# std::setw

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< cpp‎ | io‎ | manip

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.

### Parameters

n | - | new value for width

### 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
        str.width(n);
    }

### Notes

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_istream&, char*)`

- Output

  - Overloads of `basic_ostream::operator<<()` that take arithmetic types or void pointers (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)

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.

### Example

Run this code

    #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";
    }

Output:

    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"

### 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

### See also

width | manages field width (public member function of `std::ios_base`)
setfill | changes the fill character (function template)
internal left right | sets the placement of fill characters (function)
showbase noshowbase | controls whether prefix is used to indicate numeric base (function)

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