W3cubDocs

std::bit_ceil

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

value of unsigned integer type

Return value

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

Exceptions

Throws nothing.

Notes

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

Possible implementation

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

template<std::unsigned_integral T>
    requires !std::same_as<T, bool> && !std::same_as<T, char> &&
             !std::same_as<T, char8_t> && !std::same_as<T, char16_t> &&
             !std::same_as<T, char32_t> && !std::same_as<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

#include <bit>
#include <bitset>
#include <iostream>
 
int main()
{
    using bin = std::bitset<8>;
 
    for (unsigned x{0}; x != 10; ++x)
    {
        unsigned const z = std::bit_ceil(x); // `ceil2` before P1956R1
        std::cout << "bit_ceil( " << bin(x) << " ) = " << bin(z) << '\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