NAME

std::fegetround,std::fesetround - std::fegetround,std::fesetround

Synopsis

Defined in header <cfenv>
int fesetround( int round ) (1) (since C++11)
int fegetround() (2) (since C++11)

Manages the floating-point rounding direction.

1) Attempts to establish the floating-point rounding direction equal to the argument
round, which is expected to be one of the floating point rounding macros.
2) Returns the value of the floating point rounding macro that corresponds to the
current rounding direction.

Parameters

round - rounding direction, one of floating point rounding macros

Return value

1) 0 on success, non-zero otherwise.

2) the floating point rounding macro describing the current rounding direction or a
negative value if the direction cannot be determined

Notes

The current rounding mode, reflecting the effects of the most recent fesetround, can
also be queried with FLT_ROUNDS

See floating-point rounding macros for the effects of rounding.

Example

// Run this code

#include <cfenv>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <utility>
// #pragma STDC FENV_ACCESS ON

int main() {
static constexpr std::pair<const char*, const double> samples[] {
{" 12.0", 12.0}, {" 12.1", 12.1}, {"-12.1", -12.1}, {" 12.5", 12.5},
{"-12.5", -12.5}, {" 12.9", 12.9}, {"-12.9", -12.9}, {" 13.0", 13.0},
};

std::cout <<
"│ sample │ FE_DOWNWARD │ FE_UPWARD │ FE_TONEAREST │ FE_TOWARDZERO │\n";

for (const auto& [str, fp] : samples) {
std::cout << "│ " << std::setw(6) << str << " │ ";
for (const int dir : {FE_DOWNWARD, FE_UPWARD, FE_TONEAREST, FE_TOWARDZERO}) {
std::fesetround(dir);
std::cout << std::setw(10) << std::fixed << std::nearbyint(fp) << " │ ";
}
std::cout << '\n';
}
}

Output:

│ sample │ FE_DOWNWARD │ FE_UPWARD │ FE_TONEAREST │ FE_TOWARDZERO │
│ 12.0 │ 12.000000 │ 12.000000 │ 12.000000 │ 12.000000 │
│ 12.1 │ 12.000000 │ 13.000000 │ 12.000000 │ 12.000000 │
│ -12.1 │ -13.000000 │ -12.000000 │ -12.000000 │ -12.000000 │
│ 12.5 │ 12.000000 │ 13.000000 │ 12.000000 │ 12.000000 │
│ -12.5 │ -13.000000 │ -12.000000 │ -12.000000 │ -12.000000 │
│ 12.9 │ 12.000000 │ 13.000000 │ 13.000000 │ 12.000000 │
│ -12.9 │ -13.000000 │ -12.000000 │ -13.000000 │ -12.000000 │
│ 13.0 │ 13.000000 │ 13.000000 │ 13.000000 │ 13.000000 │