boost_beast/doc/5_08_custom_parsers.qbk

[/
    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:custom_parsers Custom Parsers]

While the parsers included in the library will handle a broad number of
use-cases, the __basic_parser__ interface can be subclassed to implement
custom parsing strategies: the parser processes the incoming octets into
elements according to the HTTP/1 protocol specification, while the derived
class decides what to do with those elements. In particular, users who
create exotic containers for [*Fields] may need to also create their own
parser. Custom parsers will work with all of the stream read operations
that work on parsers, as those algorithms use only the basic parser
interface. Some use cases for implementing custom parsers are:

* Inspect incoming header fields and keep or discard them.

* Use a container provided by an external interface.

* Store header data in a user-defined __Fields__ type.

The basic parser uses the Curiously Recurring Template Pattern
([@https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern CRTP]).
To declare your user defined parser, derive it from __basic_parser__.
The interface to the parser is event-driven. Member functions of the derived
class (termed "callbacks" in this context) are invoked with parsed elements
as they become available, requiring either the `friend` declaration as shown
above or that the member functions are declared public (not recommended).
Buffers provided by the parser are non-owning references; it is the
responsibility of the derived class to copy any information it needs before
returning from the callback.

[http_sample_custom_parser]

[endsect]