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Read a complete message asynchronously from a stream.
          Defined in header <boost/beast/http/read.hpp>
        
template< class AsyncReadStream, class DynamicBuffer, bool isRequest, class Body, class Allocator, class ReadHandler> async_read( AsyncReadStream& stream, DynamicBuffer& buffer, message< isRequest, Body, basic_fields< Allocator >>& msg, ReadHandler&& handler);
          This function is used to asynchronously read a complete message from a
          stream into an instance of http::message. 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 zero or more calls to the next
          layer'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 bytes from
          the stream that lie past the end of the message being read. These additional
          bytes are stored in the dynamic buffer, which must be preserved for subsequent
          reads. If the end of file error is received while reading from the stream,
          then the error returned from this function will be:
        
http::end_of_stream
              if no bytes were parsed, or
            http::partial_message
              if any bytes 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 meet the AsyncReadStream requirements. | 
| 
                     | Storage for additional bytes read by the implementation from the stream. This is both an input and an output parameter; on entry, the parser will be presented with any remaining data in the dynamic buffer's readable bytes sequence first. The type must meet the DynamicBuffer requirements. The object must remain valid at least until the handler is called; ownership is not transferred. | 
| 
                     | The container in which to store the message contents. This message container should not have previous contents, otherwise the behavior is undefined. The type must be meet the MoveAssignable and MoveConstructible requirements. The object must remain valid at least until the handler is called; ownership is not transferred. | 
| 
                     | The completion handler to invoke when the operation completes. The implementation takes ownership of the handler by performing a decay-copy. The equivalent function signature of the handler must be: void handler( error_code const& error, // result of operation std::size_t bytes_transferred // the total number of bytes transferred from the stream ); 
                    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 completion handler will receive as a parameter the total number of
          bytes transferred from the stream. This may be zero for the case where
          there is sufficient pre-existing message data in the dynamic buffer. The
          implementation will call http::basic_parser::eager with the value true on the parser passed in.
        
          Convenience header <boost/beast/http.hpp>