On Tue, 28 Dec 2010 23:34:42 -0700, Andrei Alexandrescu
<seewebsiteforem...@erdani.org> wrote:
On 12/28/10 11:54 AM, Sean Kelly wrote:
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 :-)
I agree with the spirit. In brief, I think it's fine to have a Json
formatter as long as data is provided to it as Json-friendly types
(ints, strings, arrays, associative arrays). In other words, I need to
simplify the interface to not attempt to format class and struct types -
only built-in types.
By the way, JSON doesn't support associative arrays in general. It only
supports AA in the sense that JSON objects are an array of string:value
pairs.
