std::nearbyint, std::nearbyintf, std::nearbyintl

Since C++23

Header: <cmath>

1-3) Rounds the floating-point argument num to an integer value in floating-point format, using the current rounding mode.The library provides overloads of std::nearbyint for all cv-unqualified floating-point types as the type of the parameter.(since C++23)

# Declarations

float nearbyint ( float num );
double nearbyint ( double num );
long double nearbyint ( long double num );

(until C++23)

/*floating-point-type*/
nearbyint ( /*floating-point-type*/ num );

(since C++23)

float nearbyintf( float num );

(since C++11)

long double nearbyintl( long double num );

(since C++11)

SIMD overload (since C++26)

template< /*math-floating-point*/ V >
constexpr /*deduced-simd-t*/<V>
nearbyint ( const V& v_num );

(since C++26)

Additional overloads (since C++11)

template< class Integer >
double nearbyint ( Integer num );

# Parameters

# Return value

The nearest integer value to num, according to the current rounding mode, is returned.

# Notes

The only difference between std::nearbyint and std::rint is that std::nearbyint never raises FE_INEXACT.

The largest representable floating-point values are exact integers in all standard floating-point formats, so std::nearbyint never overflows on its own; however the result may overflow any integer type (including std::intmax_t), when stored in an integer variable.

If the current rounding mode is FE_TONEAREST, this function rounds to even in halfway cases (like std::rint, but unlike std::round).

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::nearbyint(num) has the same effect as std::nearbyint(static_cast(num)).

# Example

#include <cfenv>
#include <cmath>
#include <iostream>
#pragma STDC FENV_ACCESS ON
 
int main()
{
    std::fesetround(FE_TONEAREST);
    std::cout << "rounding to nearest: \n"
              << "nearbyint(+2.3) = " << std::nearbyint(2.3)
              << "  nearbyint(+2.5) = " << std::nearbyint(2.5)
              << "  nearbyint(+3.5) = " << std::nearbyint(3.5) << '\n'
              << "nearbyint(-2.3) = " << std::nearbyint(-2.3)
              << "  nearbyint(-2.5) = " << std::nearbyint(-2.5)
              << "  nearbyint(-3.5) = " << std::nearbyint(-3.5) << '\n';
 
    std::fesetround(FE_DOWNWARD);
    std::cout << "rounding down:\n"
              << "nearbyint(+2.3) = " << std::nearbyint(2.3)
              << "  nearbyint(+2.5) = " << std::nearbyint(2.5)
              << "  nearbyint(+3.5) = " << std::nearbyint(3.5) << '\n'
              << "nearbyint(-2.3) = " << std::nearbyint(-2.3)
              << "  nearbyint(-2.5) = " << std::nearbyint(-2.5)
              << "  nearbyint(-3.5) = " << std::nearbyint(-3.5) << '\n';
 
    std::cout << "nearbyint(-0.0) = " << std::nearbyint(-0.0)  << '\n'
              << "nearbyint(-Inf) = " << std::nearbyint(-INFINITY) << '\n';
}

# See also