Non-persistent formatting of streams

Question

What's the best way (if any) to format std::*stream output in localized manner, so that formatting changes in one location won't affect the use of that stream in other locations?

That is, I'd like to set format of the stream for a single statement, not for the remaining lifetime of the stream.

The following is just a rationale for asking the above question.

Suppose you print intensity in default floating point format:

ostringstream oss;
oss << "Intensity = " << intensity << "; ";

Then print coordinates with fixed 2 digits precision:

oss << "Point = (" << fixed << setprecision(2) << pt.x << ", " << pt.y << "); ";

then, 20 lines later, print ray direction in the same fixed 2 digits format:

oss << "Direction = (" << dir.x << ", " << dir.y << "); ";

A few months later add printing of luminosity in default floating point format somewhere between printing pt and printing dir:

oss << "Luminosity = " << lum << "; ";

Oops, lum will be printed in fixed 2 digits precision because you changed oss format 20 lines before, when printing pt. Now you have to recall what you've changed in oss and rewind it for printing lum. Moreover, after fixing format for lum you'd get another problem: dir won't be printed in fixed 2 digits anymore...

Therefore I'd like to be able to format streams locally to avoid unnecessary dependencies.

πάντα ῥεῖ · Accepted Answer · 2015-08-12T21:14:55.423

You can simply save and restore stream format flags (see std::iosbase::flags()):

fmtflags oldflags = oss.flags();

// change current format flags

oss.flags(oldflags); // <<< restore to former state

If you don't like it straight forward as mentioned above, you could also use a proxy:

class local_ostream {
public:
    friend template<typename T> 
    local_ostream& operator<<(local_ostream& los, const& T value) {
        los.os_ << value;
        return los;
    }

    local_ostream(std::ostream& os) : os_(os), oldflags_(os.flags()) {
    }
    ~local_ostream() {
       os_.flags(oldflags_);
    }      

private:
    std::ostream& os_;
    std::iosbase::fmtflags oldflags_;
};

And use as follows:

 { local_ostream los(oss);
     los << "Point = (" << fixed << setprecision(2) << pt.x << ", " << pt.y << "); ";
 } // previous formatting state is restored here

Non-persistent formatting of streams

1 Answers1