std::has_single_bit(3)

✨ std::has_single_bit(3) ✨

okay so this is the C++ Standard Libary manual page for std::has_single_bit, and it is lowkey giving bit-nerd main character energy 💅. We are checking whether a number is an integral power of two, and the vibe check is immaculate. no cap.

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🌸 NAME 🌸

std::has_single_bit — std::has_single_bit (yes bestie, that is the whole name, iconic minimalism 💀)

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✨ Synopsis ✨

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

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).

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🎀 Parameters 🎀

• x — value of unsigned integer type.

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💖 Return value 💖

true if x is an integral power of two; otherwise false.

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🧚 Notes 🧚

• Feature-test macro: __cpp_lib_int_pow2 · Value: 202002L · Std: (C++20) · Feature: Integral power-of-2 operations

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👑 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;
}

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🦋 Example 🦋

// Run this code

#include <bit>
#include <bitset>
#include <cmath>
#include <iostream>

int main()
{
    for (auto u = 0u; u != 10; ++u)
    {
        std::cout << "u = " << u << " = " << std::bitset<4>(u);
        if (std::has_single_bit(u)) // `ispow2` before P1956R1
            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

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🌈 See also 🌈

• popcount — counts the number of 1 bits in an unsigned integer (C++20) (function template)
• count — returns the number of bits set to true (public member function of std::bitset<N>)
• test — accesses specific bit (public member function of std::bitset<N>)

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that's a wrap, bestie! source/version: http://cppreference.com · date: 2024.06.10 · page: std::has_single_bit(3) 💜✨ periodt.