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>
   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