C ++ Iomanip lib setfill和setw问题

Question

My Code:

#include <iostream>
#include <iomanip>

using namespace std;

int main () {
    int time;
    int honolulu, seattle, london, moscow, hongkong, auckland;

    cout << "What is the current time in Philadelphia? ";
    cin >> time;

    honolulu = (time+2400-600)%2400;
    seattle = (time+2400-300)%2400;
    london = (time+2400+500)%2400;
    moscow = (time+2400+800)%2400;
    hongkong = (time+2400+1200)%2400;
    auckland = (time+2400+1700)%2400;

    cout << endl << "Current times in other cities: " << endl;
    cout << setw (12) << left << "Honolulu:";
    cout << setw (4) << setfill('0') << honolulu << endl;
    cout << setw (12) << left << "Seattle:";
    cout << setw (4) << setfill('0') << seattle << endl;
    cout << setw (12) << left << "London:";
    cout << setw (4) << setfill('0') << london << endl;
    cout << setw (12) << left << "Moscow:";
    cout << setw (4) << setfill('0') << moscow << endl;
    cout << setw (12) << left << "Hong Kong:";
    cout << setw (4) << setfill('0') << hongkong << endl;
    cout << setw (12) << left << "Auckland:";
    cout << setw (4) << setfill('0') << auckland << endl;

    return 0;
}

Required Output :

What is the current time in Philadelphia? 0415    

Current times in other cities: 
Honolulu:   2215
Seattle:    1150
London:     9150
Moscow:     1215
Hong Kong:  1615
Auckland:   2115

My output :

What is the current time in Philadelphia? 0415    

Current times in other cities: 
Honolulu:   2215
Seattle:00001150
London:000009150
Moscow:000001215
Hong Kong:001615
Auckland:0002115

What am I doing wrong? The first line of output, Honolulu: 2215 , is correct. But the next lines have leading zeroes. I do not understand why this is happening? Is there a problem with my code or am I misunderstanding how the functions setfill and setw work?

Answer 1

Many of the iomanip objects are "sticky", that is, they stick to the stream and affect subsequent lines.

When you have this:

cout << setw (12) << left << "Seattle:";
cout << setw (4) << setfill('0') << seattle << endl;

that is going to leave the setfill active for the next line. So you might instead prefer

cout << setw (12) << setfill(' ') << left << "Seattle:";
cout << setw (4) << setfill('0') << seattle << endl;

Answer 2

The fill character is "sticky", so it remains in effect until you change it.

In your case, you want 0 as the fill for the numeric fields, but space as the fill for the character fields, so you'll have to set that explicitly, something like this:

cout << setfill(' ') << setw (12) << left << "Seattle:";

Answer 3

As mentioned in other comments many of I/O manipulators are "sticky". I personally prefer to solve this kind of problem using RAII :

class stream_format_raii {
public:
   stream_format_raii(std::ostream &stream)
      : stream_(stream)
      , state_(NULL) {
      state_.copyfmt(stream_);
   }

   ~stream_format_raii() {
      stream_.copyfmt(state_);
   }
public:
   std::ostream &stream_;
   std::ios  state_;
};

That this class does is backing up your current stream's format upon constructing and setting it back upon destructing.

You can use it this way:

void printCity(std::ostream &os, const std::string name, int time) {
   stream_format_raii back(os);
   os << std::setw(12) << std::left << (name + ":");
   os << std::setw(4) << std::setfill('0') << time;
}


int main() {
   // Same as before
   printCity(std::cout, "Honolulu", honolulu);
   // Same as before
}