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std::ceil, std::ceilf, std::ceill

Defined in header <cmath>
(1)
float       ceil ( float num );
double      ceil ( double num );
long double ceil ( long double num );
(until C++23)
constexpr /* floating-point-type */
            ceil ( /* floating-point-type */ num );
(since C++23)
float       ceilf( float num );
(2) (since C++11)
(constexpr since C++23)
long double ceill( long double num );
(3) (since C++11)
(constexpr since C++23)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer >
double      ceil ( Integer num );
(A) (constexpr since C++23)
1-3) Computes the least integer value not less than num. The library provides overloads of std::ceil for all cv-unqualified floating-point types as the type of the parameter. (since C++23)
double
(since C++11)

Parameters

num - floating point or integer value

Return value

If no errors occur, the smallest integer value not less than num, that is ⌈num⌉, is returned.

Return value
math-ceil.svg
num

Error handling

Errors are reported as specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

  • 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

Notes

FE_INEXACT may be (but is not required to be) raised when rounding a non-integer finite value.

The largest representable floating-point values are exact integers in all standard floating-point formats, so this function never overflows on its own; however the result may overflow any integer type (including std::intmax_t), when stored in an integer variable. It is for this reason that the return type is floating-point not integral.

This function (for double argument) behaves as if (except for the freedom to not raise FE_INEXACT) implemented by the following code:

#include <cfenv>
#include <cmath>
 
#pragma STDC FENV_ACCESS ON
 
double ceil(double x)
{
    int save_round = std::fegetround();
    std::fesetround(FE_UPWARD);
    double result = std::rint(x); // or std::nearbyint
    std::fesetround(save_round);
    return result;
}

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::ceil(num) has the same effect as std::ceil(static_cast<double>(num)).

Example

#include <cmath>
#include <iostream>
 
int main()
{
    std::cout << std::fixed
              << "ceil(+2.4) = " << std::ceil(+2.4) << '\n'
              << "ceil(-2.4) = " << std::ceil(-2.4) << '\n'
              << "ceil(-0.0) = " << std::ceil(-0.0) << '\n'
              << "ceil(-Inf) = " << std::ceil(-INFINITY) << '\n';
}

Output:

ceil(+2.4) = 3.000000
ceil(-2.4) = -2.000000
ceil(-0.0) = -0.000000
ceil(-Inf) = -inf

See also

(C++11)(C++11)
nearest integer not greater than the given value
(function)
(C++11)(C++11)(C++11)
nearest integer not greater in magnitude than the given value
(function)
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)
nearest integer, rounding away from zero in halfway cases
(function)
(C++11)(C++11)(C++11)
nearest integer using current rounding mode
(function)
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)
nearest integer using current rounding mode with
exception if the result differs
(function)
C documentation for ceil
Fast ceiling of an integer division — StackOverflow

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