Difference between revisions of "cpp/numeric/bit ceil"
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Revision as of 05:57, 16 March 2022
| Defined in header <bit>
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| 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).
Contents |
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 |
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> auto main() -> signed int // :() { 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
| (C++20) |
finds the largest integral power of two not greater than the given value (function template) |
