Well, I think my alternative is better if you want a stateful change, since yours doesn't accomplish that. My suggestion is certainly consistent with normal io manipulators. The point is to stream a bunch of sequences without having to repeat the format part.
I understand what you are saying and yes your version is neater. However, implementing something like this would have performance penalties (because you will need to create/store the preset values at run-time).
The penalties are small. Anyway, using std::istream/ostream have penalties, compared to using raw FILEs ;) That said, in this case there shouldn't be many penalties. Again, you can use ios_base::pword and ios_base::iword for this. It's an advanced technique, but I think it can prove very neat and flexible.
It will also change the way all sequence constructs are rendered. (n-ary types have a different rendreing so won't be affected). As I understand, this is the desired effect. But then:
You can make it possible to associate a fmt object with a delimeter object. Example: pairfmt() with openclose_formatter. basicfmt() with formatter. Etc. Then, you can store (default) delimeter objects within the stream. Once you want to output/input something, it requires a specific delim object. You'll somehow use ios_base::pword/iword to get a reference to that object.
[3] how do you extend this to formatter types that have additional delimeters? see above.
[4] how do you know when to extract the default delimeter values from the stream?
see above. Best, John -- John Torjo -- john@torjo.com Contributing editor, C/C++ Users Journal -- "Win32 GUI Generics" -- generics & GUI do mix, after all -- http://www.torjo.com/win32gui/ -- v1.4.0 - save_dlg - true binding of your data to UI controls! + easily add validation rules (win32gui/examples/smart_dlg)