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.
n | - | new value for width |
An object of unspecified type such that.
out
is an object of type std::basic_ostream<CharT, Traits>
, the expression out << setw(n)
std::basic_ostream<CharT, Traits>&
out
f(out, n)
in
is an object of type std::basic_istream<CharT, Traits>
, the expression in >> setw(n)
std::basic_istream<CharT, Traits>&
in
f(in, n)
where the function f
is defined as:
void f(std::ios_base& str, int n) { // set width str.width(n); }
The width property of the stream will be reset to zero (meaning "unspecified") if any of the following functions are called:
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.
#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"
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 oftype std::ostream or std::istream | usable with any character stream |
manages field width (public member function of std::ios_base ) |
|
changes the fill character (function template) |
|
sets the placement of fill characters (function) |
|
controls whether prefix is used to indicate numeric base (function) |
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