std::bit_ceil

 

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Bit manipulation

bit_cast (C++20)

byteswap (C++23)

Integral powers of 2

has_single_bit (C++20)

bit_ceil (C++20)

bit_floor (C++20)

bit_width (C++20)

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

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. Call to this function is permitted in constant evaluation only if the undefined behavior does not occur.

This overload participates in overload resolution only if T is an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type).

Parameters

x

-

value of unsigned integer type

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 <= 1u) return T(1); if constexpr (std::same_as<T, decltype(+x)>) return T(1) << std::bit_width(T(x - 1)); else { // for types subject to integral promotion constexpr int offset_for_ub = std::numeric_limits<unsigned>::digits - std::numeric_limits<T>::digits; return T(1u << (std::bit_width(T(x - 1)) + offset_for_ub) >> offset_for_ub); } }

Example

Run this code

#include <bit>
#include <bitset>
#include <iostream>

int main()
{
    using bin = std::bitset<8>;
    for (auto x{0U}; 0XA != x; ++x)
        std::cout << "bit_ceil( " << bin(x) << " ) = "
                  << bin(std::bit_ceil(x)) << '\n';
}

Output:

bit_ceil( 00000000 ) = 00000001
bit_ceil( 00000001 ) = 00000001
bit_ceil( 00000010 ) = 00000010
bit_ceil( 00000011 ) = 00000100
bit_ceil( 00000100 ) = 00000100
bit_ceil( 00000101 ) = 00001000
bit_ceil( 00000110 ) = 00001000
bit_ceil( 00000111 ) = 00001000
bit_ceil( 00001000 ) = 00001000
bit_ceil( 00001001 ) = 00010000

See also

bit_floor (C++20)

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