std::setprecision
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setprecision
(C++11)(C++11)
(C++20)(C++20)
(C++14)
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.
[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
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
[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 streamsof type std::ostream or std::istream |
usable with any character stream |