std::round, std::roundf, std::roundl, std::lround, std::lroundf, std::lroundl, std::llround, std::llroundf (original) (raw)

Defined in header
Rounding to floating-point types
(1)
float round ( float num ); double round ( double num ); long double round ( long double num );		(since C++11) (until C++23)
constexpr /* floating-point-type / round ( / floating-point-type */ num );		(since C++23)
float roundf( float num );	(2)	(since C++11) (constexpr since C++23)
long double roundl( long double num );	(3)	(since C++11) (constexpr since C++23)
Rounding to long
(4)
long lround ( float num ); long lround ( double num ); long lround ( long double num );		(since C++11) (until C++23)
constexpr long lround( /* floating-point-type */ num );		(since C++23)
long lroundf( float num );	(5)	(since C++11) (constexpr since C++23)
long lroundl( long double num );	(6)	(since C++11) (constexpr since C++23)
Rounding to long long
(7)
long long llround ( float num ); long long llround ( double num ); long long llround ( long double num );		(since C++11) (until C++23)
constexpr long long llround( /* floating-point-type */ num );		(since C++23)
long long llroundf( float num );	(8)	(since C++11) (constexpr since C++23)
long long llroundl( long double num );	(9)	(since C++11) (constexpr since C++23)
Additional overloads
Defined in header
template< class Integer > double round( Integer num );	(A)	(since C++11) (constexpr since C++23)
template< class Integer > long lround( Integer num );	(B)	(since C++11) (constexpr since C++23)
template< class Integer > long long llround( Integer num );	(C)	(since C++11) (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.

[edit] Parameters

num	-	floating-point or integer value

[edit] 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.

[edit] 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.

[edit] 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:

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)).

[edit] Example

#include #include #include #include #include #include // #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(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: