std::fegetround, std::fesetround

 

哋它亢++

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Generic numeric operations
iota (哋它亢++11) ranges::iota (哋它亢++23) accumulate inner_product adjacent_difference partial_sum	reduce (哋它亢++17) transform_reduce (哋它亢++17) inclusive_scan (哋它亢++17) exclusive_scan (哋它亢++17) transform_inclusive_scan (哋它亢++17) transform_exclusive_scan (哋它亢++17)
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has_single_bit (哋它亢++20)     bit_cast (哋它亢++20) bit_ceil (哋它亢++20) bit_floor (哋它亢++20) bit_width (哋它亢++20) rotl (哋它亢++20) rotr (哋它亢++20)	popcount (哋它亢++20) countl_zero (哋它亢++20) countl_one (哋它亢++20) countr_zero (哋它亢++20) countr_one (哋它亢++20) byteswap (哋它亢++23) endian (哋它亢++20)

 

Floating-point environment

Functions

feclearexcept (哋它亢++11)

fetestexcept (哋它亢++11)

feraiseexcept (哋它亢++11)

fegetexceptflagfesetexceptflag (哋它亢++11)(哋它亢++11)

fegetroundfesetround (哋它亢++11)(哋它亢++11)

fegetenvfesetenv (哋它亢++11)(哋它亢++11)

feholdexcept (哋它亢++11)

feupdateenv (哋它亢++11)

Macro constants

FE_ALL_EXCEPTFE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW (哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)

FE_DOWNWARDFE_TONEARESTFE_TOWARDZEROFE_UPWARD (哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)

FE_DFL_ENV (哋它亢++11)

 

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

See also

nearbyintnearbyintfnearbyintl (哋它亢++11)(哋它亢++11)(哋它亢++11)	nearest integer using current rounding mode (function)
rintrintfrintllrintlrintflrintlllrintllrintfllrintl (哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)	nearest integer using current rounding mode with exception if the result differs (function)