iostreams manipulator order

Question

I do not understand logic in following expression, although it works perfectly:

cout << left << setw(6) << "hello" << "there." ; 

The previous code correctly outputs what I expect: hello there.

My logic is:

cout << "hello" << left << setw(6) << "there."; 

But it outputs something unexpected: hellothere. My expectation is that first character "t" of "there" be at 7th column on output area that is after 6 columns width. In other words my concept is that "left setw(n)" should mean "n columns (spaces) from first one on output area", like some data forms with numbered columns for easy find data.

Could you please explain?

Answer 1

The setw iostreams manipulator applies to the next item that is output, and to that item only. So in the first snippet, "hello" is modified with "left, field width 6", and thus produces the following output:

|h|e|l|l|o| |

The filler character by default is the space (' '), which is what's output when there is no more input data and the field width hasn't been reached yet.

In the second snippet, only the item "there." is manipulated. Since it already consists of six output characters, the manipulators have no effect.

Answer 2

The manipulators change the state of the ostream. So they should applied before the stream is asked to output something.

In your case you ask:

• format next output left
• format next outputs with a size of 6 characters
• output 'hello'

So the stream outputs the word "hello" followed by 1 space to reach the size 6, as asked by the manipulator sequence.

You can find references there:

• std::left
• operator<<(std::string)
• std::setw

And I quote the most important sentences std::left + operator << :

if (os.flags() & ios_base::adjustfield) == ios_base::left, places os.width()-str.size() copies of the os.fill() character after the character sequence

And std::setw:

The width property of the stream will be reset to zero (meaning "unspecified") if any of the following functions are called:

Input
   operator>>(basic_istream&, basic_string&)
   operator>>(basic_ostream&, char*)

Output
  Overloads 1-7 of basic_ostream::operator<<() (at Stage 3 of num_put::put())
  operator<<(basic_ostream&, char) and operator<<(basic_ostream&, char*)
  operator<<(basic_ostream&, basic_string&)
  std::put_money (inside money_put::put())
  std::quoted (when used with an output stream)