Re: Combining fixed and scientific in stringstream

James Kanze <>
Wed, 23 Jul 2008 15:14:58 -0700 (PDT)
On Jul 23, 11:24 am, Johannes Bauer <> wrote:

Mirco Wahab schrieb:

I don't know of any trick to format the exponent in %g-mode,
so this is what I have:

In general, you can't format the exponent, neither in %g nor in
%e (and nor with ostream nor with printf).


 s << setw(5) << setprecision(3) << 1. << endl;
 s << setw(5) << setprecision(2) << 12. << endl;
 s << setw(5) << setprecision(1) << 123. << endl;

Well, yes, the output looks right - but the "setprecision"
part will be a pain to implement, probably... I just thought
maybe C++ provided some powerful means of formatting those

The problem, really is that you want the formatting to depend
(at least partially) on the value. Neither C (printf) nor C++
really support this.

You might try setting showpoint, however. According to the
standard, this should be the equivalent of using the '#' flag in
printf, which forces trailing zeros in %g. So despite its name,
it should work for that, and does with g++. But you'll still
not get E4 when the %e format is used: the standards (both C and
C++) require E+04 (i.e. the sign, even if positive, and exactly
two digits, if it fits).

An yes, Posix printf/sprintf is really powerful.

Funny, I never noticed that. The iostream formatters are a lot
more powerful, at least in terms of what you can do. And if
they don't suffice, of course, you just write a wrapper class,
and define << and >> for it.

missing in std is a printf-conformal:
   ostream& format(ostream&, "printf-format", value list)
or something like that.

Yes, it is very much indeed :-(

printf is probably the thing I miss most when using C++. There
it would be a mere "%.3lf"

That won't get the format you were requesting. What you were
requesting was basically '%#.4g" (but with a different format
for the exponent, if scientific format was used---which isn't

More generally, of course, you don't really want to specify such
details at the point of the output statement. You want to
specify what the value means, semantically, and then somewhere
globally (e.g. like in a style sheet) specify how such semantic
values are formatted. You can sort of do this with printf:

    printf( ("some text " + semanticFormat + " so on).c_str(), ... ) ;

but it hardly seems natural. Where as iostream was designed
precisely with this in mind:

    std::cout << "some text " << semanticFormat << value << " so
on" ...

The whole point of manipulators is that you write your own, for
your programs semantics, so that you don't have to specify the
physical details of formatting many times over, every time you
output a value representing the given semantics.

and the thing would be done. Thing
is, it's not typesafe.

And can't be generalized (i.e. to output to a socket, or with a
back end filter that inserts time stamps, or expands tabs). And
you more or less have to embed the physical formatting
information in with the normal text, which is totally wrong from
a software engineering point of view.

If there were some way to gernerally format strings, that'd be
quite nice. I don't want to rely on sprintf with a C++ project
though, I have my doubts... I'll see what I come up with.

You might want to have a look at Boost format. I've not had the
occasion to try it out myself, but from discussions with the
author, I think that it allows the use of manipulators for
format specifiers, but still allows keeping the text in a single
block (important for translation purposes).

James Kanze (GABI Software)
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