NAME¶

std::setprecision - std::setprecision

Synopsis¶

Defined in header <iomanip>
/*unspecified*/ setprecision( int n );

When used in an expression out << setprecision(n) or in >> setprecision(n), sets the
precision parameter of the stream out or in to exactly n.

Parameters¶

n - new value for precision

Return value¶

An object of unspecified type such that

* if out is an object of type std::basic_ostream<CharT, Traits>, the expression
out << setprecision(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
>> setprecision(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 precision
str.precision(n);
}

Example¶

// Run this code

#include <iomanip>
#include <iostream>
#include <limits>
#include <numbers>

int main()
{
constexpr long double pi{std::numbers::pi_v<long double>};

const auto default_precision{std::cout.precision()};
constexpr auto max_precision{std::numeric_limits<long double>::digits10 + 1};

std::cout << "default precision: " << default_precision << '\n'
<< "maximum precision: " << max_precision << "\n\n"
"precision: pi:\n";

for (int p{0}; p <= max_precision; ++p)
std::cout << std::setw(2) << p << " " << std::setprecision(p) << pi << '\n';

std::cout << std::setprecision(default_precision); // restore defaults
}

Output:¶

default precision: 6
maximum precision: 19

precision: pi:
0 3
1 3
2 3.1
3 3.14
4 3.142
5 3.1416
6 3.14159
7 3.141593
8 3.1415927
9 3.14159265
10 3.141592654
11 3.1415926536
12 3.14159265359
13 3.14159265359
14 3.1415926535898
15 3.14159265358979
16 3.141592653589793
17 3.1415926535897932
18 3.14159265358979324
19 3.141592653589793239

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 setprecision could only be used with streams usable with any
of type std::ostream or std::istream character stream

See also¶

fixed
scientific
hexfloat changes formatting used for floating-point I/O
defaultfloat (function)
(C++11)
(C++11)
precision manages decimal precision of floating point operations
(public member function of std::ios_base)