std::bit_ceil

 

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Defined in header <bit>
template< class T > constexpr T bit_ceil( T x );		(since C++20)

See More

Calculates the smallest integral power of two that is not smaller than x.

If that value is not representable in T, the behavior is undefined.

Parameters

x	-	a value to compare with
Type requirements
T	-	must be an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type) in order to participate in overload resolution.

Return value

The smallest integral power of two that is not smaller than x.

Exceptions

Throws nothing.

Notes

Prior to P1956R1, the proposed name for this function template was ceil2.

Feature-test macro	Value	Std	Feature
__cpp_lib_int_pow2	202002L	(C++20)	Integral power-of-2 operations

Possible implementation

See possible implementations in libstdc++ (gcc) and libc++ (clang).

template<typename T, typename ... U> concept neither = (!std::same_as<T, U> && ...); template<std::unsigned_integral T> requires neither<T, bool, char, char8_t, char16_t, char32_t, wchar_t> constexpr T bit_ceil(T x) noexcept { if (x == 0) return T(1); T bit_index = std::bit_width(T(x - 1)); if (bit_index >= std::numeric_limits<T>::digits) std::unreachable(); // UB: result is not representable in T return T(1) << bit_index; }

Example

Run this code

#include <bit>
#include <print>

int main()
{
    for (unsigned x{}; x != 0xA; ++x)
        std::print("bit_ceil({:04b}) = {:05b}\n{}",
            x, std::bit_ceil(x), std::has_single_bit(x) ? "\n" : "");
}

Output:

bit_ceil(0000) = 00001
bit_ceil(0001) = 00001

bit_ceil(0010) = 00010

bit_ceil(0011) = 00100
bit_ceil(0100) = 00100

bit_ceil(0101) = 01000
bit_ceil(0110) = 01000
bit_ceil(0111) = 01000
bit_ceil(1000) = 01000

bit_ceil(1001) = 10000

See also

bit_floor (C++20)

finds the largest integral power of 2 not greater than the given value (function template)