RE: [boost] Review : format2, yet another 'printf class'

From: Samuel Krempp (krempp_at_[hidden])
Date: 2001-07-03 11:36:39

> You mean "is strictly equivalent" to the following, right?
>
> ios_base::fmtflags old_flags(cout.flags());
> (cout << hex << n).flags(old_flags);

exactly, I forgot to unset the flags..
(well, a good example it really is a good thing to have a non-sticky
behaviour ! )

> Ok, so 'x' is not a conversion character or type indicator here, it's just a
> shortcuted form of "non-sticky" 'std::hex' manipulator that will be used to
> change a state of the output stream for the time the argument it corresponds
> to will be printed. What's more, interpretation of 'x' does not depend on
> the actual type of the supposedly affected parameter, e.g. if 'n' is an
[snip]

exactly !
I should have given the example with the pair before.
(it's easier to give an example than a formal definition of what is format
doing..)

> (and http://groups.yahoo.com/group/boost/files/format2/Readme.txt doesn't
> bring a lot of light here, mostly because of intensive mentioning of printf
> and printf-compatibility mode without going into details how it really
> works), but if you know the rules the whole picture looks quite consistent

that's because I was busy finishing work on bind_arg, set_flag, and coping
with manipulators.

the Readme.txt is already a bit better, as it gives a (very succinct)
example of behaviour with a user-defined tpye,
and also states clearly what are the effects of the flags on the
underlying streams.

I think it should now be already easier to understand the 'big picture',
as you say.

[snip]
> - I think that's cute :).

great !

> [snip]
> > 2. beyond printf usual field,
> >    use its syntax to express formatting option,
> >    where the flags' meanings are their most natural interpretations :
> >    example : range = make_pair<int,int>( n1, n2);
> > 	     int i;
> > 	     cout << format("%p1$4x %2\n") %range %i;
> > 	 will do the same as
> > 	     cout << hex << setw(4) << range << dec << setw(0)
> > 		<< " " << i << "\n";
>
> That's a nice example. I believe that you have to save and restore the state
> of the stream before and after each argument's output, instead of resetting
> it to the default one (dec, setw(0), etc.), though.

well, once again, my example is faulty.
the 2 things are not really equivalent, because format indeed restores to
the previous state, not a default state.

In fact, the whole subtlety is to set / restore only what we should, no
more (and no less, but that is not the problem)

Because manipulators passed with % might have changed the stream, and
these changes are supposed to *stay* , even when the argument appears in
several directives. ("%1 %1 %1")

I believe it's actually doing the right thing right now.
(else please let me know..)

-- 
Sam