dbus_message_get_args suggestion

[Piotr Zielinski]

Could you tell me what you think about the following suggestion?

Summary:

Introduce a variant of dbus_message_get_args from dbus-message.c, with a
different name, that differs from the original in the way it handles container
types (arrays, structs,  etc.).  For these, the new function would return the
iterator ready to iterate over the container.  For example, consider a method
call with the signature "isa{ss}".  Then we could write,

const char *string;
dbus_int32_t number;
DBusMessageIter hashiter;

dbus_message_get_args_new_variant(message, error, 
  DBUS_TYPE_STRING, &string,
  DBUS_TYPE_INT32, &number,
  DBUS_TYPE_ARRAY, &hashiter,
  DBUS_TYPE_INVALID);

and use hashiter to iterate over entries from the "a(ss}" component.


Justification:

Currently, you can read the arguments of a message either directly (with
dbus_message_get_args) or using an iterator.  Usually, the former approach is
more convenient because it requires less code and auxiliary variables.  However,
some complex types can't be handled by dbus_message_get_args.  True,
dbus_message_get_args can handle some container types (eg. an array of string),
but it will never be able to handle hash tables as standard C libraries do not
provide such data structures, and I don't think adding GLib dependencies is a
good idea.  For more complicated types, it's likely that the user will need to
handle them is a special way, which the dbus library does not implement. 
Moreover, even if the dbus library can magically do that, this would require a
dynamic allocation of complex data structures, which might be tricky to
deallocate (just think of freeing something like "a(ia{ss})").

Iterators solve this problem, but as I said before, they are not convenient for
simple types, which are most common.  It has happened to me a few times that I
had a function taking a couple of simple arguments, say "ssiii", and then I had
to add a complex one, getting say "ssiiia{ss}".  Ideally, I'd like to add just a
new entry to the dbus_message_get_args call, and get the iterator for the hash
table "a{ss}" along with five other parameters.  However, at the moment a single
parameter like "a{ss}" forces me to use iterators for the first five parameters
as well, which only clutters the code.

If you think providing a new variant of dbus_message_get_args is a good idea, I
can write a patch that implements it.

Piotr