How do I turn off std::fixed and return to the default C++ setting

Question

My code:

std::vector<double> thePoint(4);
thePoint[0] = 86;
thePoint[1] = -334.8836574; 
thePoint[2] = 24.283;
thePoint[3] = 345.67675;

ofstream file1(tempFileName, ios::trunc);
file1 << std::setprecision(16)              << thePoint[0] << " ";
file1 << std::fixed << std::setprecision(2) << thePoint[1] << " ";
file1 << std::setprecision(16)              << thePoint[2] << " ";
file1 << std::setprecision(16)              << thePoint[3];

I get:

86 -334.88 24.28300000000000 345.6767500000000

I want:

86 -334.88 24.283 345.67675

The odd formatting is needed for an interface with other picky code.

Answer 1

You should do this:

file1 << std::fixed << std::setprecision(2) << thePoint[1] << " ";
file1.unsetf(ios_base::fixed);
file1 << std::setprecision(16)              << thePoint[2];

The floatfield format flag can take any of its two possible values (using the manipulators fixed and scientific), or none of them (using ios_base::unsetf).

Answer 2

You can do it by forcing the floatfield to an empty value:

file1.setf( std::ios_base::fmtflags(), std::floatfield );

In practice, it's rare to want to, however. The usual protocol is to save the format flags, and restore them when you're through:

std::ios_base::fmtflags originalFlags = file1.flags();
//  ...
file1.flags( originalFlags );

Of course, you'd normally use RAII to do this in a real program. You should have a IOSave class in your toolbox which will save the flags, the precision and the fill character in its constructor, and restore them in the destructor.

It's also not very good practice to use std::setprection etc. directly. A better solution would be to define your own manipulators, with names like pression or volume, and use those. This is logical markup, and means that you control the format for e.g. pression from one central location, rather than having it spread throughout the program. And if you write your own manipulators, it's relatively easy to have them restore the original formatting parameters at the end of the full expression. (The manipulator objects will be temporaries, destructed at the end of the full expression.)