Re: [boost] Re: Thoughts on Format

From: Karl Nelson (kenelson_at_[hidden])
Date: 2002-01-29 14:23:41

> > This gets a lot harder when you start throwing in 
> > manipulators unfortunately.
> > That is why I am switching to a groupable format like...
> > 
> >   cout << format( fstr,  1.0, group(hex,width(10),16), "bob");
> > 
> > rather than
> > 
> >   cout << format(fstr) % 1.0 % hex % width(10) % 16 % "bob" ;
> 
> I definitely like the grouping format for local manipulators.  For
> persistent manipulators, however, it is not appropriate.  We need both.

Why should their be persistent manipulators in a format stream.  
They don't make sense because they can get moved up when reordered.

Consider if I allow persistent manipulators and just have a simple
reorder stream.

  cout << format( fstr) % 16 % hex % 16;
Okay so if fstr is "[1] = [2] base 16" thus we print
"16 = 10 base 16".  But if manipulators are persistent if 
someone attempts to reverse the order in some translation they
write "[2] ???? 16 = [1]", this implies
  cout << hex<<16 " ???? 16 = " << 16;
Thus we print "10 ???? 16 = 10".  Ouch, not what we want.

Thus no manipulators should be persistent.

> > In the first the positionally nature is there.  Plus you can now
> > see the manipulators are only to the one argument.
> > 
> > It also is good in that you can group arguments in reasonable ways...
> > 
> >   cout << format(" Change is %d", group( c>0?'+':'-', c) );
> > 
> > Thus both the char and the int get placed in the position 1.
> 
> Clever idea.  I like it.
> 
> Only the %d confuses the issue.  I would expect the %d to be applied to each
> argument, meaning for c=3 you would get: " Change is 433".  This is a good
> example of why is it best of avoid type specifiers unless you really want to
> override the current format specified by the manipulator context (e.g.
> override decimal with hex or vice verse).

I take this argument a completely different way.  Yes, type specifiers 
should not convert types, but rather they simply should not be taken
as literial.

I take the type as being very general.  
  x - stream is hex (any numbers in it are shown as hex)
  d - stream is decimal (any numbers in it are shown as decimal)
  s - string literal (stream defaults)

Thus %d on group(c>0?'+':'-',c) is "<< dec << '+' << 3".

We don't try to convert the argument.  It is also almost impossible
to do because, we would have to force the types to fix basic types
to interpret them.

Ie.

  cout << format("%.2f") % complex<int>(0,0); 

How would you know to convert 0 to float(0.0) so we get the 
proper output?

--Karl