< cpp‎ | io‎ | manip
Input/output manipulators
Floating-point formatting
Integer formatting
Boolean formatting
Field width and fill control
Other formatting
Whitespace processing
Output flushing

Status flags manipulation
Time and money I/O
Quoted manipulator
Defined in header <iomanip>
/* unspecified */ setw( int n );

When used in an expression out << std::setw(n) or in >> std::setw(n), sets the width parameter of the stream out or in to exactly n.

Some operations reset the width to zero (see below), so std::setw may need to be repeatedly called to set the width for multiple operations.


[edit] Parameters

n - new value for width

[edit] Return value

An object of unspecified type such that

  • if out is an object of type std::basic_ostream<CharT, Traits>, the expression out << setw(n)
    • has type std::basic_ostream<CharT, Traits>&
    • has value out
    • behaves as if it called f(out, n)
  • if in is an object of type std::basic_istream<CharT, Traits>, the expression in >> setw(n)
    • has type std::basic_istream<CharT, Traits>&
    • has value in
    • behaves as if it called f(in, n)

where the function f is defined as:

void f(std::ios_base& str, int n)
    // set width

[edit] Notes

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

  • Input
  • Output

The exact effects this modifier has on the input and output vary between the individual I/O functions and are described at each operator<< and operator>> overload page individually.

[edit] Example

#include <iomanip>
#include <iostream>
#include <sstream>
int main()
    std::cout << "no setw: [" << 42 << "]\n"
              << "setw(6): [" << std::setw(6) << 42 << "]\n"
              << "no setw, several elements: [" << 89 << 12 << 34 << "]\n"
              << "setw(6), several elements: [" << 89 << std::setw(6) << 12 << 34 << "]\n";
    std::istringstream is("hello, world");
    char arr[10];
    is >> std::setw(6) >> arr;
    std::cout << "Input from \"" << is.str() << "\" with setw(6) gave \""
              << arr << "\"\n";


no setw: [42]
setw(6): [    42]
no setw, several elements: [891234]
setw(6), several elements: [89    1234]
Input from "hello, world" with setw(6) gave "hello"

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 183 C++98 setw could only be used with streams of
type std::ostream or std::istream
usable with any
character stream

[edit] See also

manages field width
(public member function of std::ios_base) [edit]
changes the fill character
(function template) [edit]
sets the placement of fill characters
(function) [edit]
controls whether prefix is used to indicate numeric base
(function) [edit]