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 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
orstd::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 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.
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
See also
indicates floating-point rounding modes (enum) | |
(C++11)(C++11) | gets or sets rounding direction (function) |
C documentation for floating-point rounding macros |
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