Difference between revisions of "cpp/numeric/bit ceil"
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| − | Calculates the smallest integral power of two that is not smaller than {{ | + | Calculates the smallest integral power of two that is not smaller than {{c|x}}. |
If that value is not representable in {{tt|T}}, the behavior is undefined. Call to this function is permitted in [[cpp/language/constant expression|constant evaluation]] only if the undefined behavior does not occur. | If that value is not representable in {{tt|T}}, the behavior is undefined. Call to this function is permitted in [[cpp/language/constant expression|constant evaluation]] only if the undefined behavior does not occur. | ||
| − | {{cpp/enable_if|{{tt|T}} is an unsigned integer type (that is, {{c|unsigned char}}, {{c|unsigned short}}, {{c|unsigned int}}, {{c|unsigned long}}, {{c|unsigned long long}}, or an extended unsigned integer type)}}. | + | {{cpp/enable_if|{{tt|T}} is an unsigned integer type (that is, {{c/core|unsigned char}}, {{c/core|unsigned short}}, {{c/core|unsigned int}}, {{c/core|unsigned long}}, {{c/core|unsigned long long}}, or an extended unsigned integer type)}}. |
| − | === Parameters === | + | ===Parameters=== |
{{par begin}} | {{par begin}} | ||
| − | {{par | x | | + | {{par|x|value of unsigned integer type}} |
{{par end}} | {{par end}} | ||
| − | === Return value === | + | ===Return value=== |
| − | The smallest integral power of two that is not smaller than {{ | + | The smallest integral power of two that is not smaller than {{c|x}}. |
| − | === Exceptions === | + | ===Exceptions=== |
Throws nothing. | Throws nothing. | ||
| + | |||
| + | ===Notes=== | ||
| + | {{petty|Prior to {{stddoc|P1956R1}}, the proposed name for this function template was {{tt|ceil2}}.}} | ||
| + | |||
| + | {{feature test macro|__cpp_lib_int_pow2|std=C++20|value=202002L|[[cpp/numeric#Bit manipulation (since C++20)|Integral power-of-2 operations]]}} | ||
===Possible implementation=== | ===Possible implementation=== | ||
| − | See possible implementations in [https://github.com/gcc-mirror/gcc/blob/62c25d7adb1a5664982449dda0e7f9ca63cf4735/libstdc%2B%2B-v3/include/std/bit#L217-L248 libstdc++ (gcc)] and [https://github.com/llvm- | + | See possible implementations in [https://github.com/gcc-mirror/gcc/blob/62c25d7adb1a5664982449dda0e7f9ca63cf4735/libstdc%2B%2B-v3/include/std/bit#L217-L248 libstdc++ (gcc)] and [https://github.com/llvm/llvm-project/blob/llvmorg-14.0.4/libcxx/include/bit#L304-L321 libc++ (clang)]. |
{{eq fun|1= | {{eq fun|1= | ||
| − | template <std::unsigned_integral T> | + | template<std::unsigned_integral T> |
requires !std::same_as<T, bool> && !std::same_as<T, char> && | 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, char8_t> && !std::same_as<T, char16_t> && | ||
| − | !std::same_as<T, char32_t> && !std::same_as<T, wchar_t> | + | !std::same_as<T, char32_t> && !std::same_as<T, wchar_t> |
constexpr T bit_ceil(T x) noexcept | constexpr T bit_ceil(T x) noexcept | ||
{ | { | ||
| Line 36: | Line 41: | ||
if constexpr (std::same_as<T, decltype(+x)>) | if constexpr (std::same_as<T, decltype(+x)>) | ||
return T(1) << std::bit_width(T(x - 1)); | return T(1) << std::bit_width(T(x - 1)); | ||
| − | else { // for types subject to integral promotion | + | else |
| + | { // for types subject to integral promotion | ||
constexpr int offset_for_ub = | constexpr int offset_for_ub = | ||
std::numeric_limits<unsigned>::digits - std::numeric_limits<T>::digits; | std::numeric_limits<unsigned>::digits - std::numeric_limits<T>::digits; | ||
| Line 44: | Line 50: | ||
}} | }} | ||
| − | === Example === | + | ===Example=== |
{{example | {{example | ||
| − | + | |code= | |
| − | + | ||
#include <bit> | #include <bit> | ||
#include <bitset> | #include <bitset> | ||
#include <iostream> | #include <iostream> | ||
| − | + | int main() | |
{ | { | ||
using bin = std::bitset<8>; | using bin = std::bitset<8>; | ||
| − | + | for (auto x{0U}; 0XA != x; ++x) | |
| − | for ( | + | std::cout << "bit_ceil( " << bin(x) << " ) = " |
| − | + | << bin(std::bit_ceil(x)) << '\n'; | |
| − | + | ||
| − | + | ||
| − | std::cout << "bit_ceil( " << bin(x) << " ) = " << bin( | + | |
| − | + | ||
} | } | ||
| − | + | |output= | |
bit_ceil( 00000000 ) = 00000001 | bit_ceil( 00000000 ) = 00000001 | ||
bit_ceil( 00000001 ) = 00000001 | bit_ceil( 00000001 ) = 00000001 | ||
| Line 76: | Line 77: | ||
}} | }} | ||
| − | === See also === | + | ===See also=== |
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/numeric/dsc bit_floor}} | + | {{dsc inc|cpp/numeric/dsc bit_floor}} |
{{dsc end}} | {{dsc end}} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | ||
Latest revision as of 13:49, 2 November 2024
| 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).
Contents |
[edit] Parameters
| x | - | value of unsigned integer type |
[edit] Return value
The smallest integral power of two that is not smaller than x.
[edit] Exceptions
Throws nothing.
[edit] 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 |
[edit] 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); } } |
[edit] 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
[edit] See also
| (C++20) |
finds the largest integral power of two not greater than the given value (function template) |
