From cppreference.com
Defined in header
#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
and
to indicate one of the supported floating-point rounding modes. The implementation may define additional rounding mode constants in
, 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_DOWNWARDrounding towards negative infinity FE_TONEARESTrounding towards nearest representable value FE_TOWARDZEROrounding towards zero FE_UPWARDrounding towards positive infinity Additional rounding modes may be supported by an implementation.
The current rounding mode affects the following:
results of floating-point
outside of constant expressions
doublex=1;x/10;// 0.09999999999999999167332731531132594682276248931884765625// or 0.1000000000000000055511151231257827021181583404541015625results of standard library
std::sqrt(2);// 1.41421356237309492343001693370752036571502685546875// or 1.4142135623730951454746218587388284504413604736328125floating-point to floating-point implicit conversion and casts
doubled=1+std::numeric_limits<double>::epsilon();floatf=d;// 1.00000000000000000000000// or 1.00000011920928955078125string conversions such as
or
std::stof("0.1");// 0.0999999940395355224609375// or 0.100000001490116119384765625the library rounding functions
,
,
std::lrint(2.1);// 2 or 3The 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
,
,
,
,
,
.
As with any
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<cfenv>#include<cmath>#include<iomanip>#include<iostream>#include<string>// #pragma STDC FENV_ACCESS ONintmain(){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