Andrei Alexandrescu Wrote: > On 12/28/10 5:09 AM, Vladimir Panteleev wrote: > >> abstract interface Formatter; > > > > I'm really not sure about this interface. I can see at most three > > implementations of it (native, high-endian and low-endian variants), > > everything else being too obscure to count. I think it should be > > implemented as static structs instead. Also, having an abstract method > > for each native type is quite ugly for D standards, I'm sure there's a > > better solution. > > Nonono. Perhaps I chose the wrong name, but Formatter is really anything > that takes typed data and encodes it in raw bytes suitable for > transporting. That includes e.g. json, csv, and also a variety of binary > formats.
This one is really difficult to get right. JSON, for example, has named members of its object type. How could the name of a field be communicated to the formatter? The best I was able to do with C++ iostreams was to create an abstract formatter class that knew about the types I needed to format and have protocol-specific derived classes do the work. Here's some of the dispatching code: printer* get_printer( std::ios_base& str ) { void*& ptr = str.pword( printer::stream_index() ); if( ptr == NULL ) { str.register_callback( &printer_callback, printer::stream_index() ); ptr = new xml_printer(); } return static_cast<printer*>( ptr ); } std::ostream& operator<<( std::ostream& os, const message_header& val ) { printer* ptr = get_printer( os ); return (*ptr)( os, val ); } Actually using this code to write data to a stream looks great: ostr << header << someobj << anotherobj<< end_msg; but I'm not happy about how much specialized underlying code needs to exist. I guess what I'm saying is that a generic formatter may be great for simple formats like zip streams, CSV files, etc, but not so much for more structured output. That may be a sufficient goal for std.stream2, but if so I'd remove JSON from your list of possible output formats :-)