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>
Rounding to floating-point
types
float round ( float num
);

double round ( double num (since C++11)
); (until C++23)

long double round ( long double
num );
constexpr /*
floating-point-type */ (since C++23)
round ( /*
floating-point-type */ num );
float roundf( float num (since C++11)
); (2) (constexpr
since C++23)
long double roundl( long double (since C++11)
num ); (3) (constexpr
since C++23)
Rounding to long
long lround ( float num );

long lround ( double num ); (since C++11)
(until C++23)
long lround ( long double num
);
constexpr long lround( /* (since C++23)
floating-point-type */ num );
(since C++11)
long lroundf( float num ); (5) (constexpr
since C++23)
long lroundl( long double num (1) (since C++11)
); (6) (constexpr
since C++23)
Rounding to long long
long long llround ( float num
);

long long llround ( double num (since C++11)
); (until C++23)
(4)
long long llround ( long double
num );
constexpr long long llround( /* (since C++23)
floating-point-type */ num );
long long llroundf( float num (since C++11)
); (8) (constexpr
since C++23)
long long llroundl( long double (7) (since C++11)
num ); (9) (constexpr
since C++23)
Additional overloads
Defined in header <cmath>
template< class Integer > (since C++11)
double round( Integer num ); (A) (constexpr
since C++23)
template< class Integer > (since C++11)
long lround( Integer num ); (B) (constexpr
since C++23)
template< class Integer > (since C++11)
long long llround( Integer num (C) (constexpr
); since C++23)

1-3) Computes the nearest integer value to num (in floating-point format), rounding
halfway cases away from zero, regardless of the current rounding mode.
The library provides overloads of std::round for all cv-unqualified floating-point
types as the type of the parameter num.
(since C++23)
4-9) Computes the nearest integer value to num (in integer format), rounding halfway
cases away from zero, regardless of the current rounding mode.
The library provides overloads of std::lround and std::llround for all
cv-unqualified floating-point types as the type of the parameter num.
(since C++23)
A-C) Additional overloads are provided for all integer types, which are treated as
double.

Parameters¶

num - floating-point or integer value

Return value¶

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

Return value¶

math-round away zero.svg
num

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 num is ±∞, it is returned, unmodified.
* If num is ±0, it is returned, unmodified.
* If num 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 num 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 num is NaN, FE_INVALID is raised and an implementation-defined value is
returned.

Notes¶

FE_INEXACT may be (but is not 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 <cfenv>
#include <cmath>

#pragma STDC FENV_ACCESS ON

double round(double x)
{
const int save_round = std::fegetround();
std::fesetround(FE_TOWARDZERO);
const double result = std::rint(std::copysign(0.5 + std::fabs(x), x));
std::fesetround(save_round);
return result;
}

The additional overloads are not required to be provided exactly as (A-C). They only
need to be sufficient to ensure that for their argument num of integer type:

* std::round(num) has the same effect as std::round(static_cast<double>(num)).
* std::lround(num) has the same effect as std::lround(static_cast<double>(num)).
* std::llround(num) has the same effect as std::llround(static_cast<double>(num)).

Example¶

// Run this code

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

// #pragma STDC FENV_ACCESS ON

double custom_round(double x)
{
const int save_round = std::fegetround();
std::fesetround(FE_TOWARDZERO);
const double result = std::rint(std::copysign(0.5 + std::fabs(x), x));
std::fesetround(save_round);
return result;
}

void test_custom_round()
{
for (const double x :
{
0.0, 0.3,
0.5 - DBL_EPSILON / 2,
0.5,
0.5 + DBL_EPSILON / 2,
0.7, 1.0, 2.3, 2.5, 2.7, 3.0,
static_cast<double>(INFINITY)
})
assert(round(+x) == custom_round(+x) && round(-x) == custom_round(-x));
}

int main()
{
test_custom_round();

std::cout << std::showpos;

// 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

See also¶

floor
floorf nearest integer not greater than the given value
floorl (function)
(C++11)
(C++11)
ceil
ceilf nearest integer not less than the given value
ceill (function)
(C++11)
(C++11)
trunc
truncf
truncl nearest integer not greater in magnitude than the given value
(C++11) (function)
(C++11)
(C++11)
C documentation for
round