NAME¶

std::bit_ceil - std::bit_ceil

Synopsis¶

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

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¶

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<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¶

// Run this code

#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

See also¶

bit_floor finds the largest integral power of two not greater than the given value
(C++20) (function template)