NAME¶

std::setw - std::setw

Synopsis¶

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 usable with any
type std::ostream or std::istream character stream

See also¶

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