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FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD

From cppreference.com
< cpp‎ | numeric‎ | fenv
 
 
 
Floating-point environment
Functions
(C++11)(C++11)
(C++11)(C++11)
Macro constants
FE_DOWNWARDFE_TONEARESTFE_TOWARDZEROFE_UPWARD
(C++11)(C++11)(C++11)(C++11)
(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:

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:

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(2.1) << "\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

[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