FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD

 

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fegetexceptflagfesetexceptflag (C++11)(C++11)

fegetroundfesetround (C++11)(C++11)

fegetenvfesetenv (C++11)(C++11)

feholdexcept (C++11)

feupdateenv (C++11)

Macro constants

FE_ALL_EXCEPTFE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW (C++11)(C++11)(C++11)(C++11)(C++11)(C++11)

FE_DOWNWARDFE_TONEARESTFE_TOWARDZEROFE_UPWARD (C++11)(C++11)(C++11)(C++11)

FE_DFL_ENV (C++11)

 

Defined in header <cfenv>
#define FE_DOWNWARD     /*implementation defined*/		(since C++11)
#define FE_TONEAREST    /*implementation defined*/		(since C++11)
#define FE_TOWARDZERO   /*implementation defined*/		(since C++11)
#define FE_UPWARD       /*implementation defined*/		(since C++11)

Each of these macro constants expands to a nonnegative integer constant expression, which can be used with std::fesetround and std::fegetround to indicate one of the supported floating-point rounding modes. The implementation may define additional rounding mode constants in <cfenv>, which should all begin with FE_ followed by at least one uppercase letter. Each macro is only defined if it is supported.

Constant	Explanation
FE_DOWNWARD	rounding towards negative infinity
FE_TONEAREST	rounding towards nearest representable value
FE_TOWARDZERO	rounding towards zero
FE_UPWARD	rounding towards positive infinity

Additional rounding modes may be supported by an implementation.

The current rounding mode affects the following:

• results of floating-point arithmetic operators outside of constant expressions

double x = 1;
x/10; // 0.09999999999999999167332731531132594682276248931884765625
   // or 0.1000000000000000055511151231257827021181583404541015625

• results of standard library mathematical functions

std::sqrt(2); // 1.41421356237309492343001693370752036571502685546875
           // or 1.4142135623730951454746218587388284504413604736328125

• floating-point to floating-point implicit conversion and casts

double d = 1 + std::numeric_limits<double>::epsilon();
float f = d; //  1.00000000000000000000000
           // or 1.00000011920928955078125

• string conversions such as std::strtod or std::printf

std::stof("0.1"); // 0.0999999940395355224609375
               // or 0.100000001490116119384765625

• the library rounding functions std::nearbyint, std::rint, std::lrint

std::lrint(2.1); // 2 or 3

The current rounding mode does NOT affect the following:

• floating-point to integer implicit conversion and casts (always towards zero)
• results of floating-point arithmetic operators in expressions executed at compile time (always to nearest)
• the library functions std::round, std::lround, std::llround, std::ceil, std::floor, std::trunc

As with any floating-point environment functionality, rounding is only guaranteed if #pragma STDC FENV_ACCESS ON is set.

Compilers that do not support the pragma may offer their own ways to support current rounding mode. For example Clang and GCC have the option -frounding-math intended to disable optimizations that would change the meaning of rounding-sensitive code.

Example

Run this code

#include <iostream>
#include <iomanip>
#include <string>
#include <cfenv>
#include <cmath>
int main()
{
#pragma STDC FENV_ACCESS ON
    std::fesetround(FE_DOWNWARD);
    std::cout << "rounding down: \n" << std::setprecision(50)
              << "         pi = " << std::acos(-1.f) << '\n'
              << "stof(\"1.1\") = " << std::stof("1.1") << '\n'
              << "  rint(2.1) = " << std::rint(2.1) << "\n\n";
    std::fesetround(FE_UPWARD);
    std::cout << "rounding up: \n"
              << "         pi = " << std::acos(-1.f) << '\n'
              << "stof(\"1.1\") = " << std::stof("1.1") << '\n'
              << "  rint(2.1) = " << std::rint(2.1) << '\n';
}

Output:

rounding down: 
         pi = 3.141592502593994140625
stof("1.1") = 1.099999904632568359375
  rint(2.1) = 2
 
rounding up: 
         pi = 3.1415927410125732421875
stof("1.1") = 1.10000002384185791015625
  rint(2.1) = 3

See also

float_round_style	indicates floating-point rounding modes (enum)
fegetroundfesetround (C++11)(C++11)	gets or sets rounding direction (function)