std::popcount(3)

✨ std::popcount ✨

okay bestie, so this little icon lives in the C++ Standard Library (yes, that's the manual name, we're fancy here 💅) and honestly? it's giving "tiny function, huge brain." Let's get into it 🫶

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🌸 NAME 🌸

std::popcount — literally just std::popcount. That's the name. That's the whole name section. iconic of it, honestly 👑

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🦋 Synopsis 🦋

Defined in header <bit>

template< class T >                      (since C++20)
constexpr int popcount( T x ) noexcept;

Returns the number of 1 bits in the value of x. that's it. that's the function. no cap 💀

⚠️ okay this part matters so listen up: this overload only participates in overload resolution if T is an unsigned integer type — and "unsigned integer type" here means unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type. signed types? not invited to the party 🙈

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🎀 Parameters 🎀

• x — value of unsigned integer type

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💖 Return value 💖

The number of 1 bits in the value of x. so like, you hand it a number, it counts the 1s. very demure, very mindful 🧚

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🌈 Notes 🌈

The name popcount is a contraction for "population count" — and now you know, so iykyk ✨

Feature-test macro vibes, keep these exactly as they are:

• Feature-test macro: __cpp_lib_bitops
• Value: 201907L
• Std: (C++20)
• Feature: Bit operations

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🔥 Example 🔥

// Run this code

#include <bit>
#include <bitset>
#include <cstdint>
#include <iostream>

static_assert(std::popcount(0xFULL) == 4);

int main()
{
    for (const std::uint8_t x : {0, 0b00011101, 0b11111111})
        std::cout << "popcount( " << std::bitset<8>(x) << " ) = "
                  << std::popcount(x) << '\n';
}

lowkey that static_assert is a whole mood — 0xFULL has 4 set bits and the compiler checks it before the program even runs. obsessed 💜

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⭐ Output ⭐

popcount( 00000000 ) = 0
popcount( 00011101 ) = 4
popcount( 11111111 ) = 8

look at that. zero 1s → 0, four 1s → 4, all eight → 8. the math is mathing, bestie 🍓

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🦄 See also 🦄

• countl_zero (C++20) — counts the number of consecutive 0 bits, starting from the most significant bit (function template)
• countl_one (C++20) — counts the number of consecutive 1 bits, starting from the most significant bit (function template)
• countr_zero (C++20) — counts the number of consecutive 0 bits, starting from the least significant bit (function template)
• countr_one (C++20) — counts the number of consecutive 1 bits, starting from the least significant bit (function template)
• has_single_bit (C++20) — checks if a number is an integral power of 2 (function template)
• count — returns the number of bits set to true (public member function of std::bitset<N>)
• all, any, none — checks if all, any or none of the bits are set to true (public member function of std::bitset<N>)

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💫 that's the whole page, bestie — go count some bits and slay 💫

source: http://cppreference.com · version date: 2024.06.10 · page: std::popcount(3) — mwah, see you in the next manpage 💋🎀