std::setw

 

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setfill setw	internalleftright
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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)
internalleftright	sets the placement of fill characters (function)
showbasenoshowbase	controls whether prefix is used to indicate numeric base (function)