std::rint, std::rintf, std::rintl, std::lrint, std::lrintf, std::lrintl, std::llrint, std::llrintf

From cppreference.com

Defined in header `<cmath>`
Rounding to floating-point types
	(1)
`float rint ( float num ); double rint ( double num ); long double rint ( long double num );`			(since C++11) (until C++23)
`/* floating-point-type / rint( / floating-point-type */ num );`			(since C++23)
`float rintf( float num );`		(2)	(since C++11)
`long double rintl( long double num );`		(3)	(since C++11)
Rounding to `long`
		(4)
`long lrint ( float num ); long lrint ( double num ); long lrint ( long double num );`				(since C++11) (until C++23)
`long lrint ( /* floating-point-type */ num );`				(since C++23)
`long lrintf( float num );`			(5)	(since C++11)
`long lrintl( long double num );`			(6)	(since C++11)
Rounding to `long long`
			(7)
`long long llrint ( float num ); long long llrint ( double num ); long long llrint ( long double num );`					(since C++11) (until C++23)
`long long llrint ( /* floating-point-type */ num );`					(since C++23)
`long long llrintf( float num );`				(8)	(since C++11)
`long long llrintl( long double num );`				(9)	(since C++11)
Additional overloads
Defined in header `<cmath>`
`template< class Integer > double rint( Integer num );`				(A)	(since C++11)
`template< class Integer > long lrint( Integer num );`				(B)	(since C++11)
`template< class Integer > long long llrint( Integer num );`				(C)	(since C++11)

1-3) Rounds the floating-point argument num to an integer value (in floating-point format), using the current rounding mode. The library provides overloads of std::rint for all cv-unqualified floating-point types as the type of the parameter num.(since C++23)

4-9) Rounds the floating-point argument num to an integer value, using the current rounding mode. The library provides overloads of std::lrint and std::llrint 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, according to the current rounding mode, is returned.

Error handling

Errors are reported as specified in math_errhandling.

If the result of std::lrint or std::llrint 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::rint function:

If num is ±∞, it is returned, unmodified.
If num is ±0, it is returned, unmodified.
If num is NaN, NaN is returned.

For std::lrint and std::llrint functions:

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

POSIX specifies that all cases where std::lrint or std::llrint raise FE_INEXACT are domain errors.

As specified in math_errhandling, FE_INEXACT may be (but isn't required to be on non-IEEE floating-point platforms) raised by std::rint when rounding a non-integer finite value.

The only difference between std::rint and std::nearbyint is that std::nearbyint never raises FE_INEXACT.

The largest representable floating-point values are exact integers in all standard floating-point formats, so std::rint never overflows on its own; however the result may overflow any integer type (including std::intmax_t), when stored in an integer variable.

If the current rounding mode is:

FE_DOWNWARD, then std::rint is equivalent to std::floor.
FE_UPWARD, then std::rint is equivalent to std::ceil.
FE_TOWARDZERO, then std::rint is equivalent to std::trunc.
FE_TONEAREST, then std::rint differs from std::round in that halfway cases are rounded to even rather than away from zero.

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::rint(num) has the same effect as std::rint(static_cast<double>(num)).
std::lrint(num) has the same effect as std::lrint(static_cast<double>(num)).
std::llrint(num) has the same effect as std::llrint(static_cast<double>(num)).

Example

#include <cfenv>
#include <climits>
#include <cmath>
#include <iostream>
// #pragma STDC FENV_ACCESS ON
 
int main()
{
    std::fesetround(FE_TONEAREST);
    std::cout << "Rounding to nearest (halfway cases to even):\n"
              << "  rint(+2.3) = " << std::rint(2.3) << '\n'
              << "  rint(+2.5) = " << std::rint(2.5) << '\n'
              << "  rint(+3.5) = " << std::rint(3.5) << '\n'
              << "  rint(-2.3) = " << std::rint(-2.3) << '\n'
              << "  rint(-2.5) = " << std::rint(-2.5) << '\n'
              << "  rint(-3.5) = " << std::rint(-3.5) << '\n';
 
    std::fesetround(FE_DOWNWARD);
    std::cout << "Rounding down:\n"
              << "  rint(+2.3) = " << std::rint(2.3) << '\n'
              << "  rint(+2.5) = " << std::rint(2.5) << '\n'
              << "  rint(+3.5) = " << std::rint(3.5) << '\n'
              << "  rint(-2.3) = " << std::rint(-2.3) << '\n'
              << "  rint(-2.5) = " << std::rint(-2.5) << '\n'
              << "  rint(-3.5) = " << std::rint(-3.5) << '\n'
              << "Rounding down with lrint:\n"
              << "  lrint(+2.3) = " << std::lrint(2.3) << '\n'
              << "  lrint(+2.5) = " << std::lrint(2.5) << '\n'
              << "  lrint(+3.5) = " << std::lrint(3.5) << '\n'
              << "  lrint(-2.3) = " << std::lrint(-2.3) << '\n'
              << "  lrint(-2.5) = " << std::lrint(-2.5) << '\n'
              << "  lrint(-3.5) = " << std::lrint(-3.5) << '\n'
              << "Special values:\n"
              << "  lrint(-0.0) = " << std::lrint(-0.0) << '\n'
              << std::hex << std::showbase
              << "  lrint(-Inf) = " << std::lrint(-INFINITY) << '\n';

    // error handling
    std::feclearexcept(FE_ALL_EXCEPT);
 
    std::cout << "std::rint(0.1) = " << std::rint(.1) << '\n';
    if (std::fetestexcept(FE_INEXACT))
        std::cout << "  FE_INEXACT was raised\n";
 
    std::feclearexcept(FE_ALL_EXCEPT);
 
    std::cout << "std::lrint(LONG_MIN-2048.0) = "
              << std::lrint(LONG_MIN - 2048.0) << '\n';
    if (std::fetestexcept(FE_INVALID))
        std::cout << "  FE_INVALID was raised\n";
}

Possible output:

Rounding to nearest (halfway cases to even):
  rint(+2.3) = 2
  rint(+2.5) = 2
  rint(+3.5) = 4
  rint(-2.3) = -2
  rint(-2.5) = -2
  rint(-3.5) = -4
Rounding down:
  rint(+2.3) = 2
  rint(+2.5) = 2
  rint(+3.5) = 4
  rint(-2.3) = -2
  rint(-2.5) = -2
  rint(-3.5) = -4
Rounding down with lrint:
  lrint(+2.3) = 2
  lrint(+2.5) = 2
  lrint(+3.5) = 3
  lrint(-2.3) = -3
  lrint(-2.5) = -3
  lrint(-3.5) = -4
Special values:
  lrint(-0.0) = 0
  lrint(-Inf) = 0x8000000000000000
std::rint(0.1) = 0
std::lrint(LONG_MIN-2048.0) = 0x8000000000000000
  FE_INVALID was raised

Parameters

Return value

Error handling

Notes

Example

See also