the
[link beast.ref.boost__beast__http__basic_file_body `basic_file_body`]
type which supports both reading and
writing to a file on the file system. The interface is a class templated
on the type of file used to access the file system, which must meet the
requirements of __File__.

First we declare the type with its nested types:

[example_http_file_body_1]

We will start with the definition of the `value_type`. Our strategy
will be to store the file object directly in the message container
through the `value_type` field. To use this body it will be necessary
to call `msg.body.file().open()` first with the required information
such as the path and open mode. This ensures that the file exists
throughout the operation and prevent the race condition where the
file is removed from the file system in between calls.

[example_http_file_body_2]

Our implementation of __BodyWriter__ will contain a small buffer
from which the file contents are read. The buffer is provided to
the implementation on each call until everything has been read in.

[example_http_file_body_3]

And here are the definitions for the functions we have declared:

[example_http_file_body_4]

Files can be read now, and the next step is to allow writing to files
by implementing the __BodyReader__. The style is similar to the writer,
except that buffers are incoming instead of outgoing. Here's the
declaration:

[example_http_file_body_5]

Finally, here is the implementation of the reader member functions:

[example_http_file_body_6]

We have created a full featured body type capable of reading and
writing files on the filesystem, integrating seamlessly with the
HTTP algorithms and message container. The body type works with
any file implementation meeting the requirements of __File__ so
it may be transparently used with solutions optimized for particular
platforms. Example HTTP servers which use file bodies are available
in the example directory.

[endsect]



[endsect]
