# std::setprecision

From cppreference.com
< cpp‎ | io‎ | manip

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 <iostream>
    #include <iomanip>
    #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 of type std::ostream or std::istream | usable with any character stream

### See also

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

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/io/manip/setprecision&oldid=145871"