std::setw

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Input/output library

I/O manipulators

Print functions (C++23)

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

basic_spanbuf (C++23)

strstreambuf (C++98/26*)

basic_syncbuf (C++20)

Streams

Abstractions

ios_base

basic_ios

basic_istream

basic_ostream

basic_iostream

File I/O

basic_ifstream

basic_ofstream

basic_fstream

String I/O

basic_istringstream

basic_ostringstream

basic_stringstream

Array I/O

basic_ispanstream (C++23)

basic_ospanstream (C++23)

basic_spanstream (C++23)

istrstream (C++98/26*)

ostrstream (C++98/26*)

strstream (C++98/26*)

Synchronized Output

basic_osyncstream (C++20)

Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

 

Input/output manipulators

Floating-point formatting
showpointnoshowpoint setprecision	fixedscientifichexfloatdefaultfloat (C++11)(C++11)
Integer formatting
setbase showbasenoshowbase	dechexoct
Boolean formatting
boolalphanoboolalpha
Field width and fill control
setfill setw	internalleftright
Other formatting
showposnoshowpos	uppercasenouppercase
Whitespace processing
ws ends	skipwsnoskipws
Output flushing
flush endl flush_emit (C++20)	unitbufnounitbuf emit_on_flushnoemit_on_flush (C++20)(C++20)
Status flags manipulation
resetiosflags	setiosflags
Time and money I/O
get_money (C++11) get_time (C++11)	put_money (C++11) put_time (C++11)
Quoted manipulator
quoted (C++14)

 

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)

Follow Lee on X/Twitter - Father, Husband, Serial builder creating AI, crypto, games & web tools. We are friends :) AI Will Come To Life!

Check out: eBank.nz (Art Generator) | Netwrck.com (AI Tools) | Text-Generator.io (AI API) | BitBank.nz (Crypto AI) | ReadingTime (Kids Reading) | RewordGame | BigMultiplayerChess | WebFiddle | How.nz | Helix AI Assistant