c++ - How does std::setw work with string output? -

i trying use set width setw string output output file, however, not able make work. have me 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: above lines expected have many spaces between first , second, first second

i hardly see spaces, output comes firstsecond think missed working of setw()

note: integers, works fine just:

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

what doing wrong??.

i suspect understanding of std::setw not correct. think need more along lines of combination of:

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

what happening in code:

• uses std::setw(5) establish field width of 5 characters.
• sends "first" stream, 5 characters long, established field width consumed. no additional filling takes place.
• sends "second" stream, 6 characters long, again, entire field width consumed (and in-fact breached). again, no filling takes place

if you're intent have (with column numbers above show positions):

 col: 0123456789012345678901234567890123456789       first     second    third     fourth 

notice how each word starts on multiple of 10 boundary. 1 way using :

• a output position std::left (so fill, if goes on right achieve desired width). default strings, never hurts sure.
• a fill character of std::setfill(' '). again, default.
• a field width std::setw(10) why such 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 happens if change output location std::right ? well, identical program, changing first line :

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

we get

0123456789012345678901234567890123456789      first    second     third    fourth 

---

finally, 1 constructive way of seeing fill characters being applied changing fill char visible (ie. besides space). last 2 examples output, changing fill char std::setfill('*') produces following output:

first

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

second

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

notice in both cases, since none of individual output items breached std::setw value, total output line size each same. changed fills applied , output aligned within std::setw specification.