Resetting output flags in C++

问题

I'm intending to reset all output flags to default on the lines where I end using the resetiosflags function. It provides erroneous output when I attempt to do it in this manner, contrary to my expectations.

#include <iostream>
#include <iomanip>
using namespace std;
int
main()
{
    bool first;
    int second;
    long third;
    float fourth;
    float fifth;
    double sixth;

    cout << "Enter bool, int, long, float, float, and double values: ";
    cin >> first >> second >> third >> fourth >> fifth >> sixth;
    cout << endl;

// ***** Solution starts here ****
    cout << first << " " << boolalpha << first  << endl << resetiosflags;
    cout << second << " " << showbase << hex << second << " " << oct << second << endl << resetiosflags;
    cout << third << endl;
    cout << showpos << setprecision(4) << showpoint << right << fourth << endl << resetiosflags;
    cout << scientific << fourth << endl << resetiosflags;
    cout << setprecision(7) << left << fifth << endl << resetiosflags;
    cout << fixed << setprecision(3) << fifth << endl << resetiosflags;
    cout << third << endl;
    cout << fixed << setprecision(2) << fourth << endl << resetiosflags;
    cout << fixed << setprecision(0) << sixth << endl << resetiosflags;
    cout << fixed << setprecision(8) << fourth << endl << resetiosflags;
    cout << setprecision(6) << sixth << endl << resetiosflags;
// ***** Solution ends here ****

    cin.get();
    return 0;
}

My known alternative is de-flagging them individually by restating them, but that seems superfluous.

回答1:

/*unspecified*/ resetiosflags( std::ios_base::fmtflags mask );

std::resetiosflags() is a manipulator intended to be used in an expression such as out << resetiosfloags( flags ). Presumably what you're doing is passing in a function pointer, which gets selected by the overload of std::operator<< that takes a boolean and prints 1.

But std::resetiosflags() takes a format flags as a parameter which the precision can't be manipulated with. std::ios_base::boolalpha can, however:

std::cout << ... << std::resetiosflags(std::ios_base::boolalpha);

There's also std::noboolalpha

std::cout << ... << std::noboolalpha;

But if you need to reset the precision to its default you can just create your own manipulator for that. You can also use Boost IO State Saver.

来源：https://stackoverflow.com/questions/28257527/resetting-output-flags-in-c

标签

c++

iostream

flags

iomanip