boost_beast/doc/4_8_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.


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, first derive it from __basic_parser__:
```
template<bool isRequest>
class custom_parser
    : public basic_parser<isRequest, custom_parser<isRequest>>
{
    friend class basic_parser<isRequest, custom_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.


```
template<bool isRequest>
class custom_parser
    : public basic_parser<isRequest, custom_parser<isRequest>>
{
    friend class basic_parser<isRequest, custom_parser>;
    
    /// Called after receiving the request-line (isRequest == true).
    void
    on_request(
        string_view method,     // The method
        string_view target,     // The request-target
        int version,            // The HTTP-version
        error_code& ec);        // The error returned to the caller, if any

    /// Called after receiving the start-line (isRequest == false).
    void
    on_response(
        int status,             // The status-code
        string_view reason,     // The obsolete reason-phrase
        int version,            // The HTTP-version
        error_code& ec);        // The error returned to the caller, if any

    /// Called after receiving a header field.
    void
    on_field(
        string_view name,       // The field name
        string_view value,      // The field value
        error_code& ec);        // The error returned to the caller, if any

    /// Called after the complete header is received.
    void
    on_header(
        error_code& ec);        // The error returned to the caller, if any

    /// Called just before processing the body, if a body exists.
    void
    on_body(boost::optional<
            std::uint64_t> const&
        content_length,         // Content length if known, else `boost::none`
        error_code& ec);        // The error returned to the caller, if any

    /// Called for each piece of the body, if a body exists.
    //
    //  If present, the chunked Transfer-Encoding will be removed
    //  before this callback is invoked.
    //
    void
    on_data(
        string_view s,          // A portion of the body
        error_code& ec);        // The error returned to the caller, if any

    /// Called for each chunk header.
    void
    on_chunk(
        std::uint64_t size,     // The size of the upcoming chunk
        string_view extension,  // The chunk-extension (may be empty)
        error_code& ec);        // The error returned to the caller, if any

    /// Called when the complete message is parsed.
    void
    on_complete(error_code& ec);

public:
    custom_parser() = default;
};
```

[endsect]