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Read a complete message from a stream asynchronously using a parser.
          Defined in header <boost/beast/http/read.hpp>
        
template< class AsyncReadStream, class DynamicBuffer, bool isRequest, class Derived, class ReadHandler> void-or-deduced async_read( AsyncReadStream& stream, DynamicBuffer& buffer, basic_parser< isRequest, Derived >& parser, ReadHandler&& handler);
          This function is used to asynchronously read a complete message 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:
        
http::basic_parser::is_done returns 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:
        
http::end_of_stream
              if no octets were parsed, or
            http::partial_message
              if any octets were parsed but the message was incomplete, otherwise:
            http::end_of_stream
            | Name | Description | 
|---|---|
| 
                     | 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  | 
          The implementation will call http::basic_parser::eager with the value true on the parser passed in.
        
          Convenience header <boost/beast/http.hpp>