C++ Standard Template (STL)-Input/Output Manipulator-(std::setprecision,std::setw)

Operators are helper functions that enable code to control input/output streams using operator<< or operator>>.

Manipulators called without arguments (such as std::cout << std::boolalpha; or std::cin >> std::hex; ) are implemented as functions that accept a reference to a stream as their only argument. Special overloaded versions of basic_ostream::operator<< and basic_istream::operator>> accept pointers to these functions. These functions (or instantiations of function templates) are the only addressable functions in the standard library. (since C++20)

Manipulators called with arguments (such as std::cout << std::setw(10); ) are implemented as functions that return an object of unspecified type. These operators define their own operator<< or operator>> that performs the requested operation.

Defined in the header file <iomanip>

Change floating point precision

std::setprecision

defined in header file`<iomanip>`
/unspecified/ setprecision( int n );

When used in the expression out << setprecision(n) or in >> setprecision(n) , the set stream or outargument is exactly .inprecisionn

parameter

new value for precision

return value

Returns an object of unspecified type such that if stris an output stream name of type std::basic_ostream<CharT, Traits> or an input stream name of type std::basic_istream<CharT, Traits>, the expression str << setprecision(n) Or str >> setprecision(n) behaves like executing the following code:

str.precision(n);

Call example

#include <iostream>
#include <iomanip>
#include <cmath>
#include <limits>

int main()
{
    const long double pi = std::acos(-1.L);
    std::cout << "default precision (6): "
              << pi << std::endl
              << "std::setprecision(10): "
              << std::setprecision(10) << pi << std::endl
              << "max precision:         "
              << std::setprecision(std::numeric_limits<long double>::digits10 + 1)
              << pi << std::endl;

    return 0;
}

output

Change the width of the next input/output field

std::setw

/*unspecified*/ setw( int n );

When used in the expressions out << setw(n) or in >> setw(n) , the set flow or outargument is exactly .inwidthn

parameter

new value for width

return value

Returns an unspecified type object. If strit is the name of a std::basic_ostream<CharT, Traits> or std::basic_istream<CharT, Traits> type stream, the expression str << setw(n) or str >> setw(n ) behaves as if the following code were executed:

str.width(n);

Notice

If any of the following functions are called, the stream's width property will be set to zero (meaning "not specified"):

enter

operator>>(basic_istream&, basic_string&)
operator>>(basic_istream&, char*)

output

Overloads 1-7 of basic_ostream::operator<<() (in stage 3 of num_put::put())
operator<<(basic_ostream&, char) 和 operator<<(basic_ostream&, char*)
operator<<(basic_ostream&, basic_string&)
std::put_money (inside money_put::put())
std::quoted (when used as an output stream)

The exact effect of this modifier on input and output varies between individual I/O functions and is described separately on each operator<<overloaded operator>>page.

Call example

#include <sstream>
#include <iostream>
#include <iomanip>

int main()
{
    std::cout << "no setw:" << 42 << std::endl
              << "setw(6):" << std::setw(6) << 42 << std::endl
              << "setw(6), several elements: " << 89
              << std::setw(6) << 12 << 34 << std::endl;

    std::istringstream is("hello, world");
    char arr[10];
    is >> std::setw(6) >> arr;
    std::cout << "Input from \"" << is.str()
              << "\" with setw(6) gave \""
              << arr << "\"" << std::endl;

    return 0;
}