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
double d = 1 + std::numeric_limits<double>::epsilon(); float f = d; // 1.00000000000000000000000 // or 1.00000011920928955078125
std::strtod
or std::printf
std::stof("0.1"); // 0.0999999940395355224609375 // or 0.100000001490116119384765625
std::nearbyint
, std::rint
, std::lrint
std::lrint(2.1); // 2 or 3
The current rounding mode does NOT affect the following:
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.
#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
indicates floating-point rounding modes (enum) |
|
(C++11)(C++11) | gets or sets rounding direction (function) |
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