std::countr_one(3)

💅 std::countr_one(3)

okay bestie so this page lives in the C++ Standard Libary manual (yes that's how it's spelled in the header, don't come for me), and honestly? It's a little icon. Let's get into it.

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✨ NAME ✨

std::countr_one — literally std::countr_one, that's the whole name, we're starting strong 💖

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🌸 SYNOPSIS 🌸

Defined in header <bit>:

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

So what does it do? It returns the number of consecutive 1 bits in the value of x, starting from the least significant bit (that's the "right" side, iykyk).

Now the important little caveat: this overload only participates in overload resolution if T is an unsigned integer type — meaning unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type. No sneaky signed stuff, no cap 🫶

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🦋 PARAMETERS 🦋

• x — value of unsigned integer type

That's it. One parameter. Main character energy 👑

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⭐ RETURN VALUE ⭐

The number of consecutive 1 bits in the value of x, starting from the least significant bit. Same deal as above, just answering the question "okay but what do I get back" 💀

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🎀 NOTES 🎀

Feature-test macro vibes, here's the whole thing:

• __cpp_lib_bitops — value 201907L — Std (C++20) — Feature: Bit operations

Basically: if you're checking for bitops support, that's your macro 🔥

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🌈 EXAMPLE 🌈

Run this code, it's cute:

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

int main()
{
    for (const std::uint8_t i : {0, 0b11111111, 0b11111110, 0b11100011})
        std::cout << "countr_one( " << std::bitset<8>(i) << " ) = "
                  << std::countr_one(i) << '\n';
}

💜 Output

countr_one( 00000000 ) = 0
countr_one( 11111111 ) = 8
countr_one( 11111110 ) = 0
countr_one( 11100011 ) = 2

Look at that. 11111111 giving a full 8, and 11100011 stopping at 2 because it hits a 0 on the third bit from the right. The way the trailing ones just pile up and then get cut off 😭✨

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🧚 SEE ALSO 🧚

The whole bit-squad, all (since C++20) unless noted:

• countl_zero — counts the number of consecutive 0 bits, starting from the most significant bit (function template) — since C++20
• countl_one — counts the number of consecutive 1 bits, starting from the most significant bit (function template) — since C++20
• countr_zero — counts the number of consecutive 0 bits, starting from the least significant bit (function template) — since C++20
• popcount — counts the number of 1 bits in an unsigned integer (function template) — since C++20
• has_single_bit — checks if a number is an integral power of 2 (function template) — since C++20
• count — returns the number of bits set to true (public member function of std::bitset<N>)
• all — checks if all, any or none of the bits are set to true (public member function of std::bitset<N>)
• any — checks if all, any or none of the bits are set to true (public member function of std::bitset<N>)
• 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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lowkey obsessed with how countl_* looks left and countr_* looks right. left-to-right besties fr fr 🫶

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okay that's the page!! sourced from http://cppreference.com, dated 2024.06.10, and filed under std::countr_one(3) — go count some trailing ones, bestie, you're gonna slay 💅🎀🌸