std :: setw如何与字符串输出一起使用？

Question

I am trying to use set width setw for string output to an output file, however, I am not able to make it work. I have with me the following example.

// setw example
#include <iostream>     
#include <iomanip>      
#include <fstream>

int main () {
    std::ofstream output_file;
    output_file.open("file.txt");
    output_file << "first" <<std::setw(5)<< "second"<< std::endl;
    output_file.close();
  return 0;
}

Edit: With the above lines I expected to have many spaces between first and second , something like first second

I hardly see any spaces, the output just comes like firstsecond I think I missed the working of setw()

Note: For integers, it works fine just:

output_file << 1 <<std::setw(5)<< 2 << std::endl;

What I am doing wrong??.

Answer 1

I suspect your understanding of std::setw is simply not correct. I think you need something more along the lines of a combination of:

• std::setw for setting field width
• std::setfill for setting the fill character
• std::left , std::right , std::internal for setting the write position within the specified field width .

What is happening in your code:

• Uses std::setw(5) to establish a field width of five characters.
• Sends "first" to the stream, which is five characters long, so the established field width is completely consumed. No additional filling takes place.
• Sends "second" to the stream, which is six characters long, so again, the entire field width is consumed (and in-fact breached). Again, no filling takes place

If you're intent is to have something like this (with column numbers above to show positions):

 col: 0123456789012345678901234567890123456789
      first     second    third     fourth

Notice how each word starts on an even multiple of 10 boundary. One way to do that is by using :

• A output position std::left (so the fill, if any goes on the right to achieve the desired width). This the default for strings, but it never hurts to be sure.
• A fill character of std::setfill(' ') . Again, the default.
• A field width std::setw(10) Why such a large number? See below

Example

#include <iostream>
#include <iomanip>

int main ()
{
    std::cout << std::left << std::setfill(' ')
              << std::setw(10) << "first"
              << std::setw(10) << "second"
              << std::setw(10) << "third"
              << std::setw(10) << "fourth" << '\n';
    return 0;
}

Output (column numbers added)

0123456789012345678901234567890123456789
first     second    third     fourth

---

So what happens if we change the output location to std::right ? Well, with the identical program, changing only the first line to :

std::cout << std::right << std::setfill(' ')

we get

0123456789012345678901234567890123456789
     first    second     third    fourth

---

Finally, one constructive way of seeing where the fill characters are being applied is by simply changing the fill char to something visible (ie. something besides a space). The last two examples output, changing the fill char to std::setfill('*') produces the following output:

First

first*****second****third*****fourth****

Second

*****first****second*****third****fourth    

Notice in both cases, since none of the individual output items breached the std::setw value, the total output line size for each is the same. All that changed was where the fills were applied and the output aligned within the std::setw specification.