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Read a header from a stream asynchronously using a parser.


Defined in header <boost/beast/http/read.hpp>

    class AsyncReadStream,
    class DynamicBuffer,
    bool isRequest,
    class Derived,
    class ReadHandler>
    AsyncReadStream& stream,
    DynamicBuffer& buffer,
    basic_parser< isRequest, Derived >& parser,
    ReadHandler&& handler);

This function is used to asynchronously read a header from a stream into a subclass of http::basic_parser. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

This operation is implemented in terms of one or more calls to the stream's async_read_some function, and is known as a composed operation. The program must ensure that the stream performs no other reads until this operation completes. The implementation may read additional octets that lie past the end of the message being read. This additional data is stored in the dynamic buffer, which must be retained for subsequent reads.

If the stream returns the error boost::asio::error::eof indicating the end of file during a read, the error returned from this function will be:





The stream from which the data is to be read. The type must support the AsyncReadStream concept.


A DynamicBuffer holding additional bytes read by the implementation from the stream. This is both an input and an output parameter; on entry, any data in the dynamic buffer's input sequence will be given to the parser first.


The parser to use. The object must remain valid at least until the handler is called; ownership is not transferred.


The handler to be called when the request completes. Copies will be made of the handler as required. The equivalent function signature of the handler must be:

 void handler(
    error_code const& error,        // result of operation,
    std::size_t bytes_transferred   // the number of bytes transferred to the parser

Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using boost::asio::io_context::post.


The implementation will call http::basic_parser::eager with the value false on the parser passed in.

Convenience header <boost/beast/http.hpp>