template< class T >
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).
The smallest integral power of two that is not smaller than x.
[edit] ExceptionsThrows nothing.
[edit] NotesPrior to P1956R1, the proposed name for this function template was ceil2.
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);
}
}[edit] Example
#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[edit] See also finds the largest integral power of 2 not greater than the given value
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