std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,(3)

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

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)