Difference between revisions of "cpp/numeric/has single bit"
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Checks if {{c|x}} is an integral power of two. | Checks if {{c|x}} is an integral power of two. | ||
| − | {{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=== | ||
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===Notes=== | ===Notes=== | ||
| + | {{petty|Prior to {{stddoc|P1956R1}}, the proposed name for this function template was {{tt|ispow2}}.}} | ||
| + | |||
{{feature test macro|__cpp_lib_int_pow2|[[cpp/numeric#Bit manipulation (since C++20)|Integral power-of-2 operations]]|std=C++20|value=202002L}} | {{feature test macro|__cpp_lib_int_pow2|[[cpp/numeric#Bit manipulation (since C++20)|Integral power-of-2 operations]]|std=C++20|value=202002L}} | ||
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int main() | int main() | ||
{ | { | ||
| − | for (auto u | + | for (auto u{0u}; u != 0B1010; ++u) |
{ | { | ||
std::cout << "u = " << u << " = " << std::bitset<4>(u); | std::cout << "u = " << u << " = " << std::bitset<4>(u); | ||
| − | if (std::has_single_bit(u)) | + | if (std::has_single_bit(u)) |
std::cout << " = 2^" << std::log2(u) << " (is power of two)"; | std::cout << " = 2^" << std::log2(u) << " (is power of two)"; | ||
std::cout << '\n'; | std::cout << '\n'; | ||
Latest revision as of 13:39, 2 November 2024
| Defined in header <bit>
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| template< class T > constexpr bool has_single_bit( T x ) noexcept; |
(since C++20) | |
Checks if x is an integral power of two.
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
true if x is an integral power of two; otherwise false.
[edit] Notes
Prior to P1956R1, the proposed name for this function template was ispow2.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_int_pow2 |
202002L | (C++20) | Integral power-of-2 operations |
[edit] Possible implementation
| First version |
|---|
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 bool has_single_bit(T x) noexcept { return x && !(x & (x - 1)); } |
| Second version |
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 bool has_single_bit(T x) noexcept { return std::popcount(x) == 1; } |
[edit] Example
Run this code
#include <bit> #include <bitset> #include <cmath> #include <iostream> int main() { for (auto u{0u}; u != 0B1010; ++u) { std::cout << "u = " << u << " = " << std::bitset<4>(u); if (std::has_single_bit(u)) std::cout << " = 2^" << std::log2(u) << " (is power of two)"; std::cout << '\n'; } }
Output:
u = 0 = 0000 u = 1 = 0001 = 2^0 (is power of two) u = 2 = 0010 = 2^1 (is power of two) u = 3 = 0011 u = 4 = 0100 = 2^2 (is power of two) u = 5 = 0101 u = 6 = 0110 u = 7 = 0111 u = 8 = 1000 = 2^3 (is power of two) u = 9 = 1001
[edit] See also
| (C++20) |
counts the number of 1 bits in an unsigned integer (function template) |
| returns the number of bits set to true (public member function of std::bitset<N>)
| |
| accesses specific bit (public member function of std::bitset<N>)
|
