NAME

std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl, - std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,

Synopsis

Defined in header <cmath>
float round ( float arg ); (1) (since C++11)
(constexpr since C++23)
float roundf( float arg ); (2) (since C++11)
(constexpr since C++23)
double round ( double arg ); (3) (since C++11)
(constexpr since C++23)
long double round ( long double arg ); (4) (since C++11)
(constexpr since C++23)
long double roundl( long double arg ); (5) (since C++11)
(constexpr since C++23)
double round ( IntegralType arg ); (6) (since C++11)
(constexpr since C++23)
long lround ( float arg ); (7) (since C++11)
(constexpr since C++23)
long lroundf( float arg ); (8) (since C++11)
(constexpr since C++23)
long lround ( double arg ); (9) (since C++11)
(constexpr since C++23)
long lround ( long double arg ); (10) (since C++11)
(constexpr since C++23)
long lroundl( long double arg ); (11) (since C++11)
(constexpr since C++23)
long lround ( IntegralType arg ); (12) (since C++11)
(constexpr since C++23)
long long llround ( float arg ); (13) (since C++11)
(constexpr since C++23)
long long llroundf( float arg ); (14) (since C++11)
(constexpr since C++23)
long long llround ( double arg ); (15) (since C++11)
(constexpr since C++23)
long long llround ( long double arg ); (16) (since C++11)
(constexpr since C++23)
long long llroundl( long double arg ); (17) (since C++11)
(constexpr since C++23)
long long llround ( IntegralType arg ); (18) (since C++11)
(constexpr since C++23)

1-5) Computes the nearest integer value to arg (in floating-point format), rounding
halfway cases away from zero, regardless of the current rounding mode.
7-11,13-17) Computes the nearest integer value to arg (in integer format), rounding
halfway cases away from zero, regardless of the current rounding mode.
6,12,18) A set of overloads or a function template accepting an argument of any
integral type. Equivalent to (3), (9), or (15), respectively (the argument is cast
to double).

Parameters

arg - floating point value.

Return value

If no errors occur, the nearest integer value to arg, rounding halfway cases away
from zero, is returned.

Return value

math-round away zero.svg
Argument

If a domain error occurs, an implementation-defined value is returned.

Error handling

Errors are reported as specified in math_errhandling.

If the result of std::lround or std::llround is outside the range representable by
the return type, a domain error or a range error may occur.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

For the std::round function:

* The current rounding mode has no effect.
* If arg is ±∞, it is returned, unmodified
* If arg is ±0, it is returned, unmodified
* If arg is NaN, NaN is returned

For std::lround and std::llround functions:

* FE_INEXACT is never raised
* The current rounding mode has no effect.
* If arg is ±∞, FE_INVALID is raised and an implementation-defined value is
returned
* If the result of the rounding is outside the range of the return type,
FE_INVALID is raised and an implementation-defined value is returned
* If arg is NaN, FE_INVALID is raised and an implementation-defined value is
returned.

Notes

FE_INEXACT may be (but isn't required to be) raised by std::round when rounding a
non-integer finite value.

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

POSIX specifies that all cases where std::lround or std::llround raise FE_INEXACT
are domain errors.

The double version of std::round behaves as if implemented as follows:

#include <cmath>
#include <cfenv>
#pragma STDC FENV_ACCESS ON
double round(double x)
{
std::fenv_t save_env;
std::feholdexcept(&save_env);
double result = std::rint(x);
if (std::fetestexcept(FE_INEXACT)) {
auto const save_round = std::fegetround();
std::fesetround(FE_TOWARDZERO);
result = std::rint(std::copysign(0.5 + std::fabs(x), x));
std::fesetround(save_round);
}
std::feupdateenv(&save_env);
return result;
}

Example

// Run this code

#include <iostream>
#include <cmath>
#include <cfenv>
#include <climits>

// #pragma STDC FENV_ACCESS ON

int main()
{
// round
std::cout << "round(+2.3) = " << std::round(2.3)
<< " round(+2.5) = " << std::round(2.5)
<< " round(+2.7) = " << std::round(2.7) << '\n'
<< "round(-2.3) = " << std::round(-2.3)
<< " round(-2.5) = " << std::round(-2.5)
<< " round(-2.7) = " << std::round(-2.7) << '\n';

std::cout << "round(-0.0) = " << std::round(-0.0) << '\n'
<< "round(-Inf) = " << std::round(-INFINITY) << '\n';

// lround
std::cout << "lround(+2.3) = " << std::lround(2.3)
<< " lround(+2.5) = " << std::lround(2.5)
<< " lround(+2.7) = " << std::lround(2.7) << '\n'
<< "lround(-2.3) = " << std::lround(-2.3)
<< " lround(-2.5) = " << std::lround(-2.5)
<< " lround(-2.7) = " << std::lround(-2.7) << '\n';

std::cout << "lround(-0.0) = " << std::lround(-0.0) << '\n'
<< "lround(-Inf) = " << std::lround(-INFINITY) << '\n';

// error handling
std::feclearexcept(FE_ALL_EXCEPT);
std::cout << "std::lround(LONG_MAX+1.5) = "
<< std::lround(LONG_MAX+1.5) << '\n';
if (std::fetestexcept(FE_INVALID))
std::cout << " FE_INVALID was raised\n";
}

Possible output:

round(+2.3) = 2 round(+2.5) = 3 round(+2.7) = 3
round(-2.3) = -2 round(-2.5) = -3 round(-2.7) = -3
round(-0.0) = -0
round(-Inf) = -inf
lround(+2.3) = 2 lround(+2.5) = 3 lround(+2.7) = 3
lround(-2.3) = -2 lround(-2.5) = -3 lround(-2.7) = -3
lround(-0.0) = 0
lround(-Inf) = -9223372036854775808
std::lround(LONG_MAX+1.5) = -9223372036854775808
FE_INVALID was raised