async_read_header needs documentation and example code

[OleksandrKvl]

beast version: 181 compiler: VS2017 15.8.8

Got an error when used async_read_header() like this:

#include <boost/beast.hpp>
#include <boost/asio.hpp>

void main()
{
	namespace http = boost::beast::http;
	using tcp = boost::asio::ip::tcp;

	boost::asio::io_context ioc;
	tcp::socket s{ ioc };
	boost::beast::flat_buffer buffer{ 8192 };
	http::request<http::dynamic_body> req;
	http::request_header<> req_header;

	http::async_read_header(
		s, buffer, req_header, [](const auto & error, auto length){});

	http::async_read_header(
		s, buffer, req, [](const auto & error, auto length) {});
}

Error:

1>------ Build started: Project: beast_wrapper, Configuration: Debug Win32 ------
1>beast_wrapper.cpp
1>beast_wrapper.cpp(258): error C2672: 'boost::beast::http::async_read_header': no matching overloaded function found
1>beast_wrapper.cpp(259): error C2784: '::boost::asio::async_result<::boost::asio::decay<ReadHandler>::type,void(boost::beast::error_code,size_t)>::return_type boost::beast::http::async_read_header(AsyncReadStream &,DynamicBuffer &,boost::beast::http::basic_parser<isRequest,Derived> &,ReadHandler &&)': could not deduce template argument for 'boost::beast::http::basic_parser<isRequest,Derived> &' from 'boost::beast::http::header<true,Fields>'
1>        with
1>        [
1>            Fields=boost::beast::http::fields
1>        ]
1>\boost_1_68_0\boost\beast\http\read.hpp(386): note: see declaration of 'boost::beast::http::async_read_header'
1>beast_wrapper.cpp(261): error C2672: 'boost::beast::http::async_read_header': no matching overloaded function found
1>beast_wrapper.cpp(262): error C2784: '::boost::asio::async_result<::boost::asio::decay<ReadHandler>::type,void(boost::beast::error_code,size_t)>::return_type boost::beast::http::async_read_header(AsyncReadStream &,DynamicBuffer &,boost::beast::http::basic_parser<isRequest,Derived> &,ReadHandler &&)': could not deduce template argument for 'boost::beast::http::basic_parser<isRequest,Derived> &' from 'boost::beast::http::message<true,boost::beast::http::dynamic_body,boost::beast::http::fields>'
1>\boost_1_68_0\boost\beast\http\read.hpp(386): note: see declaration of 'boost::beast::http::async_read_header'
1>Done building project "beast_wrapper.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

What am I doing wrong, should it work with both request and request_header?

[vinniefalco]

should it work with both request and request_header?

No, you can only read into a request. Try http::request<http::empty_body> req.

[OleksandrKvl]

Thank you for fast response, same error:

1>\beast_wrapper.cpp(256): error C2672: 'boost::beast::http::async_read_header': no matching overloaded function found
1>\beast_wrapper.cpp(257): error C2784: '::boost::asio::async_result<::boost::asio::decay<ReadHandler>::type,void(boost::beast::error_code,size_t)>::return_type boost::beast::http::async_read_header(AsyncReadStream &,DynamicBuffer &,boost::beast::http::basic_parser<isRequest,Derived> &,ReadHandler &&)': could not deduce template argument for 'boost::beast::http::basic_parser<isRequest,Derived> &' from 'boost::beast::http::message<true,boost::beast::http::empty_body,boost::beast::http::fields>'
1>\boost_1_68_0\boost\beast\http\read.hpp(386): note: see declaration of 'boost::beast::http::async_read_header'

[vinniefalco]

Oh, I'm sorry about that.. you can only read into a parser. See the documentation page: https://www.boost.org/doc/libs/1_69_0/libs/beast/doc/html/beast/ref/boost__beast__http__async_read_header.html

The reason is simple, because if you only read the header there is state information that would be lost if the parser was discarded. Thus Beast requires that you use the parser oriented interface.

There's an example of parsing just the header here:

    parser<isRequest, buffer_body> p;
    read_header(stream, buffer, p, ec);

https://www.boost.org/doc/libs/1_69_0/libs/beast/doc/html/beast/using_http/parser_stream_operations/incremental_read.html

[OleksandrKvl]

It works, thank you :+1:

[vinniefalco]

I'm going to go ahead and keep this open, after renaming the title, as a reminder when I work on the documentation to provide explicit example code - thanks!

[gsanjeevkumar]

Good Work! is the example available, Please!