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Showing content from https://en.cppreference.com/w/cpp/language/../numeric/../io/manip/setprecision.html below:

std::setprecision - cppreference.com

/*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.

[edit] Parameters n - new value for precision [edit] 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);
}

[edit] Example

#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

[edit] 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 [edit] See also

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