# std::bitset::count

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< cpp‎ | utility‎ | bitset

std::size_t count() const; |  |

Returns the number of bits that are set to true.

### Parameters

(none)

### Return value

number of bits that are set to true.

### Exceptions

(none) | (until C++11)
`noexcept` specification: noexcept | (since C++11)

### Example

Run this code

    #include <iostream>
    #include <bitset>

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

        // find the first unset bit
        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 size number of bits that the bitset can hold (public member function)

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