# std::bitset<N>::count

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< cpp‎ | utility‎ | bitset

std::size_t count() const; |  | (noexcept since C++11) (constexpr since C++23)

Returns the number of bits that are set to true.

### Return value

Number of bits that are set to true.

### Example

Run this code

    #include <bitset>
    #include <iostream>

    constexpr auto popcount(unsigned x) noexcept
    {
        unsigned num{};
        for (; x; ++num, x &= (x - 1));
        return num;
    };
    static_assert(popcount(0b101010) == std::bitset<8>{0b101010}.count());

    int main()
    {
        std::bitset<8> b("00010010");
        std::cout << "Initial value: " << b << '\n';

        // Find the first unset bit
        std::size_t idx = 0;
        while (idx < b.size() && b.test(idx))
            ++idx;

        // Continue setting bits until half the bitset is filled
        while (idx < b.size() && b.count() < b.size() / 2)
        {
            b.set(idx);
            std::cout << "Setting bit " << idx << ": " << b << '\n';
            while (idx < b.size() && b.test(idx))
                ++idx;
        }
    }

Output:

    Initial value: 00010010
    Setting bit 0: 00010011
    Setting bit 2: 00010111

### See also

size | returns the number of bits that the bitset holds (public member function)
popcount (C++20) | counts the number of 1 bits in an unsigned integer (function template)

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