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FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD

From cppreference.com
< cpp‎ | numeric‎ | fenv
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.

On most implementations, these macro constants expand to the values equal to the values of FLT_ROUNDS and std::float_round_style

Constant Explanation
FE_DOWNWARD rounding towards negative infinity
FE_TONEAREST rounding towards nearest integer
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:

double x = 1;
x/10; // 0.09999999999999999167332731531132594682276248931884765625
   // or 0.1000000000000000055511151231257827021181583404541015625
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
std::stof("0.1"); // 0.0999999940395355224609375
               // or 0.100000001490116119384765625
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 constant expressions (always to nearest)
  • the library functions std::round, std::lround, 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.

[edit] Example

#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(2.3) << "\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(2.3) << '\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

[edit] See also

indicates floating-point rounding modes
(enum) [edit]
(C++11)(C++11)
gets or sets rounding direction
(function) [edit]
C documentation for floating-point rounding macros