# std::round, std::roundf, std::roundl, std::lround, std::lroundf, std::lroundl, std::llround, std::llroundf

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< cpp‎ | numeric‎ | math

Defined in header `<cmath>` |  |
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 `<cmath>` |  |
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.

### 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
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

floorfloorffloorl (C++11)(C++11) | nearest integer not greater than the given value (function)
ceilceilfceill (C++11)(C++11) | nearest integer not less than the given value (function)
trunctruncftruncl (C++11)(C++11)(C++11) | nearest integer not greater in magnitude than the given value (function)
C documentation for round

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